A simple biological method for determination of small amounts of tuberculostatic agents in fluids.

The authors describe the use of the vertical diffusion test for determining the quantity of certain tuberculostatic drugs in body fluids and the degree of mycobacterial susceptibility to these drugs, using tubercle bacilli as test organisms. It is found to be a reliable assay method for isoniazid, para-aminosalicylic acid, ethionamide and ethambutol when carried out on a modification of Middlebrook 7H-10 solid medium containing 1.5% Oxoid Ion-Agar No. 2.As a drug-susceptibility test, it yields results expressed quantitatively in degrees of sensitivity or resistance and is therefore recommended for use only in laboratories not equipped for more comprehensive testing. The method may also be used for ascertaining the level of tuberculostatic activity of a patient's serum against his own mycobacteria.