In most countries with a high TB burden, direct smear microscopy remains the principal laboratory support available on a timely basis for initial diagnosis and treatment decisions.
Unfortunately, microscopy lacks the sensitivity to identify TB in more than about half of all patients; and the problem is particularly acute in subjects co-infected with HIV who tend to have sputum levels of the bacterium undetectable by this method.
Moreover, microscopy provides no patient-specific guidance on the likelihood of drug resistance, typically a very time consuming process. Whether the patient’s TB is multidrug resistant (MDR) is a critical consideration in deciding on the appropriate course of therapy both for previously treated patients and for new patients living with HIV/AIDS or in other settings where MDR-TB is likely to be prevalent.
Primary Uses and Value of Diagnostics in the Developing World1
Consequently, initial treatment decisions must often be made “in the dark”, or perhaps without ever knowing if the patient has MDR-TB. Guidance is typically limited to statistics on prevalence for locales and patient groups — statistics whose reliability currently leaves much to be desired, as MDR-TB is still badly underdiagnosed and underreported. Even in settings where comparatively rapid, high-sensitivity methods for bacterial identification and drug resistance testing are available, turnaround may be too slow to influence initial therapy decisions because laboratories simply cannot handle the volume of testing required.
1. The Diagnostics Innovation Map: Medical Diagnostics for the Unmet Needs of the Developing World, Page 14. 2010