Pseudometabolic acidosis caused by underfill of Vacutainer tubes

To determine how serum bicarbonate and anion gap are affected by sample size in a 10-mL red-top (clot) Vacutainer tube at the fixed sample volumes of 10, 3, and 1 mL.

Venous phlebotomy on consecutive emergency department patients; three tubes drawn in random order. The first 20 patients had unvented tubes, and the last ten had the tubes vented within one minute of the draw.

University hospital ED.

Thirty ED patients.

All blood samples had electrolytes determined within one hour of phlebotomy. To approximate the ED setting, the time to analysis was not controlled, but each triple draw had simultaneous analysis.

Mean bicarbonate in mmol/L (with sample size) was 21.7 (10 mL), 19.4 (3 mL), and 16.3 (1 mL) (r2 = .86, P = .0001). Anion gap in mmol/L was 16.7 (10 mL), 17.5 (3 mL), and 19.1 (1 mL) (r2 = .84, P = .0077). Venting of Vacutainer tubes did not significantly change these results.

Underfill of 10-mL Vacutainer tubes causes a significant decline in bicarbonate and an increase in anion gap that may be mistaken for a metabolic acidosis. To correct for these effects, the bicarbonate should be increased by 0.5 to 0.6 mmol/L, and the anion gap should be decreased by 0.2 to 0.3 mmol/L for every milliliter of air above the sample in a 10-mL Vacutainer tube. Venting the tubes will not correct this effect. All tubes should be filled completely to avoid creating a pseudometabolic acidosis.