The effect of assay methods on plasma levels and pharmacokinetics of theophylline: HPLC and EIA.

1 The effects of two assay methods, HPLC and EIA, on theophylline plasma levels (n = 254) and pharmacokinetics (n = 6) were examined on blood samples obtained from asthmatic patients and normal adult male volunteers. 2 The standard curves obtained and the plasma level values during plasma level monitoring of theophylline measured with two methods are well correlated each other within concentration range of 2.5 to 20.0 microgram/ml (P less than 0.001). EIA, however in general, appears to give a lower value than HPLC. 3 The values of plasma theophylline half-life and clearance analyzed using a two-compartment open system model were found different between the two assay methods and dissimilar to those previously reported by using ultraviolet method. 4 The sampling end-point and/or assay sensitivity particularly to measure a lower level of theophylline (less than 2.5 microgram/ml) can influence the pharmacokinetics for a potent bronchodilator with a narrow therapeutic margin, theophylline.