MTB in Dehradun

What is a MTB?

This molecular test detects the DNA of Mycobacterium tuberculosis directly in a clinical sample, and its value lies in speed and sensitivity relative to the older methods. Sputum smear microscopy, long the mainstay in India, misses a substantial proportion of cases because it needs a high bacterial load to see anything, and it cannot distinguish tuberculosis from other mycobacteria. Culture is the reference standard and provides full drug sensitivities, but it takes two to six weeks because the organism grows extremely slowly. PCR sits between them: results within a day or two, considerably more sensitive than smear, and specific for the tuberculosis complex. Its limitations follow from what it measures. It detects DNA whether the organism is alive or dead, so it can remain positive for months after successful treatment and cannot be used to judge whether therapy is working. Sensitivity is lower in extrapulmonary disease, where bacterial loads are small, so a negative result there does not exclude tuberculosis. And unless the specific assay includes resistance markers, it says nothing about which drugs will work.

Why take this test?

A TB PCR is advised where active tuberculosis is suspected: a cough lasting more than two to three weeks, fever particularly in the evenings, drenching night sweats, unexplained weight loss, blood in the sputum, or an abnormal chest X-ray. It is especially valuable where smear microscopy has been negative but suspicion remains high, which is common, and in extrapulmonary disease affecting lymph nodes, pleura, abdomen, bone or the nervous system, where obtaining a diagnosis by any other means is difficult. It is also used in children, who often cannot produce sputum and have low bacterial loads. Where drug resistance is a concern, ask your doctor about an assay that includes rifampicin resistance, since knowing that at diagnosis rather than weeks later changes the entire regimen. Tuberculosis is notifiable in India and treatment is available free through the national programme, so a positive result should go to a doctor immediately rather than being managed privately in isolation.

• Cough persisting beyond two to three weeks
• Evening fever, night sweats or unexplained weight loss
• Blood in the sputum, or an abnormal chest X-ray
• A negative sputum smear with continuing clinical suspicion
• Suspected extrapulmonary tuberculosis
• Suspected tuberculosis in children, who rarely produce sputum

FAQs: MTB in Dehradun