A molecular test system is a machine that finds an infection from the genetic material of the germ, its DNA or RNA, instead of growing the germ or looking for it under a microscope. The best-known kind in India is the cartridge-based test for tuberculosis (TB). Indian doctors call it CBNAAT, short for cartridge-based nucleic acid amplification test. It gives a result in under two hours, and it is used across India's national TB programme.
The older tests are slow, or they miss cases. Looking for TB germs in sputum, the mucus coughed up from the lungs, under a microscope misses many patients. Growing the germs in a culture takes weeks, because TB germs grow slowly. A molecular test is faster, because it copies the germ's DNA instead of waiting for the germ to grow.
The method is called the polymerase chain reaction (PCR). The machine heats and cools the sample again and again. In each cycle, a chemical reaction doubles one short, chosen piece of the germ's DNA. After about 40 cycles, one copy has become billions of copies, enough to detect with a dye that glows. If the TB germ's DNA is not in the sample, nothing is copied and nothing glows. Reading genetic material in this way is part of genomics.
The cartridge is what makes the test usable outside a specialist lab. Every step, from breaking open the germs to the PCR, happens inside one sealed plastic cartridge. A technician adds the sample, closes the lid and puts the cartridge into the machine. So a district lab with no molecular biologist can run the test safely.
Consider a district TB centre in Gaya, Bihar. A 34-year-old farm worker has had a cough for three weeks and has lost weight. An AI radiology model has read his chest X-ray and flagged signs of TB. At 10 a.m., the technician puts his sputum sample into a cartridge. At 11:50 a.m., the machine reports two things: TB germs are present, and they are resistant to rifampicin. Rifampicin is one of the two main medicines used against TB. So the doctor starts treatment for drug-resistant TB the same day, instead of the standard treatment, which would not have worked.
The machine sends the result to the laboratory information system, and from there to the patient's record in the hospital EHR. India's TB programme also records each patient's result and treatment in its own national online system. The same machines can test for other infections, such as the amount of HIV in the blood, with a different cartridge.
A molecular test differs from a clinical lab analyser. An analyser measures substances made by the body, such as sugar or a hormone. A molecular test looks for the genes of a germ.