The site of the PPD skin test is itchy.

Itching at the site of the PPD skin test may indicate a possible infection with Mycobacterium tuberculosis, prior vaccination with Bacillus Calmette-Guérin (BCG), or a normal immune response. Further evaluation is required, including a CT scan, blood tests, and antibody testing to confirm the diagnosis.

1. Mycobacterium tuberculosis infection

When a patient is infected with Mycobacterium tuberculosis, itching, redness, and swelling may occur after the PPD skin test, necessitating further examination. If induration is detected during follow-up testing and its diameter exceeds 5 mm, the result is considered positive. Specifically, an induration diameter of 5–9 mm is classified as (+), 10–19 mm as (++), and greater than 20 mm—or the presence of blistering, necrosis, or lymphangitis at the injection site—as strongly positive. A strongly positive result suggests active pulmonary tuberculosis. This condition is primarily treated with anti-tuberculosis medications and, in some cases, surgery.

2. Prior BCG vaccination

In individuals who have received the BCG vaccine, the presence of antibodies can trigger an immune reaction leading to PPD test site itching. This is not a cause for concern, and the itching typically resolves spontaneously.

3. Normal immune response

If the area of redness and itching measures less than 5 mm in diameter, it is generally considered a normal immune response to the purified protein derivative (PPD) injected during the test. The redness and itching result from the release of inflammatory mediators and the accumulation of inflammatory cells at the injection site. No treatment is necessary, as symptoms usually resolve on their own over time.