Direct drug susceptibility test for tubercle bacilli by the sputum swab culture method

To develop a simple, inexpensive method for testing direct drug susceptibility of tubercle bacilli to isoniazid (INH) and streptomycin (SM) which can be adopted for use even in remote parts of the world.

Using 239 smear-positive sputum specimens obtained from an equal number of patients, a comparison was made between the direct swab susceptibility test and the standard indirect method for INH and SM using Löwenstein-Jensen (L-J) medium.

There was 95% agreement of results for INH by 6 weeks and 90% for SM by 8 weeks; 96% of INH-resistant cultures could be detected in 5 weeks and 91% of SM-resistant cultures by 8 weeks. The discrepancies in the two tests were virtually symmetrically distributed at 6 and 8 weeks. The speed and efficiency of detection of resistance by the swab method was also assessed in relation to the standard indirect method. For INH, 96% of the cultures were detected by the fifth week, while 66% were detected as early as 2 weeks and 93% by 4 weeks. With SM, 84% were detected by 4 weeks, 89% by 5 weeks and 91% by 8 weeks.

This study has indicated the usefulness of the swab method for testing the drug susceptibility of tubercle bacilli. As this method is simple and easy, and does not even require a centrifuge, it has the potential of application even in the remote parts of the world. The material used, Cetavlon (Cetrimide), is inexpensive and easily water soluble, and more importantly, aqueous solutions are self-sterilizing. It should be stressed, however, that the results obtained with this test take the same time as conventional methods, and it can therefore not be considered as a rapid test.