A microbiological assay for the measurement of methotrexate has been designed, using a chloramphenicol resistant mutant of Lactobacillus casei as the test organism. The method is simple, precise and inexpensive. Results are not significantly different from those obtained using a methotrexate radioassay kit employing a competitive protein binding technique. However, where patients are being treated with co-trimoxazole, the competitive protein binding technique is unsuitable for measuring methotrexate levels due to the cross reactivity of the binder. This is not a problem with the microbiological assay described here, which is able to measure methotrexate in the presence of large doses of trimethoprim. The methods were not affected by the presence of seven commonly encountered antibiotics. These had negligible effect on either method up to concentrations of 100 mg/l.
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