How to Interpret a Tuberculosis Skin Test: Understanding PPD Results and Next Steps
What Is the TB Skin Test (PPD)?
The tuberculosis skin test, commonly known as the PPD (Purified Protein Derivative) test, is a widely used diagnostic tool to determine whether someone has been exposed to the bacteria that cause tuberculosis. During the test, a small amount of PPD solution is injected just under the skin, typically on the inner surface of the left forearm. The reaction at the injection site is evaluated 48 to 72 hours later to assess possible exposure to Mycobacterium tuberculosis.
How to Read Your PPD Test Results
Interpreting the results primarily depends on the size of the induration — the raised, hardened area at the injection site — not redness. A healthcare provider will measure the diameter of this firm bump in millimeters. Generally, an induration of 5 mm or more may be considered positive in high-risk individuals, such as those with HIV or recent contact with active TB cases.
Indurations larger than 10 mm are often interpreted as positive in people without known risk factors, while those with a history of intravenous drug use or who live in crowded environments may also fall into this category. In low-risk individuals, a reaction exceeding 15 mm might indicate latent TB infection.
Signs of a Strong Positive Reaction
A particularly strong response, characterized by an induration greater than 15–20 mm, along with visible symptoms like blistering, ulceration, intense itching, or significant swelling, may suggest a high likelihood of TB infection. While these severe local reactions do not automatically confirm active tuberculosis, they strongly indicate immune sensitization to the TB bacterium and warrant further medical evaluation.
Distinguishing Between Latent and Active TB
If the skin test shows minimal reaction — for example, less than 2 mm of induration and no signs of irritation — the result is generally considered negative. However, a negative PPD does not completely rule out TB, especially in immunocompromised individuals whose bodies may not mount a detectable response.
To reach an accurate diagnosis, doctors often recommend additional testing. This may include a chest X-ray or CT scan to check for abnormalities in the lungs, sputum tests to detect the presence of TB bacteria, and blood-based interferon-gamma release assays (IGRAs), which offer higher specificity than the skin test alone.
Next Steps After a Positive Result
Receiving a positive TB test doesn't mean you have active tuberculosis. It may indicate latent TB infection, where the bacteria are present but inactive and non-contagious. Still, treatment is often recommended to prevent progression to active disease.
Standard treatment for latent TB usually involves several months of antibiotic therapy, commonly with isoniazid or a combination regimen. For active TB disease, a longer course — typically six months or more — of multiple antibiotics such as rifampin, isoniazid, pyrazinamide, and ethambutol is required to fully eradicate the bacteria and prevent drug resistance.
Why Early Detection Matters
Early screening and accurate interpretation of TB tests play a crucial role in controlling the spread of this infectious disease. Left untreated, latent TB can become active, especially under conditions of weakened immunity. Prompt diagnosis and adherence to prescribed treatments significantly reduce both individual health risks and public health burdens.
Regular testing is especially important for healthcare workers, immigrants from high-prevalence countries, and individuals living in close quarters such as prisons or shelters. By understanding how to interpret PPD results and following up with comprehensive diagnostics, patients and providers can work together to manage TB effectively and safely.