Radioenzymatic assay for trimethoprim in very small serum samples

A modification of the methotrexate radioassay kit (supplied by New England Enzyme Center) enabled determination of trimethoprim levels in 5-microliter serum samples. An excellent correlation between this assay and high-pressure liquid chromatography assay was found. These preliminary results suggest that with this method rapid determination of trimethoprim levels in very small samples (5 to 10 microliters) can be achieved.