Comparison of various measures of sensitivity of M. tuberculosis to ethambutol.

A sensitivity test for ethambutol was standardised, and performed on cultures isolated from patients before and after the start of chemotherapy with ethambutol and isoniazid. A discrimination type of approach was employed and three criteria of resistance were evolved: (a) growth of 20 colonies or more on ethambutol 4 microgram/ml, using a standard inoculum; (b) a proportion of 25% or more on ethambutol 1.4 microgram/ml; (c) a proportion of 5% or more on ethambutol 2 microgram/ml. The MIC definition appears to be the best in view of its simplicity and insensitivity of the classification (as sensitive or resistant) to variations in inoculum size.