The Tuberculosis (TB) skin test, also known as the Mantoux tuberculin skin test or PPD test, screens for Mycobacterium tuberculosis infection. It indicates if a person’s immune system has encountered the bacteria, rather than diagnosing active TB disease. This preliminary tool helps identify individuals who may need further evaluation. Understanding the visual cues at the test site is key to interpreting its outcome.
Administering the Test
Administering the TB skin test involves injecting a small amount of tuberculin purified protein derivative (PPD) into the top layer of the skin, usually on the inner forearm. A trained healthcare professional uses a fine needle to create a small, pale elevation, called a wheal, at the injection site. This temporary wheal flattens and disappears within minutes as the fluid absorbs. Individuals should avoid touching, scratching, or covering the site, as disturbing it can compromise the test’s integrity. This initial appearance is a normal part of the procedure and does not indicate the test result.
Understanding Your Test Site
Interpreting the TB skin test requires observation of the injection site 48 to 72 hours after administration, when any immune response will be most apparent. The key indicator for a reaction is induration: a firm, raised, palpable area distinct from the surrounding skin. Redness or bruising can occur from the injection or mild irritation, but these alone do not signify a positive result. A healthcare professional will feel for induration and measure its diameter using a ruler.
A negative TB skin test shows little to no induration at the injection site. Any firm, raised area present measures less than 5 millimeters. While minor redness, a faint mark, or a small bruise might be visible, the absence of distinct, palpable firmness indicates the immune system has not reacted to the tuberculin. This outcome suggests the person has not been infected with Mycobacterium tuberculosis or has not developed a sufficient immune response.
Conversely, a positive TB skin test is characterized by a measurable induration, signifying an immune response to the TB bacteria. The specific size of this firm, raised area, measured in millimeters, determines a positive result, with the threshold varying based on an individual’s risk factors for TB infection or progression. A positive reaction signifies that the body’s immune system has developed a response to the TB bacteria, indicating either a latent TB infection (LTBI) or active TB disease.
Positive Test Thresholds
An induration of 5 millimeters or more is considered positive for individuals with:
Compromised immune systems (e.g., HIV).
Recent close contact with active TB cases.
Organ transplants.
Chest X-ray findings suggestive of prior TB disease.
An induration of 10 millimeters or more is considered positive for individuals with:
Certain medical conditions that increase risk (e.g., diabetes, kidney failure).
Recent immigration from countries with high TB prevalence.
Healthcare workers.
Residents and employees of high-risk congregate settings (e.g., correctional facilities, nursing homes).
Children under the age of four.
An induration of 15 millimeters or more is considered positive for people with no known risk factors for TB.
Factors Affecting Appearance
Several factors can influence the TB test site’s appearance. A common cause of a false positive reaction is previous vaccination with the Bacille Calmette-Guérin (BCG) vaccine, administered in some countries to prevent TB. This can induce induration even without Mycobacterium tuberculosis infection. Exposure to non-tuberculous mycobacteria, environmental bacteria related to Mycobacterium tuberculosis, can also cause a similar positive reaction.
Conversely, certain conditions can lead to a false negative result, where an infected individual does not show a reaction. A weakened immune system, due to conditions like HIV, immunosuppressive medications, or severe illness, can prevent the body from responding to the tuberculin. Very recent TB infection, where the immune system has not yet developed a detectable response, or recent live virus vaccinations can also temporarily suppress the reaction.
Occasionally, individuals might experience redness, itching, or swelling at the test site without significant induration. This is a non-specific irritation or a mild allergic reaction to PPD solution components, not an indication of TB infection. Improper test administration, such as incorrect PPD injection, can also lead to an unreliable appearance, making accurate interpretation challenging.
What Happens After Interpretation
Once a healthcare professional interprets the TB test site, subsequent steps depend on the result. If the test is negative, showing no significant induration, no further action is needed. This outcome suggests the individual has not been infected with TB bacteria or their immune system has not reacted to the test. For recent known exposure or strong clinical suspicion, a repeat test might be recommended after several weeks to allow for potential immune response development.
A positive TB skin test result necessitates further diagnostic evaluation to determine if the person has latent TB infection (LTBI) or active TB disease, as the skin test only indicates exposure. Common follow-up tests include a chest X-ray for active lung disease and specific blood tests, known as Interferon-Gamma Release Assays (IGRAs). These blood tests measure the immune response to TB antigens and are not affected by prior BCG vaccination.
If active TB disease is suspected, additional tests like sputum smears and cultures are performed to confirm Mycobacterium tuberculosis. Treatment is recommended for latent TB infection to prevent progression to active disease. Active TB disease requires a specific course of multi-drug antibiotic treatment to cure the infection and prevent its spread.