Know your test

Sputum Smear/AFB Smear

Smear Microscopy is a WHO endorsed test for TB testing. Smear microscopy involves collecting a biological sample (usually sputum or some other clinical material), fixing it thinly on a glass slide and then staining it with a dye that binds specifically to mycobacteria (making them easier to identify under a microscope). Smear microscopy for TB is sometimes called AFB testing, because mycobacteria are ‘acid-fast bacilli’ (AFB). This means that certain dyes adhere to the waxy coat of mycobacteria and remain visible even after rinsing with water and being briefly treated with a solution of acid-alcohol that strips the dye from the rest of the smear (WHO 2004).

For example, when performing the most widely used staining method, Ziehl-Neelsen staining, the smear is covered with carbolfuchsin dye for a few minutes. After heating, rinsing with water and the acid-alcohol treatment, the smear is counterstained with another dye, methylene blue (to colour the background of the smear for contrast) and then rinsed again (WHO 2004). Once dried, if there was a substantial concentration of TB bacilli in the sputum sample, some of the bacilli on the slide should show up under the microscope as red characteristically rod-shaped organisms against the blue background.


The GeneXpert test is a new WHO endorsed molecular test for TB which diagnoses TB by detecting the presence of TB bacteria, as well as testing for resistance to the drug Rifampicin. It detects the DNA in TB bacteria. It uses a sputum sample and can give a result in less than 2 hours. It can also detect the genetic mutations associated with resistance to the drug Rifampicin. Any IPAQT lab would charge you ₹2000 or less for a GeneXpert test.

Hain-Line Probe Assay

Line probe assays were endorsed by the WHO in 2008 for molecular detection of drug resistance from smear-positive patients at risk of MDR-TB. LPAs use a PCR/molecular technique to identify members of the MTB while simultaneously identifying drug-resistant strains associated with resistance. Any IPAQT lab would charge you ₹1600 or less for a LPA with two drug resistance.

Liquid culture

Liquid culture is considered as a gold standard WHO endorsed test for TB diagnosis and drug susceptibility testing. The reports take 2-4 weeks. Any IPAQT lab would charge you ₹900 or less for a normal culture.