Tuberculin Skin Test and IGRA
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Direct answer
The tuberculin skin test and interferon-gamma release assay are the two tests for detecting infection with Mycobacterium tuberculosis in children, and neither proves active disease. The Mantoux test measures induration at 48 to 72 hours, whereas IGRA is a blood test that uses antigens absent from BCG. A positive result must be read against the child's risk factors and clinical picture, since BCG and nontuberculous mycobacteria can both distort it.
What you must remember
- Mantoux technique: 0.1 mL of 5 TU purified protein derivative is injected intradermally, usually on the forearm, and read at 48 to 72 hours.
- Reading: only the induration, the palpable raised area, is measured in millimetres across its transverse diameter, not the surrounding erythema.
- Cut-offs: Indian practice (NTEP, IAP) calls 10 mm or more positive, and 5 mm or more in HIV or other immunocompromised children; the US tiered scheme uses 5, 10 and 15 mm by risk group, so follow the guideline named in the question.
- False positives: recent BCG vaccination, nontuberculous mycobacteria and cross-reaction with environmental mycobacteria.
- False negatives: miliary or severe tuberculosis, HIV, malnutrition, measles, steroid therapy and very young infants.
- IGRA: a whole-blood test that measures interferon-gamma release after stimulation with ESAT-6 and CFP-10, which BCG does not carry.
- IGRA limits: indeterminate results are more common in young and malnourished children, and the test does not distinguish latent from active disease.
Choosing between the two tests for a vaccinated contact
A 4-year-old is a household contact of an adult with cavitary tuberculosis and received BCG at birth. The clinician must choose between the two tests, and the comparison below shows the trade-offs. The contact is high risk, so a positive result would lead to preventive therapy once active disease is excluded.
| Feature | Tuberculin skin test | IGRA |
|---|---|---|
| Sample | Intradermal injection | Blood sample sent to the laboratory |
| Reading | Return visit at 48 to 72 hours | Result from a single visit |
| Effect of BCG | Can give a positive reaction | No effect |
| Reader dependence | Yes, induration is measured by hand | No |
| Main limitation | False negatives in malnutrition or immunosuppression | Indeterminate results in young or malnourished children |
IGRA is preferred in BCG-vaccinated children and where the return visit is unlikely to happen. Both tests support the diagnosis, and neither replaces a clinical assessment of the child. Where IGRA is unavailable, the tuberculin test remains the practical option, and the choice follows cost and local laboratory capacity.
Where the exam tries to catch you
The questions here test interpretation more than technique.
- Trap: reading the redness at the injection site as the result Correct: only the induration is measured, in millimetres, and the erythema is ignored.
- Trap: assuming a negative tuberculin test excludes tuberculosis Correct: a malnourished or immunosuppressed child can have a false negative, so clinical suspicion still drives management.
- Trap: attributing every positive reaction in a BCG-vaccinated child to infection Correct: recent BCG can cause a positive reaction, and IGRA is preferred because BCG does not affect it.
Frequently asked questions
What induration cut-off is used for a tuberculin skin test in a child with HIV?
A child with HIV is considered positive at an induration of 5 mm or more, because the risk of progression to disease is high in this group. Cut-offs are set by the guideline in use and are lower for high-risk children than for children with no known risk factors.
Why is IGRA preferred in BCG-vaccinated children?
IGRA is preferred in BCG-vaccinated children because the antigens it uses, ESAT-6 and CFP-10, are absent from BCG, so vaccination does not cause a false positive. The test also needs only one visit, avoiding the reading at 48 to 72 hours that the tuberculin test requires.
Why does malnutrition cause false-negative tuberculin results?
Malnutrition causes false-negative tuberculin results because it impairs the cell-mediated immune response that the skin test depends on. Measles, other viral infections and steroid therapy have the same effect, so a negative tuberculin test does not exclude infection.
From what age can IGRA be used in children?
IGRA is generally used in children from about two years of age, with caution in younger children because indeterminate results are more common. Local guidance on age and testing setting is followed, and the result is always interpreted alongside the clinical picture.