Worldwide more than one and a half million people die each year from tuberculosis, a treatable disease, and the incidence is rising. Major difficulties with current strategies for control are delays in diagnosis and delays in identifying infections that are resistant to some drugs. At present, cultures are set up on a solid medium in an environment that supports the slow-growing mycobacteria. Labs in developing countries, using the best methods they have, average 26 days to get a positive culture for tuberculosis from sputum samples and almost as long again to find the best drug treatment. Labs in developed countries using an automated culture method average 13 days to diagnosis and another 9 days to identify the best treatment. During this time the patients may spread their infection and can deteriorate.

The New England Journal of Medicine has reported a study carried out in Lima, Peru of a new culture method for the diagnosis of TB called the Microscopic-Observation Drug-Susceptibility (MODS) assay. The lead author, Dr David Moore from London, worked with colleagues from Baltimore, New Orleans and Lima to study 3760 sputum samples. The new assay is based on three principles:

• The tuberculosis mycobacterium grows faster in liquid medium than on solid medium
• Characteristic cord formation can be seen microscopically at an early stage
• Growth in the presence of drugs allows drug-susceptibility to be tested at the same time as bacterial growth is detected

Briefly, decontaminated, digested, and homogenised sputum is cultured in liquid medium in a series of wells, some containing drugs, and inspected daily by microscope through a plastic cover for the presence or absence of a characteristic cord-like growth.

The authors found that a single MODS culture of a sputum sample takes only about 7 days to diagnose TB and at the same time it finds the best treatment.and identifies multidrug-resistant tuberculosis.

MODS is cheaper and more sensitive than the existing gold-standard methods. It should save many lives.