Validity of a two-point acetaminophen pharmacokinetic study

The pharmacokinetics of a single 650-mg intravenous dose of acetaminophen were determined in 82 volunteers using multiple (13 or more) plasma acetaminophen concentrations measured by high pressure liquid chromatography during 24 h after dosage. Kinetic values from the complete study were compared with kinetic estimates based on only two data points: (a) the 2- and 6-h points only; and (b) the 3 and 6-h points only. For elimination half-life, values from the complete study (mean 2.42 h) were highly correlated (r = 0.87 and 0.84) with methods a and b (means 2.41 and 2.43 h), with regression slopes of 1.00 and 0.99, respectively. For clearance, the complete study values (mean 312 ml/min) were highly correlated (r = 0.97 and 0.97) with method a and b values, but both two-point methods significantly overestimated clearance (means 350 and 355 ml/min) by an average of 13 and 14%, respectively. Results for volume of distribution were similar to those for clearance. Although acetaminophen elimination half-life can be estimated with reasonable precision using a two-point blood-sampling procedure, clearance and volume of distribution values using the two-point method overestimate the actual values.