# Latent Tuberculosis Infection > Latent tuberculosis infection for NEET-PG Medicine: TST and IGRA interpretation, NTEP preventive treatment with 3HP, 6H and 4R regimens. - Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/latent-tuberculosis-infection - Exam / course: NEET-PG · Subject: Medicine - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Latent Tuberculosis Infection", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/latent-tuberculosis-infection ## Direct answer A positive tuberculin or interferon-gamma test in a person with no symptoms and a normal chest radiograph means latent tuberculosis infection (LTBI) — infected, neither ill nor infectious. Its purpose in practice is to identify candidates for preventive treatment: household contacts, people living with HIV, and patients starting tumour-necrosis-factor blockers or transplantation, who carry the highest risk of progression to active disease. India's programme offers weekly isoniazid-rifapentine for three months (3HP) or daily isoniazid for six months (6H), with four months of rifampicin (4R) as an alternative. ## What you must remember - Rule out active tuberculosis first: symptom screen and chest radiography for every contact, with microbiology when suspicious — preventive therapy alone for undiagnosed active disease breeds resistance. - Tuberculin skin test: five tuberculin units intradermally (Mantoux), transverse induration read at 48 to 72 hours; 10 mm or more is the accepted positive cut-off in India, with around 5 mm in severe immunosuppression per many guidelines; bacille Calmette-Guerin boosts smaller readings. - Interferon-gamma release assays use antigens absent from BCG and most non-tuberculous mycobacteria; they too detect infection only and are preferred where BCG confounds the skin test and before biologics. - Whom to treat: programme priorities are household contacts (youngest and most recent converters at highest progression risk) and people living with HIV; clinic priorities add anti-tumour-necrosis-factor therapy, transplantation, dialysis, silicosis and prolonged high-dose steroids. - Regimens: 3HP — isoniazid plus rifapentine once weekly for 12 doses, programme-preferred; 6H — daily isoniazid for six months; 4R — daily rifampicin for four months after isoniazid intolerance or isoniazid-resistant exposure; 3HR is a further alternative. - Counselling: isoniazid can cause hepatitis (stop for jaundice or transaminase rise); pyridoxine prevents neuropathy in at-risk groups; rifapentine and rifampicin interact with oral contraceptives, warfarin and antiretrovirals; 3HP commonly causes a transient flu-like reaction. - In high-burden India, testing and treating the general population is not recommended — the intervention targets contacts and high-risk groups. ## A contact walks into the clinic A six-year-old, household contact of a newly diagnosed sputum-positive uncle, is brought for screening. Before any test, the rule that outranks all others: exclude active tuberculosis first — symptom screen and chest radiograph, with microbiology if either raises suspicion — because giving preventive therapy alone to undiagnosed active disease breeds resistance. She is well and the film is clean, so place the Mantoux: five tuberculin units intradermally, transverse induration read at 48 to 72 hours (never erythema), 10 mm or more the accepted positive cut-off in India. Hers measures 14 mm — infected. An interferon-gamma release assay would confirm without BCG confounding, but it too detects infection only, never disease. Now treat: the programme-preferred 3HP — isoniazid plus rifapentine once weekly for 12 doses — 6H daily isoniazid and 4R daily rifampicin (after isoniazid intolerance or an isoniazid-resistant source) are the alternatives. Counsel the parents about isoniazid hepatitis, give pyridoxine in at-risk groups, and warn that rifapentine and rifampicin undermine oral contraceptives, warfarin and antiretrovirals. In high-burden India, testing and treating the general population is explicitly not recommended — contacts and high-risk groups are the target. ## Where students slip The dominant error is reading a positive tuberculin or interferon-gamma test as disease and reaching for four drugs — the tests speak to infection, and the answer is a single preventive regimen once active disease is excluded. The mechanics of the Mantoux supply the next tier of traps: induration not erythema, the 48 to 72 hour window, the 10 mm Indian cut-off with lower thresholds in immunosuppression, and BCG inflating readings. And the sequencing question — "young contact, positive test, normal radiograph, next step?" — is answered with preventive therapy, not with repeat testing or with full treatment. ## Frequently asked questions ### How is the tuberculin skin test performed and read? Five tuberculin units intradermally into the forearm, with the transverse induration measured at 48 to 72 hours; 10 mm or more is positive for most Indian risk groups. ### Why prefer an interferon-gamma release assay? It uses antigens absent from the BCG vaccine and needs one visit — though it still detects infection, not disease. ### What must precede preventive therapy? Exclusion of active tuberculosis by symptom screen and chest radiography, with sputum testing if either is abnormal. ### What is the 3HP regimen? Twelve once-weekly doses of isoniazid and rifapentine over three months — the short preventive course preferred under current programme guidance. ### What are the alternatives to 3HP? Daily isoniazid for six months (6H) or rifampicin for four months (4R), the latter suiting isoniazid intolerance or resistant source cases. ### Is a positive test alone an indication for four-drug therapy? Never — it indicates infection without disease, managed with one preventive regimen once active tuberculosis is excluded.