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

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.

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.

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).

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.