The Tuberculin Skin Test (TST), also known as the Mantoux or PPD test, screens for latent tuberculosis infection (LTBI). Purified Protein Derivative (PPD) is a standardized solution of proteins derived from the Mycobacterium tuberculosis bacterium. The injection detects an immune response indicating prior exposure, not active disease. A positive TST means the immune system recognized the PPD antigens, suggesting dormant TB bacteria are present. This screening tool helps identify and treat latent infection before it progresses to a transmissible, active disease state.
The Tuberculin Skin Test Procedure
The TST uses the Mantoux technique, involving an intradermal injection into the skin’s top layer. A healthcare professional injects 0.1 milliliters of the PPD solution, typically into the inner forearm. A successful injection creates a pale, raised bubble, or wheal, usually 6 to 10 millimeters in diameter, which disappears shortly after administration.
This procedure introduces mycobacterial antigens directly into the skin, initiating a localized cell-mediated delayed hypersensitivity reaction in previously exposed individuals. The patient must return for a reading between 48 and 72 hours after the injection, when the immune response reaches its peak.
If the reading is not performed within the 72-hour timeframe, the result is invalid because the reaction may fade, leading to inaccuracy. The healthcare worker measures the diameter of the induration—the firm, raised, hardened area at the injection site. Measurement must focus only on the induration, not surrounding redness (erythema), which does not indicate a positive reaction. The exact measurement in millimeters is recorded, and only a trained professional can interpret the result.
Interpreting TST Results
TST interpretation depends on the individual’s risk factors for TB exposure and progression to active disease, not a single fixed measurement. The induration size is stratified into three positive cutoffs to maximize the test’s predictive value. This risk-stratification ensures that high-risk individuals are identified while minimizing false-positive results in low-risk populations.
An induration of five millimeters or greater is positive for individuals in the highest-risk categories.
5 mm Positive Cutoff
This threshold applies to people who are immunosuppressed, such as those with HIV infection, organ transplant recipients, or patients receiving high-dose corticosteroids. It also applies to recent close contacts of a person with active TB and those whose Chest X-ray shows evidence of old, healed TB lesions.
A larger induration of ten millimeters or greater is positive for the medium-risk group.
10 mm Positive Cutoff
This group includes recent immigrants from high-prevalence countries, injection drug users, and residents or employees of high-risk congregate settings (e.g., hospitals, prisons, homeless shelters). People with underlying health conditions that increase progression risk, such as diabetes, chronic renal failure, or certain cancers, are also positive at this threshold.
Only an induration of fifteen millimeters or greater is positive for people with no known TB risk factors. Because the probability of infection is low in this group, a larger reaction is required for confirmation. If the induration is less than the cutoff for the individual’s risk group, the test is considered negative.
Factors Affecting TST Accuracy
The TST is an indirect measure of the immune response, making it susceptible to false-positive or false-negative results. A common cause of a false-positive result is previous vaccination with the Bacille Calmette-Guérin (BCG) vaccine, used widely in high-TB burden countries. The BCG vaccine contains a weakened strain of Mycobacterium bovis that can cause the immune system to react to PPD proteins, even without M. tuberculosis infection.
False-positive results may also occur from infection with non-tuberculous mycobacteria, which are common in the environment and share antigens with TB bacteria. Conversely, a false-negative result (where the infected individual tests negative) can be caused by a weakened immune system, known as anergy. This is often seen in individuals with advanced HIV infection, those on immunosuppressive medications, or the elderly, whose immune systems cannot mount a proper PPD response.
Timing also matters; recently infected individuals (within 8 to 10 weeks) may not have developed the necessary immune response for a positive test. Technical errors, such as injecting the PPD solution too deeply (subcutaneously instead of intradermally) or measuring redness instead of induration, also lead to inaccurate results. Therefore, a positive or negative TST must always be evaluated within the context of a patient’s medical history and risk profile.
What Happens After a Positive TST
A positive TST suggests TB infection, but further steps are required to determine if the disease is latent or active. The immediate goal is to rule out active TB, which is contagious and requires a multi-drug treatment regimen. This process starts with a comprehensive medical evaluation, physical examination, and a Chest X-ray.
The Chest X-ray looks for lung abnormalities characteristic of active pulmonary TB, such as infiltrates, cavities, or fibrotic changes. If the patient has symptoms of active disease (e.g., persistent cough, fever, unexplained weight loss), additional tests like sputum samples for culture and smear may be ordered. The diagnosis is confirmed as Latent TB Infection (LTBI) only if the physical exam and Chest X-ray are clear of active disease.
Blood tests known as Interferon-Gamma Release Assays (IGRAs), such as Quantiferon, may also be performed. IGRAs are often preferred for individuals who received the BCG vaccine because they are more specific and less likely to yield a false-positive result. Once active disease is excluded, LTBI treatment is initiated to prevent the dormant bacteria from reactivating. Treatment regimens typically involve preventative medication, such as isoniazid, rifampin, or a combination of both, for a period ranging from a few months up to nine months.