Effect of the size of evacuated blood collection tubes on total carbon dioxide concentration in equine plasma

Objective-To determine whether plasma total CO₂ concentrations would vary with the size of the evacuated tube used to collect blood samples. Design-Randomized crossover study. Animals-Convenience sample of 20 healthy adult horses. Procedures-Jugular venous blood was collected from horses in random order into 8 types of evacuated tubes: 2-mL glass, 2- or 3-mL plastic or plastic plasma separator, 4- or 6-mL plastic, and 10-mL glass or plastic. Total CO₂ concentrations in plasma were measured with a biochemistry analyzer. Data were analyzed via repeated-measures ANOVA and multivariate regression. Results-The air volume-to-blood volume ratio was significantly higher and consequently, plasma total CO₂ concentration was significantly lower when blood was collected into 2-mL glass tubes and 2- or 3-mL plastic tubes than when the other 5 types of evacuated tubes were used. Concentrations in the other tube types were statistically equivalent. A linear relationship was detected between total CO₂ concentration and air volume-to-blood volume ratio. Conclusions and Clinical Relevance-Blood samples should be collected into evacuated tubes with a small air volume-to-blood volume ratio whenever an accurate estimate of plasma total CO₂ concentration is required.