Latent Tuberculosis Infection
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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.