Spectrophotometric assay of antipyrine in plasma: a reevaluation.

The linearity, sensitivity, and replicability of the spectrophotometric antipyrine assay were reevaluated. Standard curves are always linear through concentrations of at least 50 microgram/ml, whether prepared in water, dilute acid, or plasma. Sensitivity limits are 1 to 2 microgram of antipyrine per ml of plasma in healthy individuals. Within-day coefficients of variation for identical samples ranged from 12.3% at 2 microgram/ml to 1.2% at 50 microgram/ml. Identical samples assayed two years apart deviated by a mean of 3.8%, indicating high between-day replicability as well as stability during prolonged storage at -20 degrees C. Thus the spectrophotometric assay for antipyrine is suitable for most clinical pharmacokinetic studies.