Effect of phlebotomy technique on serum bicarbonate values.

Abstract

To determine whether the following phlebotomy technique factors result in spuriously low serum bicarbonate values: 1) small needle size, 2) prolonged tourniquet time, and 3) underfilling of Vacutainer tubes. Thirty-eight healthy young adult student volunteers from Brown University (mean age = 22 years) were randomly assigned to 1 of 2 (24-ga or 20-ga) IV needle size groups. With a tourniquet applied, the subjects had blood sampled initially and then at 3 intervals 1 minute apart. In addition, the initial blood sample was separated into four 4-mL Vacutainer tubes in blood volumes of 0.5, 1, 2, and 3 mL. Serum bicarbonate values (mmol/L) were compared using these techniques. 1) Small needle size did not affect serum bicarbonate values (p > 0.60); 2) prolonged tourniquet time resulted in a statistically significant serum bicarbonate elevation (32.0, 32.5, 33.8, and 33.6, respectively; p < 0.01); and 3) underfilling of Vacutainer tubes resulted in significantly lower serum bicarbonate values for all 4 Vacutainer blood volume groups (21.4, 23.0, 25.0, and 26.2, respectively; p < 0.001). Small needle size does not affect serum bicarbonate values. Prolonged tourniquet time results in a statistically significant elevation of serum bicarbonate, although this elevation may not be clinically meaningful. Underfilling of Vacutainer tubes significantly influences the accuracy of serum bicarbonate values.