Simple low-cost test for drug-resistant tuberculosis

Researchers at the London School of Hygiene and Tropical Medicine in London have developed a simple test kit that may be used to identify Mycobacterium tuberculosis isolates resistant to rifampicin. The kit is suitable for screening large numbers of tubercular strains and also has the potential for use in testing susceptibility to streptomycin. The end-point of the test is a simple colour change, and results are available within 48 hours.

The test uses mycobacteriophage D29, a type of virus which only infects and multiplies within live mycobacteria. Rifampicin stops infection of the mycobacteria by the bacteriophage in susceptible strains but not in drug-resistant bacteria. Thus, if the bacteriophage multiply when rifampicin is present, the bacteria under test are drug resistant.

This technology of rapid phage-based detection was first demonstrated two years ago and further cost-analysis work was presented at the World Congress on Lung Health held in Florence in September 2000. Dr Ruth McNerney, who was involved in developing the method, said: ‘‘... costs can be minimized by the use of a 96-well plate format. This enables efficient processing of a large number of samples and requires only reduced reagent volumes.’’ Because the reagents needed are cheap (about US$ 0.35 per assay), the major cost being the labour, the test may be suitable for developing countries.

Tests currently in use for screening for drug-resistant tuberculosis compare growth of the bacteria with and without the drug. However, since the bacteria grow slowly the tests take up to two weeks to complete. Other new rapid tests have been developed but they are too expensive for parts of the world where annual spending on health may be less than US$ 10 per head.

The cost of the new test compares favourably with the expense of time-consuming methods of susceptibility testing currently in use in low-income countries.

Tudor Toma, London

World Health Organization Genebra - Genebra - Switzerland