Falsely raised plasma theophylline concentrations in renal failure.

We studied the validity of plasma theophylline measurements in renal failure. Patients with increased serum creatinine receiving theophylline had drug measurements by four methods: high pressure liquid chromatography (HPLC) fluorescence polarization immunoassay (FPIA), enzyme immunoassay (EIA) and dipstick immunoassay (DI). The mean +/- SD (mumol/l) for plasma theophylline in the 18 patients with renal impairment was: HPLC 63.4 +/- 36.7; EIA 64.4 +/- 35.9; FPIA 70 +/- 3.43; DI 122 +/- 65.9. A significant correlation (r = 0.94, p less than 0.01) was obtained between serum creatinine and the difference in theophylline concentrations between HPLC and DI. Plasma theophylline results should be interpreted with caution in patients with renal failure.