The Tuberculin Purified Protein Derivative (PPD) test, also widely recognized as the Mantoux test or tuberculin skin test, serves as a common screening tool in healthcare. Its purpose is to determine if an individual has been exposed to Mycobacterium tuberculosis, the bacterium causing tuberculosis (TB). A positive PPD test indicates prior exposure to the TB bacteria, but it does not definitively diagnose active, contagious TB disease. The PPD test helps identify individuals who may carry the bacteria without showing symptoms.
The Tuberculin PPD Testing Procedure
The PPD testing procedure involves two visits to a healthcare provider. During the initial appointment, a small amount of tuberculin solution, 0.1 milliliters, is injected just beneath the skin. This injection is usually administered on the inner surface of the forearm, using a fine needle, such as a 27-gauge tuberculin syringe. A correctly performed injection creates a small, pale, raised bump, known as a wheal, measuring approximately 6 to 10 millimeters in diameter, indicating the solution has entered the correct skin layer.
After the initial injection, patients are advised to leave the injection site undisturbed, avoiding scratching or covering the area. A follow-up visit is required 48 to 72 hours later. At this appointment, a healthcare professional will examine the injection site. This involves visual inspection and palpation to measure the diameter of any firm, raised swelling.
Interpreting Test Results
The interpretation of a Tuberculin PPD test result focuses on measuring the diameter of the induration, which is the firm, raised, and hardened area at the injection site. Redness (erythema) surrounding the induration is not considered when determining the result. The healthcare provider will measure the indurated area in millimeters.
The threshold for a positive result varies depending on an individual’s risk factors for tuberculosis. An induration of 5 millimeters or more is considered positive for individuals with compromised immune systems, such as those with HIV, organ transplant recipients, or people receiving immunosuppressive medications like long-term steroids. It also applies to individuals who have had close contact with a person diagnosed with active TB.
An induration of 10 millimeters or more is positive for:
Recent immigrants from countries where TB is prevalent.
Residents and employees in high-risk environments like prisons or homeless shelters.
Healthcare workers.
Intravenous drug users.
Children under four years of age.
For healthy individuals with no known risk factors for TB, an induration of 15 millimeters or more is positive. A negative result is indicated by little to no induration at the injection site.
Factors That Influence PPD Test Accuracy
Several factors can influence PPD test accuracy. The Bacille Calmette-Guérin (BCG) vaccine, commonly administered in many countries with high TB prevalence, is a significant factor. This vaccine can cause a false-positive PPD result because it introduces components that can elicit an immune response similar to Mycobacterium tuberculosis. While the reactivity from the BCG vaccine may diminish over time, repeated PPD tests can sometimes “boost” this reaction, leading to a misleading positive result.
False-negative PPD test results can also occur, meaning an individual who is truly infected does not show a reaction. Recent TB exposure is one reason; it can take 2 to 8 weeks after initial infection for the immune system to develop a detectable reaction. Testing too soon can yield a false-negative. Individuals with severely compromised immune systems, sometimes referred to as anergy, may also experience false-negative results. This can happen in people with conditions like HIV, severe malnutrition, or those taking medications that suppress the immune response, as their bodies may not mount a strong enough reaction.
Next Steps After a Positive Result
A positive PPD test indicates exposure to the TB bacteria, but it typically suggests a latent TB infection (LTBI) rather than active disease. In LTBI, the bacteria are present but are inactive, causing no symptoms and not being contagious. Since the PPD test cannot differentiate between latent infection and active TB disease, further medical evaluation is necessary.
Follow-up after a positive PPD test usually involves a chest X-ray. This imaging test assesses the lungs for active TB disease. If the chest X-ray is normal and there are no active TB symptoms (e.g., persistent cough, fever, or unexplained weight loss), LTBI is supported. An Interferon-Gamma Release Assay (IGRA), a blood test, may also be performed. IGRAs measure the immune response to TB antigens and are useful, especially if there is a history of BCG vaccination or if the PPD result is ambiguous. Treatment for latent TB infection is often recommended to prevent progression to active, contagious TB disease.