Impact of blood volume, air exposure duration, transport duration, and testing delay on plasma total carbon dioxide...

Exposing blood specimens to air reduces plasma total carbon dioxide (TCO₂). We evaluated the degree of TCO₂ reduction attributed to open collection of neonatal blood in BD microtainers® (microtainers), microtainer transport duration and delayed testing of open plasma aliquots.

Venous blood was aliquoted into open microtainers in a 3x4 factorial design to simulate combined effects of blood volume (0.2-0.6 mL) and air exposure duration (0-5 min), with blood drawn in vacutainers as a control. Separate effects of in-hospital transport duration (0-120 min; whole blood), off-site transport duration (0-24 h; centrifuged whole blood), and the duration plasma aliquots remained open (0-120 min) were evaluated by repeated testing. Findings were analyzed using repeated-measures ANOVA and Student's T-tests.

In the factorial experiment, mean plasma TCO₂ in microtainers was on average 3.5 mmol/L lower than in vacutainers. Smaller blood volume but not greater air exposure duration significantly (p < 0.05) reduced TCO₂. Mean TCO₂ in filled (0.6 mL; 1-5 min air exposure) microtainers was on average 2.9 mmol/L lower than in vacutainers. Simulated off-site transport of microtainers containing centrifuged whole blood significantly reduced TCO₂ (4 h; mean change = -1.5 mmol/L), as did delayed testing of aliquoted plasma (15 min; mean change = -1.3 mmol/L).

Plasma TCO₂ decreased with reduced microtainer blood volume, extended off-site transport duration of centrifuged whole blood and testing delay of aliquoted plasma. To minimize TCO₂ reduction, microtainers should be fully filled and tested rapidly. Laboratories should also consider whether an interpretive comment, correction factor or separate reference intervals are appropriate for these tests.