IT is well known that the results of isoniazid sensitivity tests by the indirect method
have prognostic significance in the treatment of tuberculosis with regimens containing
isoniazid (Tuberculosis Chemotherapy Centre, Madras, 1960 ; Devadatta et al., 1961) ;
the indirect method is, however, complex and time-consuming, and therefore not
feasible in institutions with limited facilities.
Direct sensitivity tests are attractive because they involve only one stage of handling
(i.e. cultures need not be set up), are simpler and less expensive, and consume less time.
Although the direct sensitivity test introduced by Middlebrook and co-workers (Middlebrook
and Cohn, 1958 ; Russell and Middlebrook, 1961) using 7H10 agar medium
yielded satisfactory results, it has certain disadvantages. Thus, the medium employed is
expensive, the in are not readily available in this country and, in our experience
at Madras, losses due to contamination can be considerable.
This paper describes a direct sensitivity test for isoniaid using Lowenstein-
Jensen medium, and compares the findings obtained by this method with those obtained
by the indirect method