A rapid, sensitive and specific radioimmunoassay for methotrexate.

A rapid, sensitive, radioimmunoassay for methotrexate has been developed using tritiated methotrexate and an antiserum raised in rabbits against the immunogen, bovine serum albumin containing 34 methotrexate residues per protein molecule. Separation of bound and free drug was attained using donkey anti-rabbit precipitating serum. The method can accurately measure as little as 50 fmol methotrexate in capillary blood samples and is not interfered with by vast excesses of natural folates nor citrovorum rescue factor. The method can handle 30 samples per day with the results available after 6 h. This makes it suitable for monitoring serum methotrexate levels after high dose therapy when a result is required within 24 h to provide helpful information regarding impending toxicity and the need for extended citrovorum rescue factor or maintenance of urinary pH.