Accuracy of base excess - An in vitro evaluation of the Van Slyke equation

Objectives: To evaluate the precision, bias and CO2 invariance of base exce ss as determined by the Van Slyke equation over a wide PCO2 range at normal and low hemoglobin concentrations. Design: Prospective in vitro study. Setting: University research laboratory. Subjects: Normal human blood, both undiluted and diluted with plasma. Interventions: Two experiments were conducted. In the first, blood unmodifi ed or after adding HCl or sodium bicarbonate was rendered hypercarbic (PCO2 >70 torr) by gas equilibration. Rapid PCO2 reduction in greater than or eq ual to 10 steps to a final PCO2 less than or equal to 20 torr was then perf ormed. In the second experiment, blood unmodified or diluted to a hemoglobi n concentration of similar to 4 G% was mixed anaerobically (9:1, vol:vol) w ith varying concentrations of lactic acid in saline (0-250 mmol/L). Measurements and Main Results: In the first experiment, blood gas analysis at each step during the progressive PCO2 reduction revealed that base exces s remained nearly constant (SD all specimens less than or equal to 0.6 mmol /L) whereas PCO2 changed by >80 torr. In the second experiment, simultaneou s blood gas and plasma lactate analyses showed that changes in base excess correlated closely with changes in both plasma and whole blood lactate conc entrations (r(2) greater than or equal to 0.91) despite concurrent PCO2 ele vations as great as 200 torr. Quantification by base excess of change in wh ole blood lactate concentration was precise with slight negative bias (mean negative bias, 1.1 +/- 1.9 mmol/L) in both diluted and undiluted blood. Th ere was significant underestimation of change in plasma lactate concentrati on in undiluted blood, presumably because base excess is a whole blood vari able. Conclusions:Base excess calculated using the Van Slyke equation accurately quantifies metabolic (nonrespiratory) acidbase status in blood in vitro. Th is accuracy is little affected by large simultaneous alterations in PCO2, o r by very low hemoglobin concentrations similar to that used to calculate s tandard base excess.