Biological variation of blood acid-base status: consequences for analytical goal-setting and interpretation of results.

Analytical, intra-individual, and interindividual components of variation have been determined for pH, pCO2, bicarbonate, base excess, and total CO2 in capillary specimens of whole blood from a cohort of 14 healthy subjects. Calculated analytical goals (SD) are pH less than or equal to 0.008, pCO2 less than or equal to 0.9 mmHg, bicarbonate less than or equal to 0.5 mmol/L, base excess less than or equal to 0.5 mmol/L, and total CO2 less than or equal to 0.5 mmol/L. Because pH and base excess vary little between individuals, population-based reference values for them will be of utility in interpretation of results; pCO2, bicarbonate, and total CO2 show more marked variation between individuals, and reference values for them will be of more limited use. The generally applicable differences required for two results to be significantly different (P less than or equal to 0.05) when goals are met--which is currently feasible--are pH: 0.04, pCO2: 0.745 kPa (5.6 mmHg), bicarbonate: 2.6 mmol/L, base excess: 2.2 mmol/L, and total CO2: 2.7 mmol/L.