A radiometric assay for antigiardial drugs.

Susceptibility of Giardia lamblia trophozoites to 3 antigiardial drugs was determined morphologically by 24h parasite counts and radiometrically by uptake of [3H]thymidine and by rapid 4h microassays of motility and viability. Growth of Giardia trophozoites in liquid culture correlated well with uptake of [3H]thymidine during a 72h period (r = 0.91, P less than 0.01). ED50 and MIC for metronidazole, mepacrine and sodium fusidate were similar in both morphological and radiometric growth assays and, except for mepacrine, results were similar to previously reported drug efficacies obtained with more labour intensive methods. Motility and viability 4h microassays were insensitive and time-consuming and cannot be recommended for routine screening of antigiardial drugs. The radiometric growth assay is therefore a simple, objective measure of antigiardial drug activity which could be used to determine drug sensitivity profiles of clinical isolates or for screening new antigiardial drugs.