What to Expect from a Tuberculosis Skin Test: Understanding PPD Results and Interpretation
When evaluating potential tuberculosis (TB) infection, one of the most commonly used screening tools is the tuberculin skin test, also known as the PPD (purified protein derivative) test. This test helps determine whether a person has been exposed to the bacteria that cause TB. The procedure involves injecting a small amount of PPD just under the skin, typically on the forearm. The results are then assessed 48 to 72 hours later by measuring the size of the induration—firm swelling—at the injection site.
Interpreting PPD Test Results
The reaction at the injection site is measured based on the diameter of the hardened area, not redness. A result with less than 5 millimeters (mm) of induration is generally considered negative, indicating no significant immune response to the TB bacteria. However, interpretation can vary depending on individual risk factors.
PPD Reaction Scale and Meaning
A reaction between 5 and 10 mm is classified as (+), which may be considered positive in high-risk individuals such as those with HIV, recent exposure to active TB, or organ transplant recipients. A 10–15 mm reaction is marked as (++), often interpreted as a moderate positive, especially in people with underlying health conditions. Reactions from 15 to 20 mm are recorded as (+++), suggesting a stronger likelihood of TB exposure. Any induration larger than 20 mm is labeled (++++) and strongly indicates sensitization to TB antigens. Additionally, even if the measurement is smaller, the presence of blistering (vesiculation), ulceration, or lymphangitis (inflammation of lymph vessels) also qualifies as a (++++) result due to the intensity of the immune response.
Limitations and Considerations of the PPD Test
It's important to understand that the PPD test does not diagnose active tuberculosis disease—it only indicates whether someone has been infected with the TB bacterium at some point. Moreover, the test has limitations. For instance, individuals who received the BCG (Bacillus Calmette-Guérin) vaccine during childhood may show a false-positive result. The BCG vaccine, widely used in countries with high TB prevalence, can trigger an immune response to the PPD antigen even in the absence of actual TB infection.
False-negative results are also possible, particularly among people with weakened immune systems. Conditions such as advanced HIV/AIDS, malnutrition, recent viral infections, or certain cancers can suppress the body's ability to react to the PPD injection. Diabetic patients, those undergoing immunosuppressive therapy, or individuals on long-term corticosteroids may not mount a detectable response—even if they are infected with TB.
PPD as Part of a Broader Diagnostic Process
Due to these variables, the PPD skin test should never be used in isolation to confirm or rule out TB infection. Instead, it serves as an initial screening tool. Abnormal results must be followed up with additional testing, such as chest X-rays, sputum cultures, or interferon-gamma release assays (IGRAs), which are blood tests more specific to TB and less influenced by prior BCG vaccination.
In summary, while the TB skin test is a valuable and cost-effective method for detecting latent TB infection, its results require careful clinical interpretation. Healthcare providers must consider a patient's medical history, risk factors, vaccination status, and overall immune function when assessing PPD outcomes. Accurate diagnosis ensures timely treatment for those who need it while preventing unnecessary interventions for those who don't.