For patients with increased risk of treatment failure or relapse, TB treatment monitoring is critical to provide early recognition of developing problems and adaptation of the treatment. However, in many low-income countries, it is not feasible to perform drug susceptibility testing (DST) on all new cases, and current tests are far from perfect for monitoring treatment progress. Microscopy for acid-fast bacilli remains the recommended technique, but, particularly if the test is carefully executed, once a patient is in treatment, most positive smears will represent dead bacilli, resulting in poor predictive test values.
Under guidance from The Union, projects run by the Damien Foundation Bangladesh have piloted another approach – using vital staining with fluorescein diacetate (FDA). This widely applicable test, tried decades ago for leprosy but hardly ever for TB, is being used in their regional reference laboratories on referred sputum from various types of retreatment cases, as well as in patients with delayed conversion of routine smears.
This forgotten technique allows differentiation of living cells, which render the FDA fluorescent, as dead organisms can't cleave off the acetate and remain invisible. A retrospective analysis of the results over the last four years showed that this test was very useful to avoid unnecessary culture and rapid DST, as well as the false declaration of treatment failure, mainly in patients on treatment for the first time.
Used for routine AFB smear-positive cases from the third month of treatment onwards, a negative FDA test excluded growth on culture as well as multi-drug resistance (MDR-TB) in over 95% of cases. As MDR-TB prevalence is low in the Bangladesh population, the FDA smear was highly predictive of culture-positivity and MDR-TB if still positive late during first treatment, or at any time during retreatment with first-line drugs.
In regions with higher prevalence of MDR-TB and few mycobacterioses, FDA smears might even allow presumptive MDR-TB diagnosis. Other applications, such as substitution of cultures for MDR-TB treatment monitoring, are being studied. The technique is safe and simple, but still requires an LED fluorescence microscope and cold storage of the reagent, limiting its decentralisation in resource-poor settings.
This story is based on a fast-track article from the International Journal of Tuberculosis and Lung Disease, which is available online to subscribers only at http://dx.doi.org/10.5588/ijtld.11.0166