Tuberculin skin test and IGRA !
- Topic
Two tests show whether a person has had a tuberculosis infection: one on the skin (tuberculin), one on blood (IGRA). Neither says if the disease is active.
Updated 2026-11-01
See it in 3D: Skin
What it is
The tuberculin skin test (PPD, Mantoux) injects purified protein derivative into the dermis and measures, at 48 to 72 hours, the induration caused by type IV hypersensitivity. IGRAs mix blood with Mycobacterium tuberculosis-specific peptides and measure the interferon gamma released by memory T cells. Both detect infection, latent or active, and disease must then be ruled out with clinical findings, chest X-ray and bacteriology.
Tuberculin skin test versus IGRA
| Feature | Tuberculin (PPD) | IGRA |
|---|---|---|
| Mechanism | Type IV hypersensitivity in the skin | Interferon gamma released by T cells in blood |
| Visits | Two (placement and reading at 48 to 72 hours) | One (blood draw) |
| Effect of BCG | Can give false positives | No: uses antigens absent from BCG (ESAT-6, CFP-10) |
| Preferred use | Children under 5 in CDC guidance; settings without a laboratory | BCG-vaccinated people aged 5 and older; people who may not return for the reading |
| Latent versus active | Does not distinguish | Does not distinguish |
In the lab
- Induration is measured in millimetres, not redness.
- CDC cut-offs: 5 mm in HIV, recent contacts, fibrotic lesions or immunosuppression; 10 mm in groups at exposure risk; 15 mm with no risk factors.
- False negatives (anergy): advanced HIV, malnutrition, immunosuppression, severe illness, very recent infection.
- IGRA results: positive, negative or indeterminate (or borderline and invalid depending on the method).
Test yourself
Think of the answer, then tap the question to see it.
Which type of hypersensitivity underlies the tuberculin skin test?
Type IV or delayed-type, driven by memory T cells and macrophages; that is why it is read at 48 to 72 hours.
Why is an IGRA preferred in a BCG-vaccinated adult?
It uses antigens (ESAT-6 and CFP-10) not present in BCG, so the vaccine causes no false positives.
What is measured when reading a tuberculin test?
The transverse diameter of the induration in millimetres, not the redness.
Does a positive IGRA confirm active tuberculosis?
No. It shows infection; clinical assessment, chest X-ray and bacteriological testing are needed to rule disease in or out.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Serology: IgM vs IgG, seroconversion, avidity
- Vaccine types: live, inactivated, subunit, toxoid, mRNA, vector
- Flow cytometry and the CD4 count
- Type IV hypersensitivity (delayed, T cells)
- Granulomatous inflammation
- Interferon-γ
- CD4⁺ helper T cell
Related conditions
Microorganisms
Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.
Sources: Clinical Testing and Diagnosis for Tuberculosis | Tuberculosis (TB) | CDC (Centers for Disease Control and Prevention (CDC), 2024); Tuberculosis Screening (MedlinePlus (U.S. National Library of Medicine), 2024).
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Representative model, for educational purposes only.