The Tuberculin Skin Test (TST), often called the Mantoux test, screens individuals who may have been infected with the bacteria that cause tuberculosis (TB). This test involves injecting a small amount of purified protein derivative (PPD) just beneath the skin, typically on the forearm. The body’s immune response is then observed to determine if the person has previously encountered Mycobacterium tuberculosis. For accurate results, a trained healthcare professional must examine the injection site within 48 to 72 hours following the administration of the PPD solution.
Visual Characteristics of a Reaction
A positive reaction at the 48-hour mark is characterized by a specific skin change at the injection site. The primary feature healthcare professionals look for is induration, a firm, raised, and dense area of swelling. This induration represents a localized delayed-type hypersensitivity reaction where immune cells have migrated to the site. When gently touched, the induration feels hard or thickened, distinguishing it from normal skin tissue. Redness (erythema) may also be present, but only the diameter of the palpable, raised induration is measured to determine the test outcome.
Determining a Positive Result
The determination of a positive TST result depends on measuring the induration across its widest point, perpendicular to the long axis of the forearm. The measurement is recorded in millimeters, and there are three primary thresholds used to define positivity. These thresholds are directly related to an individual’s risk factors for TB infection and progression to disease. Healthcare providers must utilize these risk categories to interpret the size of the reaction correctly.
An induration of 5 millimeters or greater is considered a positive result for the highest-risk populations. This includes individuals who are immunocompromised, such as those with HIV or organ transplants, and those who have had recent close contact with a person diagnosed with infectious TB disease. This lower threshold also applies to patients whose chest X-ray shows fibrotic changes that suggest old, healed TB.
The intermediate threshold for a positive result is an induration of 10 millimeters or greater. This measurement applies to people with moderate-risk factors, such as recent immigrants from countries where TB is common, injection drug users, or residents and employees of high-risk congregate settings like hospitals and prisons. People with certain medical conditions, including diabetes, end-stage renal disease, or a history of significant weight loss, are also placed in this 10-millimeter category.
For persons who have no known risk factors for TB infection, the cut-off for a positive result is an induration of 15 millimeters or greater. This highest threshold ensures that low-risk individuals are not unnecessarily classified as positive due to minor skin reactions.
What a Positive Test Indicates
A positive TST result signifies that the person’s immune system has been sensitized by the Mycobacterium tuberculosis bacteria. This means the individual has been infected at some point, but the test does not distinguish between the two states of infection. The result merely confirms exposure and the presence of a delayed hypersensitivity response to the PPD antigens.
The two states of infection are Latent TB Infection (LTBI) and active TB Disease. In LTBI, the bacteria are present but inactive; the person does not feel sick, has no symptoms, and cannot spread the bacteria. Active TB Disease occurs when the bacteria overcome the immune system, multiply, and become symptomatic and contagious.
Symptoms of active TB disease can include a prolonged cough, chest pain, coughing up blood, unexplained weight loss, and night sweats. A positive TST result necessitates further evaluation to determine which of these two conditions is present. A person with LTBI is at risk of progressing to active disease, which is why treatment is often recommended.
Next Steps After a Positive Result
Following a positive TST, the immediate step is to consult with a healthcare provider for a medical evaluation. The priority is to rule out active TB disease, as this requires immediate treatment. This evaluation typically includes a clinical assessment for symptoms and a chest X-ray to look for signs of lung damage or active infection.
Additional tests may be ordered to confirm the diagnosis or differentiate between latent and active infection. This can involve a blood test, known as an Interferon-Gamma Release Assay (IGRA), such as QuantiFERON-TB Gold. If the person is exhibiting pulmonary symptoms, sputum samples may be collected for laboratory analysis to check for active bacteria. If the evaluation rules out active TB disease, the provider will discuss preventative treatment for LTBI—a course of antibiotics designed to eliminate the inactive bacteria and reduce the risk of progression to active disease.