Written By: N. Lothungbeni Humtsoe | Published : March 23, 2026 4:01 PM IST
Medically Verified By: Dr. Niranjan Patil
Drug-resistant tuberculosis (TB) refers to a severe form of Tuberculosis where the bacteria is no longer affected by the anti-TB drugs. It normally occurs as a result of under-treatment, lack of doses or misuse of medication. Consequently the infection will be more difficult to treat and can be transferred more easily. The World Health Organization (WHO) states that drug-resistant Tuberculosis is one of the significant societal issues and therefore early diagnosis and adequate medication are critical to curb its transmission.
"Drug-resistant TB (DR- TB) is diagnosed by identifying the presence of Mycobacterium tuberculosis bacteria in a patient's sample. It is followed by performing specific tests to determine which anti-TB drugs may no longer kill those bacteria," Dr. Niranjan Patil, Associate Vice President of Metropolis Healthcare Limited, Mumbai, explained. "Some of the commonly used anti-TB drugs are Rifampicin, Isoniazid (INH), Ethambutol and Pyrazinamide. Whereas in drug-resistant tuberculosis people often take Levofloxacin, Moxifloxacin, Linezolid, Clofazamine, Bedaquiline, Pretomanid, Delamanid, etc." Here are a few types of drug-resistant tuberculosis:
According to Dr. Patil, for the diagnosis of drug-resistant tuberculosis, two conditions need to be addressed, i.e., confirm the presence of TB infection and test for drug resistance, either by a phenotypic method, genotypic or both.
The doctor outlines that confirmation of TB infection is primarily a laboratory-based microbiological diagnosis. However, a microscopic examination (Ziehl-Neelsen-stained smears or Fluorescent-stained smears) method is the quickest and most reasonable approach to confirm TB infection. Although it is important to note that anti-TB drug resistance cannot be determined by this method.
If you want to take a test for drug susceptibility and resistance, then you can either try for phenotypic or genotypic. Explaining phenotypic drug susceptibility test (DST), Dr. Patil explains, "TB pathogens from a patient's specimen are cultivated in a solid or liquid culture medium containing anti-TB drugs. If they keep growing with the drug in the medium then that TB strain is identified as resistant to that specific anti-TB drug. Culture-based drug susceptibility test is still the gold standard in determining the diagnosis of resistance to TB."
He further explained that genotypic test seeks certain specific or known genetic mutations within TB bacteria that cause drug resistance, for example, kat G and inh Afor Isoniazid, gyro for fluoroquinolones. Whereas the line probe assay (LPA) is a molecular test that looks for mutations in genes to identify drug resistance. It works on the principle of reverse hybridization which can be found as first line and second line, with results within 1-2 days.
Dr. Patil also discusses the Whole Genome Sequencing (WGS) which is a TB Next Generation Sequencing (TB NGS) technology that is used for TB Whole Genome Sequencing. All mutations are detected for the existing 18 anti-TB drugs. In India under the National TB elimination programme (NTEP) universal drug susceptibility testing is mandatory to be done on all laboratory diagnosed TB cases to rule out drug resistance before or at treatment initiation.
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