Comparative determination of sulfonamide concentration in serum by chemical and microbiological assay

Analysis of sulfamethoxazole in serum from patients treated with trimethoprim-sulfamethoxazole was performed with the agar diffusion assay which is supposed to measure only unconjugated sulfonamide and with the spectrophotometric method which enables one to measure both unconjugated and conjugated sulfonamide. The two methods gave serum concentrations in the same range when used in patients with intact kidney function. In patients with decreased renal function the chemical method yielded higher values, probably due to accumulation of conjugated sulfamethoxazole. The agar diffusion assay gives an accurate answer if the clinical question concerns the therapeutic effect only. The spectrophotometric analytical method has a broader potential since it may be employed irrespective of the clinical question that motivated the analysis.