The Tuberculin Skin Test (TST), or Mantoux test, screens for infection with the bacterium Mycobacterium tuberculosis, the cause of tuberculosis (TB). The test introduces a small amount of purified protein derivative (PPD) into the skin to see if the immune system recognizes it. A positive reaction indicates the body has been exposed to the bacteria, but does not necessarily mean the person has active disease. This quick procedure is a crucial initial step in identifying individuals who require further evaluation for TB infection.
The Intradermal Injection Technique
The TST uses an intradermal injection, placing the PPD directly beneath the epidermis but above the deeper dermis. This technique differs from typical injections that penetrate muscle or fat. A fine 27-gauge needle is inserted at a shallow angle (five to fifteen degrees) to ensure the medication remains in the upper skin layers.
The correct administration of the 0.1 milliliter dose of PPD is immediately confirmed by the formation of a small, pale, raised area called a wheal. This wheal, typically six to ten millimeters in diameter, indicates the fluid has been correctly deposited within the dermis. The integrity of the wheal validates the test, confirming the antigen is in the correct location for the immune response.
The procedure should ideally result in minimal to no blood at the injection site. If a slight pinprick of blood appears upon needle withdrawal, it is considered normal and does not invalidate the test. This small amount of blood usually means the needle nicked a capillary or was inserted fractionally deeper than intended. Healthcare providers gently blot any blood rather than pressing on the site, as pressure could disperse the PPD and compromise the wheal’s accuracy.
Interpreting the Reaction
The TST result is not read immediately, as the immune system requires time to mount a delayed-type hypersensitivity reaction. A trained healthcare professional must examine the site within the mandatory window of 48 to 72 hours after PPD administration. If the patient does not return within this timeframe, the test is considered invalid and must be repeated.
The evaluator measures only the diameter of the induration, which is the firm, raised, localized swelling. Redness (erythema) alone is not measured and may simply indicate mild, non-specific irritation. The skin is palpated to find the edges of the induration, and a ruler is used to measure its diameter across the forearm in millimeters.
The interpretation of the measurement depends on the patient’s individual risk factors for TB infection.
Positive Induration Measurements
- Five millimeters or more is considered positive for high-risk groups, such as people living with HIV, recent close contacts of individuals with active TB, or those who are significantly immunosuppressed.
- Ten millimeters or more is considered positive for individuals with elevated risk, such as recent immigrants from high-prevalence countries or healthcare workers.
- Fifteen millimeters or more is considered positive for people with no known risk factors for TB.
Follow-Up Procedures After a Positive Test
A positive TST indicates infection with Mycobacterium tuberculosis but does not distinguish between latent TB infection (LTBI) and active TB disease. Further diagnostic steps are necessary to determine the appropriate medical management. The initial positive finding triggers a comprehensive evaluation.
The next step frequently involves a confirmatory blood test, specifically an Interferon-Gamma Release Assay (IGRA), such as the QuantiFERON-TB Gold. IGRAs measure the immune response to TB proteins and are less likely to be affected by prior BCG vaccination, offering a more specific confirmation of infection. Diagnostic imaging is also a standard follow-up, with a chest X-ray performed to look for characteristic abnormalities or lesions in the lungs that suggest active TB disease.
If the chest X-ray is clear and the patient has no symptoms of active disease, they are typically diagnosed with latent TB infection and may be offered preventive treatment. If the X-ray is abnormal or symptoms like a persistent cough or unexplained weight loss are present, further testing, such as sputum samples, is required to confirm active TB disease, which requires a different and more extensive treatment regimen.