Assay for trimethoprim in serum and urine by means of ion-pair chromatography.

We describe a rapid, precise, and reliable procedure for assay of trimethoprim in serum and urine by ion-pair chromatography. Trimethoprim concentrations in urine are determined by an externally standardized, direct-injection procedure; assay in serum involves a simple preliminary extraction and internal standardization. The assays are suitable for pharmacokinetic studies and have been applied to determination of trimethoprim concentrations in serum and urine during therapy with Co-trimoxazole, a sulfonamide/trimethoprim preparation.