A positive TB (tuberculosis) skin test typically appears as a raised, firm bump (induration) at the site of the injection on the forearm, usually measured 48 to 72 hours after the test is administered. The key factor in determining a positive result is the size of this induration, not redness or color change around the site. The interpretation of a positive test depends on the size of the induration and the individual's risk factors:
- An induration of 15 mm or more is considered positive in anyone, regardless of risk factors.
- An induration of 10 mm or more is positive for people with certain risk factors such as recent immigrants from high TB prevalence countries, healthcare workers, children under 4, or people who use injected drugs.
- An induration of 5 mm or more is positive for people with higher risk factors like recent contact with someone with TB, HIV infection, organ transplant recipients, or those on immunosuppressive therapy.
- Indurations less than 5 mm are generally considered negative
The bump is firm and raised, and the healthcare provider measures the diameter of the induration across the forearm perpendicular to the arm's length. The redness alone is not used to determine positivity. The bump usually fades within a few weeks
. A positive test indicates TB exposure or infection but does not distinguish between latent (inactive) TB infection and active TB disease. Further tests such as chest X-rays and sputum analysis are needed to diagnose active TB
. In summary, a positive TB test looks like a raised, firm bump on the skin at the injection site, with the size of the bump meeting or exceeding specific thresholds based on risk factors
