Minimizing discard volume to 2 ml before blood sampling from a central venous catheter: a prospective observational study in India

Abinaya Pushparaj; Lakshmi Ramamoorthy; Manu Ayyan S; Rajeswari Murugesan; Hmar Thiak Lalthanthuami

doi:10.4266/acc.000792

Articles

Page Path: HOME > Acute Crit Care > Volume 41(1); 2026 > Article

Original Article Nursing Minimizing discard volume to 2 ml before blood sampling from a central venous catheter: a prospective observational study in India: Acute and Critical Care 2026;41(1):174-182.
DOI: https://doi.org/10.4266/acc.000792
Published online: February 9, 2026

Abinaya Pushparaj¹

, Lakshmi Ramamoorthy¹

, Manu Ayyan S²

, Rajeswari Murugesan³

, Hmar Thiak Lalthanthuami¹

¹College of Nursing, Jawaharlal Institute of Postgraduate Medical Education and Research (JIPMER), Puducherry, India

²Department of Emergency Medicine and Trauma, Jawaharlal Institute of Postgraduate Medical Education and Research (JIPMER), Puducherry, India

³Department of Biostatistics, Jawaharlal Institute of Postgraduate Medical Education and Research (JIPMER), Puducherry, India

Corresponding author: Lakshmi Ramamoorthy College of Nursing, Jawaharlal Institute of Postgraduate Medical Education and Research (JIPMER), Puducherry 605006, India Tel: +91-984-239-9340 Email: laxmi_ramamoorthy@yahoo.com

• Received: November 14, 2023 • Revised: October 30, 2025 • Accepted: November 14, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

4,218 Views
92 Download
1 Web of Science
1 Crossref

prev next

Full Article

Download PDF

Abstract
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
KEY MESSAGES
NOTES
REFERENCES

Abstract

Background
Central venous catheters (CVCs) are essential for the treatment of critically ill patients. The practice of discarding varying volumes of blood before sampling from CVCs is widely adopted globally. However, there is no established consensus regarding the appropriate volume of blood to discard before sampling. Therefore, this study aims to compare serum electrolyte levels and hemogram measurements obtained via direct venous puncture with those derived from the proximal lumen of the CVC using a discard volume of 2 ml.
Methods
Using a prospective observational study design, 105 patients from intensive care units of a tertiary care center were selected through a convenience sampling method. The parameters analyzed included serum electrolytes (sodium, potassium, and calcium) and complete blood counts (hemograms). Intraclass correlation and Bland-Altman plot analysis were employed to assess agreement between the two sampling methods.
Results
The study showed high agreement between the two methods, with a P-value of <0.001, except for white blood cell. The presence of acidosis as a clinical confounder was the primary reason for the differences in agreement for potassium (P=0.018), hematocrit (P=0.006), mean corpuscular volume (P=0.041), mean corpuscular hemoglobin (P=0.034), mean corpuscular hemoglobin concentration (P=0.031), neutrophil (P=0.045), and eosinophil (P=0.553).
Conclusions
This study suggests that discarding 2 ml of blood, rather than 3 ml or more, before routine sampling from CVCs is sufficient and may help to reduce iatrogenic blood loss in critically ill patients.
Key Words: blood cell count; central venous catheters; critical illness; discard volume; electrolytes; eosinophils; hematocrit; intensive care units; neutrophils; tertiary care centers

INTRODUCTION

Patients admitted to an intensive care unit (ICU) often undergo various invasive and non-invasive procedures. A common procedure is blood collection through direct venous puncture for diagnosis and treatment. Using blood samples from an indwelling central venous catheter (CVC) can eliminate the need for peripheral vein puncture, which may be contraindicated due to the patient's condition or medication [1]. CVCs are vital for critically ill patients, enabling safe administration of drugs that cannot be given peripherally, supporting intravenous fluid resuscitation, collecting venous blood samples, and closely monitoring hemodynamic stability in cases of septic shock, cardiogenic shock, decompensated heart failure, and pulmonary hypertension [2].

Despite the benefits of CVC placement, concerns about the risks of catheter-related bloodstream infections and central line-associated bloodstream infections continue. Strict adherence to aseptic techniques and Centers for Disease Control and Prevention guidelines helps clinicians and nurses manage factors related to the catheter, the patient, and technical aspects in critically ill patients with CVCs [3]. Several methods are available for collecting blood samples from CVCs. Each method has its own advantages and potential complications. These include the discard method, reinfusion method, push-pull method, and dead space method. In the reinfusion method, clinicians reinfuse the initial volume of blood collected back into the patient after taking laboratory samples. This practice helps reduce the risk of anemia by limiting blood loss. The push-pull or mixing method does not require discarding the initial sample. Instead, before collecting the sample, the clinician draws the patient’s blood through the CVC lumen and reinfuses it back into the catheter approximately three times [4].

The discard method is the most used technique. The discard method in central venous blood sampling is the procedure of withdrawing and discarding an initial volume of blood from a CVC before collecting the sample. The type and size of CVC determine how much blood should be discarded before sampling. When collecting blood samples from the CVC, we usually discard 5 to 10 mL before taking the actual sample. This helps prevent dilution, contamination from leftover heparinized fluid, or the presence of infusion substances. However, there is no agreed standard for blood to be discarded from the central line before sampling [5,6].

In critically ill patients, repeated blood draws can cause anemia from iatrogenic blood loss. Compared with direct venous puncture, blood loss is higher when sampling from a CVC using the discard method, which increases the need for blood transfusions [1]. A study found that, in critically ill patients, the mean blood loss ranged from 18 to 377 ml per day, with discard volumes accounting for approximately 24% to 30% [7-9]. Impaired bone marrow function and chemotherapy-induced effects on red blood cell production further increase the risk of anemia, especially in oncology patients [10-12].

On the other hand, serum laboratory values also showed no significant difference between CVC samples with different discard volumes (5 ml and 10 ml) and venipuncture samples [9,10]. In another study, Blood samples collected from the proximal lumen of a triple-lumen CVC using a 2-ml discard volume, without stopping infusions in the other lumens, yielded complete blood count, chemistry, and coagulation test results that were comparable with samples obtained through direct venous puncture [1]. Similarly, the results showed the reliability of the blood samples drawn with a discard volume of 1.5 ml [9]. However, evidence regarding the reliability of using a 2-ml or 1.5-ml discard volume from the proximal lumen of a CVC remains limited. Currently, there is no established standard discard volume before collecting a blood sample from a CVC. Therefore, this study aims to determine the agreement between hemogram and electrolyte measurements obtained from samples collected by direct venipuncture and those from the proximal lumen of a CVC using a 2-ml discard volume.

MATERIALS AND METHODS

The study received approval from the Research Monitoring Committee (CON/NRMC/M.Sc./2021/MSN/1) and the Institute Ethics Committee for Human Studies (CON/IEC/M.Sc./2021/MSN/1) of Jawaharlal institute of Post Graduate Medical Education and Research Institute, Puducherry, India. It was conducted in accordance with the Helsinki Declaration. Informed consent was obtained from each participant’s legally authorized representative. Participants’ confidentiality and anonymity were maintained, and they had the right to withdraw from the study at any time. All participant data were securely stored and handled with strict confidentiality.

Study Design and Sampling

The prospective observational study was conducted to assess the agreement between hemogram and electrolyte measurements obtained from samples collected by direct venipuncture and from the proximal lumen of a CVC using a 2-ml discard volume. The study involved patients admitted to the emergency room and trauma ICUs of a public tertiary care center in South India over seven months from September 2022 to March 2023. A total of 105 samples were selected through convenience sampling. The sample size was estimated based on an expected intraclass correlation coefficient (ICC) of 0.70 between blood samples from direct venipuncture and the proximal CVC, using a 2-ml discard volume, with a null hypothesis of 0.50, a 5% level of significance, and 90% power. Inclusion Criteria were patients aged 18 years or older with a triple-lumen CVC. Exclusion criteria included patients whose proximal lumen of the CVC was obstructed or used for administering vasoactive or inotropic drugs, blood, or blood products. Direct venipuncture was not feasible in patients with bandages, casts, or peripheral catheters.

Sample collections were performed concurrently with clinically indicated samples. Parameters assessed included serum electrolytes (sodium, potassium, and calcium) and hemogram indices such as hemoglobin, red blood cells (RBC), hematocrit (HCT), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), red cell distribution width-standard deviation (RDW-SD), white blood cell (WBC) count, neutrophils, eosinophils, basophils, lymphocytes, monocytes, absolute eosinophil count (AEC), absolute neutrophil count (ANC), and platelet count.

Data Collection Procedure

Blood samples were collected from both direct venous puncture and the proximal lumen of the CVC for each patient. All samples were labeled correctly and analyzed in the biochemistry laboratory. First, blood was drawn by direct venous puncture from the peripheral veins of either the right or left arm. After preparing the necessary equipment and performing hand hygiene, the skin at the puncture site was cleaned with a 0.5% chlorhexidine swab. Using a new 5-ml syringe, 4 ml of blood was withdrawn from the direct venous puncture site and transferred into two separate vials: 2 ml for hemogram testing and 2 ml for serum electrolyte analysis, and labeled as the first study sample.

Subsequently, a second blood sample was collected from the CVC's proximal lumen. Following equipment preparation and hand hygiene, the hub of the central line was cleaned with an alcohol prep pad for 15 seconds and allowed to dry. A 5-ml syringe was attached to the catheter, and 2 ml of blood was gently aspirated and discarded. A new 5-ml syringe was then used to collect an additional 4 ml of blood, which was transferred into two separate vials—one for hemogram testing and the other for serum electrolytes—labeled as the second study sample, with the timing of sample transport recorded in accordance with institutional policy. After blood withdrawal, the proximal lumen of the CVC was flushed with 5 ml of heparinized saline in a pulsatile manner, as per institutional protocol. The hub was cleaned with an alcohol prep pad, and all used syringes and discarded materials were properly disposed of.

Statistical Analysis

The agreement between the two measurement methods was assessed using ICC and Bland-Altman plot analysis. The relationship between differences in blood parameters (using a intraclass correlation coefficient) and clinical confounders was analyzed using analysis of variance, the Kruskal-Wallis test, and the Wilcoxon signed-rank test.

RESULTS

The male participants comprised a larger portion (60%), while females accounted for about 40% of the 105 participants. The mean age was 46.10±14.44 years. The most common site for CVC placement was the right subclavian vein (51 cases, 48.5%), while the right femoral vein was the least commonly used site (2 cases, 1.9%) (Table 1).

The level of agreement for serum parameters, including sodium, potassium, calcium, hemoglobin, RBC, HCT, MCV, MCH, MCHC, RDW-SD, neutrophil, eosinophil, basophil, lymphocyte, monocyte, AEC, ANC, and platelet count, was assessed using ICC. All parameters showed a statistically significant level of agreement (P<0.001), except for WBC, which was not significant (P=0.482) (Table 2).

The agreement between samples obtained by direct venous puncture and from the proximal lumen of the CVC, using a 2-ml discard volume, was evaluated through a parameter-wise Bland–Altman plot analysis. Few points lie outside the limits, indicating high agreement between the two methods for measuring sodium, potassium, calcium, hemoglobin, RBCs, and platelets, with ICCs of 0.954, 0.818, 0.862, 0.956, 0.937, and 0.965 (P<0.001), respectively. The non-random scattering of points above and below zero on the Bland-Altman plot indicates a consistent bias in WBC measurement between the direct venous puncture and the CVC 2 mL discard method, with an ICC of 0.009 (P=0.482) (Figure 1).

When comparing blood parameters collected through venipuncture and the 2 ml discard method from the CVC at various insertion sites, significant differences were noted in calcium, MCV, and WBC. CVCs inserted via the right internal jugular vein showed the smallest variances in potassium, MCV, and WBC levels, whereas left subclavian CVCs displayed the largest differences for all three parameters (Table 3). Among the 32 patients with acidosis, a significant difference between the two methods was observed for several blood parameters, including potassium (P=0.018), HCT (P=0.006), MCV (P=0.041), MCH (P=0.034), MCHC (P=0.031), and neutrophil count (P=0.045) (Table 4).

DISCUSSION

This study compared serum electrolyte and hemogram measurements obtained by direct venous puncture with those from the CVC proximal lumen using a discard volume of 2 ml. It was observed that the results were comparable with both methods, and a larger discard volume—typically 3 ml or more—is usually unnecessary for routine testing in critically ill patients, thereby minimizing unnecessary blood loss.

Vital monitoring of critically ill patients usually relies on laboratory tests, with venipuncture as the most common method for blood sampling. Repeated venipuncture can lead to complications such as hematoma, phlebitis, and nerve damage. For patients with CVCs, blood sampling through the CVCs' lumens provides an alternative that avoids multiple peripheral needle sticks. Among the available techniques for blood sampling through CVCs' lumens, the discard method is considered the most reliable and is used worldwide. However, the optimal discard volume remains a topic of debate. Some studies recommend volumes ranging from 2 ml to as high as 9 ml, depending on the analytes, sampling technique, and the type or size of CVC used [1,5,13-19].

A CVC is a thin, flexible tube inserted into a large vein, usually in the neck or chest. The internal volume of the catheter system, including the lumen and its connectors, is called the dead space or purge volume [9]. Before drawing a blood sample, this volume must be flushed out to prevent dilution or contamination from residual IV fluids or flush solutions [8]. Although discard volume has decreased, it would be more useful to compare it relative to the lumen's purge volume used for extraction by calculating the ratio of discard volume to that purge volume. This comparison helps determine whether the discard volume is larger or smaller due to differences in CVC sizes. In this study, the purge volume for priming the proximal lumen of the CVC catheter was 0.34 mL, resulting in a nearly 6:1 ratio of discard volume to the catheter's dead space, consistent with other studies [2]. However, since the flush volume is often not reported, this ratio cannot be systematically used to compare disparities across different studies [1,5,9,20]. Similarly, an observational study in critically ill patients found that discarding 2 ml from CVCs—containing 1.61 ml of blood and approximately 0.39 ml of dead space—showed excellent consistency with direct venipuncture for key analytes, including sodium, potassium, leukocytes, and glucose [1].

The current study demonstrated nearly perfect agreement across all parameters examined when using two methods: the 2 mL discard from the proximal lumen of the CVC and direct venous puncture. A similar study by Villalta-García et al. [1] found that blood samples obtained either by direct puncture or from the proximal lumen of a triple-lumen CVC with a 2-ml discard volume in 54 critically ill patients demonstrated excellent reliability. The ICC and Bland-Altman plots ranged from 0.868 to 0.998 for electrolytes and Hemogram parameters, indicating that the blood collection method introduced minimal systematic errors both statistically and clinically, and that both methods yield comparable results. Additionally, the current literature reports very slight differences in hemoglobin, potassium, and creatinine levels that are not clinically significant.

In our study, patients with acidosis demonstrated significant differences in potassium levels and hematological parameters, including MCV, MCHC, HCT, neutrophil, and eosinophil counts between the two blood collection methods. Because electrolyte levels in the blood can be affected by acid–base balance, the electrolyte values measured from the two methods may be influenced by the participant’s acid-base status. These results suggest that although a 2-ml discard volume is generally reliable, acid-base disturbances can subtly impact specific hematologic and electrolyte measurements. Since acidosis can cause potassium to move into extracellular compartments and alter red blood cell shape, this may result in differences in HCT, MCV, MCH, and MCHC values between sampling methods. Therefore, cautious interpretation is essential in patients with metabolic or respiratory disorders, including those with acidosis. However, the confounding effects of acid-base imbalance and hemodynamic instability are not well studied in the literature.

Overall, the data clearly show a statistically significant agreement between the 2-ml discard volume technique from the proximal lumen of the CVC and the direct venous puncture method for serum electrolytes and hemogram parameters, except for WBC counts. These findings support existing studies that compared a 2–3-ml discard volume from the proximal lumen of a CVC with direct venous puncture [5,8,9,21]. In summary, these results support using a 2-ml discard volume when collecting blood samples from CVCs for routine clinical analysis.

This study's findings have important practical implications. Reducing the discard volume from 3–5 ml to 2 ml can help prevent unnecessary blood loss, especially in patients who need frequent laboratory tests, such as those in ICUs. Repeated blood sampling often causes iatrogenic anemia, which is linked to increased transfusions, longer hospital stays, and higher morbidity. Implementing a standard 2 ml discard protocol could lower this risk while preserving diagnostic accuracy. Additionally, standardizing discard volume procedures might reduce variations among institutions and simplify nursing protocols. Since the proximal lumen of a CVC is often used for blood draws, confirming that a minimal discard volume of 2 ml is sufficient improves both safety and efficiency in clinical workflows.

This single-center study, limited by its sample size, may have restricted the generalizability of its findings. Patients with severe instability or complex acid–base disturbances were underrepresented, and the small acidosis subgroup diminished the study's statistical power. Factors affecting preanalytical conditions—such as residual heparin, flushing methods, and draw speed—were not systematically recorded. Only basic hematologic and electrolyte parameters were examined. As an observational study, unrecognized confounding variables could not be excluded. These limitations suggest that a 2-ml discard volume may be sufficient for routine testing; however, larger, multicenter studies are needed to confirm this.

The main findings of this study indicate that serum parameters collected through direct venous puncture and a 2-ml discard volume from a CVC exhibit high agreement, without compromising diagnostic accuracy, in critically ill patients. This supports the practice of discarding 2 ml during blood sampling from CVCs to minimize iatrogenic blood loss among the patients in critical care units.

KEY MESSAGES

▪ Drawing blood samples through a central venous catheter is a common practice in intensive care units. It is necessary to discard varying volumes of blood—typically 5–10 ml—to prevent interference with test results and avoid iatrogenic blood loss.

▪ This study's results support using a 2-ml discard volume from central venous catheters before blood sampling instead of the current practices of discarding 3 ml, 5 ml, or larger amounts to decrease iatrogenic blood loss and related complications.

NOTES

CONFLICT OF INTEREST

No potential conflict of interest relevant to this article was reported.

FUNDING

This study was supported by the Jawaharlal Institute of Postgraduate Medical Education and Research (JIPMER) Intramural Research Fund.

ACKNOWLEDGMENTS

We acknowledge all patients and staff of the emergency care unit for their kind cooperation and support during the study.

AUTHOR CONTRIBUTIONS

Conceptualization: AP, LR, MAS, RM, HTL. Data curation: AP, LR, MAS, RM, HTL. Formal analysis: LR, MAS, RM, HTL. Funding acquisition: AP, LR. Methodology: AP, LR, MAS, RM, HTL. Project administration: LR, MAS. Visualization: AP, LR, MAS, RM, HTL. Writing - Original draft: AP, LR, HTL. Writing - review & editing: AP, LR, MAS, RM, HTL. All authors read and agreed to the published version of the manuscript.

Figure 1.

Bland-Altman plots for blood parameters with the difference between direct venous puncture and 2 ml discarded central venous catheter samples plotted against the mean values. SD: standard deviation; RBC: red blood cells; WBC: white blood cell; MCV: mean corpuscular volume.

Table 1.

Distribution of demographic and clinical variables of the participants (n=105)

Variable	Value
Sex
Male	63 (60.0)
Female	42 (40.0)
Diagnosis
Neurological system	60 (57.1)
Cardiovascular system	15 (14.3)
Gastrointestinal system	30 (28.6)
Site of central venous catheter placement
Right subclavian vein	51 (48.5)
Left subclavian vein	7 (6.7)
Right femoral vein	2 (1.9)
Right intrajugular vein	45 (42.9)
Arterial blood gas values
pH	7.40±0.07
PCO₂ (mm Hg)	37.79±24.60
PO₂ (mm Hg)	165.29±78.11
Bicarbonate (mEq/L)	21.81±4.01
Age (yr)	46.10±14.44

Values are presented as number (%) or mean±standard deviation.

Table 2.

Intraclass correlation for agreement between venipuncture and 2 ml discard method from central venous catheter on selected blood parameters (n=105)

Blood parameter	ICC	95 % CI	P-value
Sodium (mEq/L)	0.954	0.933 to 0.969	<0.001
Potassium (mEq/L)	0.818	0.732 to 0.876	<0.001
Calcium (mg/dl)	0.862	0.798 to 0.907	<0.001
Hemoglobin (g/dl)	0.956	0.936 to 0.970	<0.001
Red blood cells (×10⁶/μl)	0.937	0.907 to 0.957	<0.001
Hematocrit (%)	0.959	0.939 to 0.972	<0.001
Mean corpuscular volume (fL)	0.986	0.979 to 0.990	<0.001
Mean corpuscular hemoglobin (pg)	0.985	0.978 to 0.990	<0.001
Mean corpuscular hemoglobin concentration (g/dl)	0.909	0.866 to 0.938	<0.001
Red cell distribution width to standard deviation (%)	0.733	0.607 to 0.819	<0.001
White blood cell (×10³/μl)	0.009	–0.459 to 0.327	0.482
Neutrophils (%)	0.898	0.849 to 0.930	<0.001
Eosinophils (%)	0.964	0.948 to 0.976	<0.001
Basophils (%)	0.856	0.788 to 0.902	<0.001
Lymphocytes (%)	0.744	0.624 to 0.826	<0.001
Monocytes (%)	0.95	0.926 to 0.966	<0.001
Absolute eosinophil count (×10³/μl)	0.776	0.670 to 0.848	<0.001
Absolute neutrophil count (×10³/μl)	0.986	0.974 to 0.988	<0.001
Platelet count (×10³/μl)	0.965	0.949 to 0.976	<0.001

ICC: intraclass correlation coefficient.

Table 3.

Comparison of difference in blood parameters collected using venipuncture and 2 ml discard method from central venous catheter for different insertion sites (vein)

Parameter	Right subclavian	Left subclavian	Right femoral	Right internal jugular	P-value
Sodium (mEq/L)	0.00 (–1.00 to 1.00)	0.00 (–1.00 to 3.00)	–0.50 (–1.00 to 0.00)	0.00 (–1.00 to 2.00)	0.494
Potassium (mEq/L)	–0.07 (–0.32 to 0.04)	0.03 (–0.31 to 0.31)	–0.01 (–0.06 to 0.04)	–0.07 (–0.37 to 0.10)	0.717
Calcium (mg/dl)	–0.08 (–0.26 to 0.05)	–0.28 (–0.38 to –0.04)	–0.19 (–0.28 to –0.11)	0.00 (–0.12 to –0.19)	0.022
Hemoglobin (g/dl)	–0.3 0 (–0.60 to 0.10)	–0.20 (–0.80 to 0.30)	0.10 (0.00 to 0.20)	–0.10 (–0.30 to 0.30)	0.186
Red blood cells (×10⁶/μl)	–0.14 (–0.21 to 0.00)	–0.14 (–0.36 to 0.08)	0.01 (–0.02 to 0.04)	–0.01 (–0.12 to 0.06)	0.061
Hematocrit (%)	–0.80 (–2.00 to 0.00)	–0.50 (–3.00 to 0.80)	–0.05 (–0.10 to 0.00)	–0.30 (–1.00 to 0.40)	0.210
Mean corpuscular volume (fL)	–0.30 (–1.00 to 0.20)	0.40 (0.20 to 2.00)	0.40 (0.20 to 0.60)	–0.10 (–0.75 to 0.60)	0.017
Mean corpuscular hemoglobin (pg)	0.10 (–0.20 to 0.40)	0.30 (–1.20 to 1.00)	0.80 (0.20 to 1.40)	0.00 (–0.20 to 0.30)	0.447
Mean corpuscular hemoglobin concentration (g/dl)	0.00 (–0.20 to 0.50)	–0.10 (–1.70 to 0.90)	0.90 (0.10 to 1.70)	0.10 (–0.25 to 0.70)	0.623
Red cell distribution width–standard deviation (%)	0.00 (–0.20 to 0.00)	0.00 (–2.00 to 0.10)	–0.15 (–0.20 to –0.10)	0.00 (–0.10 to 0.10)	0.622
White blood cells (×10³/μl)	–0.32 (–0.88 to 0.00)	–1.06 (–4.44 to 0.13)	–0.25 (–0.54 to 0.03)	0.0 (–0.35 to 0.52)	0.023
Neutrophils (%)	0.30 (–0.10 to 1.10)	0.30 (0.10 to 1.20)	–0.05 (–0.40 to 0.30)	0.30 (–0.85 to 1.30)	0.848
Eosinophils (%)	0.00 (–0.10 to 0.00)	0.00 (–0.90 to 0.00)	0.05 (0.00 to 0.10)	0.00 (–0.10 to 0.10)	0.783
Basophils (%)	0.00 (0.00 to 0.00)	0.00 (0.00 to 0.10)	0.00 (0.00 to 0.00)	0.00 (0.00 to 0.00)	0.589
Lymphocytes (%)	0.00 (–0.50 to 0.30)	–0.50 (–1.70 to 0.80)	0.30 (0.20 to 0.40)	0.00 (–0.65 to 0.55)	0.485
Monocytes (%)	0.00 (–0.50 to 0.10)	–0.20 (–0.60 to 0.40)	0.05 (0.00 to 0.10)	0.00 (–0.40 to 0.20)	0.712
Absolute eosinophil count (×10³/μl)	0.00 (–0.01 to 0.00)	–0.02 (–0.16 to 0.02)	0.01 (0.00 to 0.01)	0.00 (–0.02 to 0.01)	0.524
Absolute neutrophil count (×10³/μl)	0.29 (–0.53 to 0.04)	–0.18 (–1.83 to 1.15)	–0.23 (–0.55 to0.09)	–0.01 (–0.29 to 0.39)	0.115
Platelet count (×10³/μl)	–1.00 (–10.00 to 9.00)	–2.00 (–13.00 to 72.00)	2.00 (1.00 to 3.00)	4.00 (–6.50 to 20.00)	0.595

Values are presented as median (interquartile range).

P-values were calculated using the Kruskal-Wallis test.

Table 4.

Comparison of blood parameters collected using venipuncture and 2-ml discard method from central venous catheter among patients with acidosis (n=32)

Blood parameter	Venipuncture	Two ml discard method	P-value
Sodium (mEq/L)	139.50 (136.00 to 142.75)	139.00 (134.25 to 144.00)	0.374
Potassium (mEq/L)	3.84 (3.56 to 4.55)	3.77 (3.34 to 4.15)	0.018
Calcium (mg/dl)	8.25 (7.41 to 8.49)	8.23 (7.56 to 8.41)	0.729
Hemoglobin (g/dl)	10.00 (8.45 to 11.55)	9.95 (8.50 to 11.47)	0.069
Red blood cell (×10⁶/μl)	3.71 (3.16 to 4.39)	3.69 (3.16 to 4.05)	0.056
Hematocrit (%)	31.80 (25.97 to 35.75)	29.95 (25.92 to 34.25)	0.006
Mean corpuscular volume (fL)	87.00 (82.85 to 90.07)	86.60 (83.57 to 89.85)	0.041
Mean corpuscular hemoglobin (pg)	28.55 (27.12 to 29.72)	28.65 (27.12 to 29.72)	0.034
Mean corpuscular hemoglobin concentration (g/dl)	32.50 (31.72 to 33.37)	32.65 (31.80 to 33.67)	0.031
Red cell distribution width-standard deviation (%)	14.40 (13.32 to 15.30)	14.35 (13.22 to 15.50)	0.705
White blood cell (×10³/μl)	12.25 (9.89 to 16.54)	12.05 (10.04 to 15.87)	0.178
Neutrophil (%)	80.50 (76.47 to 88.60)	81.95 (76.40 to 89.50)	0.045
Eosinophil (%)	0.30 (0.00 to 2.80)	0.25 (0.00 to 3.15)	0.553
Basophil (%)	0.25 (0.20 to 0.40)	0.30 (0.20 to 0.40)	0.755
Lymphocyte (%)	9.65 (7.52 to 12.77)	9.85 (6.60 to 12.20)	0.085
Monocyte (%)	4.75 (2.82 to 7.42)	4.60 (2.87 to 6.57)	0.095
Absolute eosinophil count (×10³/μl)	0.03 (0.00 to 0.28)	0.02 (0.00 to 0.33)	0.202
Absolute neutrophil count (×10³/μl)	10.00 (7.34 to 14.18)	10.28 (7.23 to 14.00)	0.480
Platelet count (×10³/μl)	162.50 (128.75 to 267.25)	183.00 (114.75 to 274.75)	0.681

Values are presented as median (interquartile range).

P-values were calculated using the Wilcoxon signed rank test.

REFERENCES

1. Villalta-García P, López-Herránz M, Mazo-Pascual S, Honrubia-Fernández T, Jáñez-Escalada L, Fernández-Pérez C, et al. Reliability of blood test results in samples obtained using a 2-mL discard volume from the proximal lumen of a triple-lumen central venous catheter in the critically ill patient. Nurs Crit Care 2017;22:298-304.Article PubMed
2. Moureau NL. Drawing blood through a central venous catheter. Nursing 2004;34:28.Article
3. Nahla KS, Manal IS, Gehan MA. Central line-related bloodstream infections and microbiological study in an Egyptian Ministry of Health hospital. Afr Health Sci 2020;20:158-67.Article PubMed PMC PDF
4. Hess S, Decker M. Comparison of the single-syringe push-pull technique with the discard technique for obtaining blood samples from pediatric central venous access devices. J Pediatr Oncol Nurs 2017;34:381-6.Article PubMed PDF
5. Lalthanthuami HT, Kumari MJ, Venkateswaran R, Lakshmi PR, Ramamoorthy L. Performance of 3 mL versus 5 mL discarded volume for blood sampling from central venous access device. J Lab Physicians 2021;13:112-7.Article PubMed PMC
6. Adlard K. Examining the push-pull method of blood sampling from central venous access devices. J Pediatr Oncol Nurs 2008;25:200-7.Article PubMed PDF
7. Byrne D. Comparing the push-pull versus discard blood sample method from adult central vascular access devices. J Infus Nurs 2016;39:130-5.Article PubMed
8. Cicolini G, Simonetti V, Di Nicola M, Palma E. Comparison of blood samples values by direct venipuncture and central venous catheters with 5 or 10 ml wasted blood. J Clin Nurs 2012;21:281-4.Article PubMed
9. Castro-Olmo FJ, Morales-Fernández P, Alcaide-Martín MJ, Fernández-Puntero B, Quintana-Díaz M, Aceña-Gil V, et al. Is minimising waste volume for drawing blood samples in critically ill patients feasible? Enferm Intensiva (Engl Ed) 2023;34:19-26.Article PubMed
10. Vincent JL, Baron JF, Reinhart K, Gattinoni L, Thijs L, Webb A, et al. Anemia and blood transfusion in critically ill patients. JAMA 2002;288:1499-507.Article PubMed
11. MacGeorte L, Steeves L, Steeves RH. Comparison of the mixing and reinfusion methods of drawing blood from a Hickman catheter. Oncol Nurs Forum 1988;15:335-8.PubMed
12. Coene KL, Roos AN, Scharnhorst V. Iatrogenic anemia/twenty-five million liters of blood into the sewer: comment. J Thromb Haemost 2015;13:1160-1.Article PubMed PDF
13. Wyant S, Crickman R. Determining the minimum discard volume for central venous catheter blood draws. Clin J Oncol Nurs 2012;16:454-8.Article
14. Winokur EJ, Pai D, Rutledge DN, Vogel K, Al-Majid S, Marshall C, et al. Blood culture accuracy: discards from central venous catheters in pediatric oncology patients in the emergency department. J Emerg Nurs 2014;40:323-9.Article PubMed
15. Barton SJ, Chase T, Latham B, Rayens MK. Comparing two methods to obtain blood specimens from pediatric central venous catheters. J Pediatr Oncol Nurs 2004;21:320-6.Article PubMed PDF
16. Weiss M, Fischer J, Boeckmann M, Freimueller R, Baenziger O. Eliminating discard volumes in neonatal and pediatric blood sampling from arterial catheters: a comparison of three simple blood-conserving aspiration techniques. Pediatr Crit Care Med 2002;3:134-40.Article PubMed
17. Cole M, Price L, Parry A, Picton S, Waters F, Marshall S, et al. A study to determine the minimum volume of blood necessary to be discarded from a central venous catheter before a valid sample is obtained in children with cancer. Pediatr Blood Cancer 2007;48:687-95.Article PubMed
18. Ponticelli E, Clari M, Volpes M, Valenti L, Angioni M, Granero D, et al. Complete blood count collected via venipuncture versus peripherally inserted central catheter in hematological patients: a comparison of 2 methods. Cancer Nurs 2022;45:E36-42.Article
19. Jeon M, Han A, Kang H, Lee KH, Lee JH, Lee JH, et al. A comparison of coagulation test results from heparinized central venous catheter and venipuncture. Blood Coagul Fibrinolysis 2020;31:145-51.Article PubMed
20. Jun WH, Gwak YS, Han HJ, Lee G. Comparison of laboratory values obtained via central venous catheters and venipuncture for better clinical decisions regarding sampling procedures of intensive care unit patients. J Clin Nurs 2015;24:866-8.Article PubMed PDF
21. Carini M, Micheletti M, Martellosio G, Caravaggi E, Portesi N, Biasiotto G, et al. A case of discrepant laboratory results in samples obtained from a central venous catheter and peripheral veins: when solving a pre-analytical mystery could improve patient care. Biochem Med (Zagreb) 2022;32:031001.Article PubMed PMC

Figure & Data

References

Citations

Citations to this article as recorded by

Assessment of minimum discard volume for central venous catheter sampling: Validation via busulfan therapeutic drug monitoring in pediatric transplant patients
Fang Ye, Huiying Ye, Dongwei Cui, Xuanxuan Zhang, Sailan Luo, Yuan Luo, Sandip Patil
Frontiers in Pediatrics.2026;[Epub] CrossRef

PubReader
ePub Link

Cite

CITE: export Copy Format; Close

Download Citation

Download a citation file in RIS format that can be imported by all major citation management software, including EndNote, ProCite, RefWorks, and Reference Manager.

Format:

RIS — For EndNote, ProCite, RefWorks, and most other reference management software
BibTeX — For JabRef, BibDesk, and other BibTeX-specific software

Include:

Citation for the content below
Citation and abstract for the content below

Minimizing discard volume to 2 ml before blood sampling from a central venous catheter: a prospective observational study in India

Acute Crit Care. 2026;41(1):174-182. Published online February 9, 2026

DOI: https://doi.org/10.4266/acc.000792

XML Download

Figure

Minimizing discard volume to 2 ml before blood sampling from a central venous catheter: a prospective observational study in India

Figure 1. Bland-Altman plots for blood parameters with the difference between direct venous puncture and 2 ml discarded central venous catheter samples plotted against the mean values. SD: standard deviation; RBC: red blood cells; WBC: white blood cell; MCV: mean corpuscular volume.

Figure 1.

Minimizing discard volume to 2 ml before blood sampling from a central venous catheter: a prospective observational study in India

Variable	Value
Sex
Male	63 (60.0)
Female	42 (40.0)
Diagnosis
Neurological system	60 (57.1)
Cardiovascular system	15 (14.3)
Gastrointestinal system	30 (28.6)
Site of central venous catheter placement
Right subclavian vein	51 (48.5)
Left subclavian vein	7 (6.7)
Right femoral vein	2 (1.9)
Right intrajugular vein	45 (42.9)
Arterial blood gas values
pH	7.40±0.07
PCO₂ (mm Hg)	37.79±24.60
PO₂ (mm Hg)	165.29±78.11
Bicarbonate (mEq/L)	21.81±4.01
Age (yr)	46.10±14.44

Blood parameter	ICC	95 % CI	P-value
Sodium (mEq/L)	0.954	0.933 to 0.969	<0.001
Potassium (mEq/L)	0.818	0.732 to 0.876	<0.001
Calcium (mg/dl)	0.862	0.798 to 0.907	<0.001
Hemoglobin (g/dl)	0.956	0.936 to 0.970	<0.001
Red blood cells (×10⁶/μl)	0.937	0.907 to 0.957	<0.001
Hematocrit (%)	0.959	0.939 to 0.972	<0.001
Mean corpuscular volume (fL)	0.986	0.979 to 0.990	<0.001
Mean corpuscular hemoglobin (pg)	0.985	0.978 to 0.990	<0.001
Mean corpuscular hemoglobin concentration (g/dl)	0.909	0.866 to 0.938	<0.001
Red cell distribution width to standard deviation (%)	0.733	0.607 to 0.819	<0.001
White blood cell (×10³/μl)	0.009	–0.459 to 0.327	0.482
Neutrophils (%)	0.898	0.849 to 0.930	<0.001
Eosinophils (%)	0.964	0.948 to 0.976	<0.001
Basophils (%)	0.856	0.788 to 0.902	<0.001
Lymphocytes (%)	0.744	0.624 to 0.826	<0.001
Monocytes (%)	0.95	0.926 to 0.966	<0.001
Absolute eosinophil count (×10³/μl)	0.776	0.670 to 0.848	<0.001
Absolute neutrophil count (×10³/μl)	0.986	0.974 to 0.988	<0.001
Platelet count (×10³/μl)	0.965	0.949 to 0.976	<0.001

Parameter	Right subclavian	Left subclavian	Right femoral	Right internal jugular	P-value
Sodium (mEq/L)	0.00 (–1.00 to 1.00)	0.00 (–1.00 to 3.00)	–0.50 (–1.00 to 0.00)	0.00 (–1.00 to 2.00)	0.494
Potassium (mEq/L)	–0.07 (–0.32 to 0.04)	0.03 (–0.31 to 0.31)	–0.01 (–0.06 to 0.04)	–0.07 (–0.37 to 0.10)	0.717
Calcium (mg/dl)	–0.08 (–0.26 to 0.05)	–0.28 (–0.38 to –0.04)	–0.19 (–0.28 to –0.11)	0.00 (–0.12 to –0.19)	0.022
Hemoglobin (g/dl)	–0.3 0 (–0.60 to 0.10)	–0.20 (–0.80 to 0.30)	0.10 (0.00 to 0.20)	–0.10 (–0.30 to 0.30)	0.186
Red blood cells (×10⁶/μl)	–0.14 (–0.21 to 0.00)	–0.14 (–0.36 to 0.08)	0.01 (–0.02 to 0.04)	–0.01 (–0.12 to 0.06)	0.061
Hematocrit (%)	–0.80 (–2.00 to 0.00)	–0.50 (–3.00 to 0.80)	–0.05 (–0.10 to 0.00)	–0.30 (–1.00 to 0.40)	0.210
Mean corpuscular volume (fL)	–0.30 (–1.00 to 0.20)	0.40 (0.20 to 2.00)	0.40 (0.20 to 0.60)	–0.10 (–0.75 to 0.60)	0.017
Mean corpuscular hemoglobin (pg)	0.10 (–0.20 to 0.40)	0.30 (–1.20 to 1.00)	0.80 (0.20 to 1.40)	0.00 (–0.20 to 0.30)	0.447
Mean corpuscular hemoglobin concentration (g/dl)	0.00 (–0.20 to 0.50)	–0.10 (–1.70 to 0.90)	0.90 (0.10 to 1.70)	0.10 (–0.25 to 0.70)	0.623
Red cell distribution width–standard deviation (%)	0.00 (–0.20 to 0.00)	0.00 (–2.00 to 0.10)	–0.15 (–0.20 to –0.10)	0.00 (–0.10 to 0.10)	0.622
White blood cells (×10³/μl)	–0.32 (–0.88 to 0.00)	–1.06 (–4.44 to 0.13)	–0.25 (–0.54 to 0.03)	0.0 (–0.35 to 0.52)	0.023
Neutrophils (%)	0.30 (–0.10 to 1.10)	0.30 (0.10 to 1.20)	–0.05 (–0.40 to 0.30)	0.30 (–0.85 to 1.30)	0.848
Eosinophils (%)	0.00 (–0.10 to 0.00)	0.00 (–0.90 to 0.00)	0.05 (0.00 to 0.10)	0.00 (–0.10 to 0.10)	0.783
Basophils (%)	0.00 (0.00 to 0.00)	0.00 (0.00 to 0.10)	0.00 (0.00 to 0.00)	0.00 (0.00 to 0.00)	0.589
Lymphocytes (%)	0.00 (–0.50 to 0.30)	–0.50 (–1.70 to 0.80)	0.30 (0.20 to 0.40)	0.00 (–0.65 to 0.55)	0.485
Monocytes (%)	0.00 (–0.50 to 0.10)	–0.20 (–0.60 to 0.40)	0.05 (0.00 to 0.10)	0.00 (–0.40 to 0.20)	0.712
Absolute eosinophil count (×10³/μl)	0.00 (–0.01 to 0.00)	–0.02 (–0.16 to 0.02)	0.01 (0.00 to 0.01)	0.00 (–0.02 to 0.01)	0.524
Absolute neutrophil count (×10³/μl)	0.29 (–0.53 to 0.04)	–0.18 (–1.83 to 1.15)	–0.23 (–0.55 to0.09)	–0.01 (–0.29 to 0.39)	0.115
Platelet count (×10³/μl)	–1.00 (–10.00 to 9.00)	–2.00 (–13.00 to 72.00)	2.00 (1.00 to 3.00)	4.00 (–6.50 to 20.00)	0.595

Blood parameter	Venipuncture	Two ml discard method	P-value
Sodium (mEq/L)	139.50 (136.00 to 142.75)	139.00 (134.25 to 144.00)	0.374
Potassium (mEq/L)	3.84 (3.56 to 4.55)	3.77 (3.34 to 4.15)	0.018
Calcium (mg/dl)	8.25 (7.41 to 8.49)	8.23 (7.56 to 8.41)	0.729
Hemoglobin (g/dl)	10.00 (8.45 to 11.55)	9.95 (8.50 to 11.47)	0.069
Red blood cell (×10⁶/μl)	3.71 (3.16 to 4.39)	3.69 (3.16 to 4.05)	0.056
Hematocrit (%)	31.80 (25.97 to 35.75)	29.95 (25.92 to 34.25)	0.006
Mean corpuscular volume (fL)	87.00 (82.85 to 90.07)	86.60 (83.57 to 89.85)	0.041
Mean corpuscular hemoglobin (pg)	28.55 (27.12 to 29.72)	28.65 (27.12 to 29.72)	0.034
Mean corpuscular hemoglobin concentration (g/dl)	32.50 (31.72 to 33.37)	32.65 (31.80 to 33.67)	0.031
Red cell distribution width-standard deviation (%)	14.40 (13.32 to 15.30)	14.35 (13.22 to 15.50)	0.705
White blood cell (×10³/μl)	12.25 (9.89 to 16.54)	12.05 (10.04 to 15.87)	0.178
Neutrophil (%)	80.50 (76.47 to 88.60)	81.95 (76.40 to 89.50)	0.045
Eosinophil (%)	0.30 (0.00 to 2.80)	0.25 (0.00 to 3.15)	0.553
Basophil (%)	0.25 (0.20 to 0.40)	0.30 (0.20 to 0.40)	0.755
Lymphocyte (%)	9.65 (7.52 to 12.77)	9.85 (6.60 to 12.20)	0.085
Monocyte (%)	4.75 (2.82 to 7.42)	4.60 (2.87 to 6.57)	0.095
Absolute eosinophil count (×10³/μl)	0.03 (0.00 to 0.28)	0.02 (0.00 to 0.33)	0.202
Absolute neutrophil count (×10³/μl)	10.00 (7.34 to 14.18)	10.28 (7.23 to 14.00)	0.480
Platelet count (×10³/μl)	162.50 (128.75 to 267.25)	183.00 (114.75 to 274.75)	0.681

Table 1. Distribution of demographic and clinical variables of the participants (n=105)

Values are presented as number (%) or mean±standard deviation.

Table 2. Intraclass correlation for agreement between venipuncture and 2 ml discard method from central venous catheter on selected blood parameters (n=105)

ICC: intraclass correlation coefficient.

Table 3. Comparison of difference in blood parameters collected using venipuncture and 2 ml discard method from central venous catheter for different insertion sites (vein)

Values are presented as median (interquartile range).

P-values were calculated using the Kruskal-Wallis test.

Table 4. Comparison of blood parameters collected using venipuncture and 2-ml discard method from central venous catheter among patients with acidosis (n=32)