A positive TB skin test shows as a raised, hard bump at the injection site. The size in millimeters determines if it is positive.
Reading
Doctors read the bump (induration) 48 to 72 hours after injection, measuring the hard, raised area, not redness.
Positive cutoffs
- 5 mm or more: HIV, recent contact, organ transplant.
- 10 mm or more: health care workers, recent immigrants, children under 4.
- 15 mm or more: anyone.
What it means
Exposure to TB bacteria. It doesn’t confirm active disease.
Next steps
Chest X-ray and sputum tests.
Alternative
IGRA blood test.
How the test is read
In a tuberculin skin test (Mantoux), 0.1 ml of tuberculin PPD is injected under the skin of the forearm, and a trained professional reads the induration (firm swelling) at 48 to 72 hours. Redness alone does not count. The width of the hard area is measured across the forearm in millimeters.
- 5 mm or more is positive for people with HIV, recent TB contact, organ transplant or immunosuppression, or fibrotic chest X-ray changes.
- 10 mm or more is positive for recent immigrants from high-prevalence countries, healthcare workers, injection drug users, children under 4 and those with certain medical conditions.
- 15 mm or more is positive for anyone, including people with no risk factors.
What a positive result means
A positive test shows exposure to TB bacteria, not necessarily active disease. Follow-up includes a chest X-ray, symptom review and sputum tests if needed. Latent TB infection can be treated with regimens such as 3 months of rifapentine and isoniazid (3HP) or 4 months of rifampin. Prior BCG vaccination can cause false positives, and blood tests (IGRA, such as QuantiFERON) are not affected by BCG.
How the test is read
For the TB skin test (Mantoux), a clinician injects tuberculin and reads the raised, firm swelling (induration) 48 to 72 hours later, not redness. Measurement in millimeters across the forearm. Positive threshold is 5 mm, 10 mm or 15 mm depending on risk group.
What a positive result means
A positive test shows exposure, not necessarily active disease. Follow up includes chest X ray and sometimes a blood test (IGRA). Latent TB can be treated to prevent disease.
Reading the result
A positive reaction is an area of hardened swelling at the injection site, measured across the forearm, not the redness. At least 5 mm is positive in HIV positive people, close contacts of TB patients and those with fibrotic chest changes. At least 10 mm is positive for healthcare workers, immigrants from high prevalence countries and people with diabetes. 15 mm or more is positive for anyone.
Follow up
BCG vaccination can cause false positives, which is why blood tests (IGRA) are preferred for vaccinated people. Chest X ray and sputum tests look for active disease. Latent TB treatment options include 3 months of isoniazid and rifapentine weekly or 4 months of rifampin. About 5 to 10 percent of latent infections progress to disease.
Based on data from the CDC
