The Tuberculin Skin Test (TST), also known as the Mantoux or PPD test, screens individuals for previous exposure to the bacterium Mycobacterium tuberculosis. This test involves injecting a small amount of purified protein derivative (PPD) into the top layer of the skin, typically on the forearm. The body’s immune reaction is assessed 48 to 72 hours later to determine if infection has occurred. Correctly interpreting the result requires understanding the specific visual signs.
The Typical Appearance of a Test Site
After the PPD injection, the test site will almost always show some immediate, localized reaction that is not indicative of a positive result. Within the first few hours, it is expected to see a small area of redness (erythema) surrounding the injection point, often alongside minor swelling or slight bruising. This simply reflects the body’s normal response to the needle and injected fluid.
Some individuals may also report mild itching or tenderness. These common signs of irritation do not determine whether the TST is positive or negative, as the diagnostic assessment focuses on a specific physical change beneath the skin’s surface.
Identifying the Key Visual Marker: Induration
The definitive sign indicating a positive TST result is induration, which differs fundamentally from simple surface redness. Induration is a firm, dense, and raised area of swelling caused by immune cells migrating to the injection site. This delayed-type hypersensitivity response means it takes time for the body’s T-cells to mount a visible reaction to the tuberculin proteins.
To properly identify this marker, the healthcare provider must physically palpate the injection site by gently running fingers across the forearm. Induration feels like a distinct, hard bump or knot beneath the skin, not soft, spongy swelling. The boundary of this palpable firmness determines the measurement, even if surrounding redness extends further out.
A negative result typically presents as a flat, soft area of skin. In contrast, a potentially positive result involves this distinct, hardened elevation that resists compression. Only the specific diameter of the firm, raised induration is measured for interpretation; visible surface redness (erythema) is ignored.
How Measurement Determines a Positive Result
Once the induration is identified, the healthcare professional uses a millimeter ruler to measure its diameter across the forearm at its widest point. Interpreting the measurement is not based on a single, universal size, but depends entirely on the patient’s underlying medical history and likelihood of recent exposure. This approach recognizes that different groups require different levels of sensitivity for a positive diagnosis.
5 Millimeters or More
The lowest threshold for a positive reading is an induration of five millimeters or more. This applies exclusively to individuals with the highest risk of developing active disease. This category includes people with compromised immune systems (such as those living with HIV), recent close contacts of an active TB case, or patients who have received organ transplants. For these individuals, a smaller reaction is significant because their immune response might be suppressed.
10 Millimeters or More
An induration of ten millimeters or more is considered positive for a moderate-risk group with an elevated chance of exposure or progression to active disease. This group includes:
- Recent immigrants from countries where tuberculosis is common.
- Intravenous drug users.
- Residents or employees of high-risk settings like hospitals, nursing homes, or prisons.
- Children under four years old.
- Those with certain medical conditions, such as diabetes or silicosis.
15 Millimeters or More
An induration measuring fifteen millimeters or more is considered a positive result for any person with no known risk factors for tuberculosis. This highest threshold is applied to the general population to ensure that smaller reactions are not misinterpreted as true infection.
The measurement process requires accuracy; the ruler must be held perpendicular to the arm, and only the firm, raised area should be measured. Measurement should not include any surrounding area of non-firm redness or swelling. The TST must be read by a trained healthcare professional to ensure the diagnosis is based strictly on the palpable diameter of the induration.
Next Steps Following a Positive TST
A positive TST result indicates the individual has been infected with Mycobacterium tuberculosis at some point, resulting in latent tuberculosis infection (LTBI). In this state, the bacteria are dormant, and the person does not exhibit symptoms or transmit the infection to others.
To determine if the infection is latent or active, further diagnostic testing is required. This typically involves a chest X-ray to look for signs of active disease in the lungs, followed by blood tests known as Interferon-Gamma Release Assays (IGRAs). IGRAs are often preferred because they are generally unaffected by prior Bacillus Calmette-Guérin (BCG) vaccination, which can sometimes cause a false positive TST result.
If the chest X-ray is clear, the diagnosis remains LTBI. Treatment is often recommended to prevent the infection from progressing to active disease. Treatment involves taking a specific antibiotic regimen for several months to eliminate the dormant bacteria.