The Tuberculin Skin Test (TST), also known as the Mantoux or PPD (Purified Protein Derivative) test, determines if a person has been infected with Mycobacterium tuberculosis, the bacterium that causes tuberculosis. This two-step procedure involves injecting a small amount of tuberculin protein into the skin and checking for a reaction several days later. The TST detects TB infection, which often exists in a latent, non-contagious state, rather than diagnosing active, symptomatic disease. Reading the TST accurately requires careful physical examination and interpretation based on the patient’s medical history.
The Physical Technique for Reading the Skin Test
The test must be read within the designated time frame, precisely 48 to 72 hours following the initial injection. If the patient returns outside this window, the result may be unreliable, and the test may need to be repeated. The reading must be performed under good light, with the forearm slightly flexed at the elbow, allowing for optimal inspection of the injection site.
The health care provider must look for a localized reaction at the injection site, distinguishing between two types of swelling. The reaction measured is induration, a firm, raised, and hardened area on the skin caused by an immune response. Induration is the only factor that determines the test result.
The second type of swelling, erythema, is simple redness or bruising around the site, and it is not measured for the test result. To identify the exact boundaries of the induration, the reader gently sweeps a fingertip across the forearm, moving from the normal skin toward the injection site. This palpation technique helps locate the margins of the raised tissue.
Once the edges of the palpable induration are located, a fine-tipped pen or marker can be used to lightly mark the widest points of the raised area across the forearm. The measurement is then taken using a flexible ruler in millimeters (mm) and must be transverse, meaning perpendicular to the long axis of the forearm. If the measurement falls between two lines on the ruler, the smaller measurement should be recorded. A measurement of zero millimeters is recorded if no induration is felt.
Interpreting Measurements Based on Patient Risk
The size of the measured induration alone does not determine a positive result; interpretation depends entirely on the patient’s medical history and specific risk factors. A positive TST is defined by three different threshold measurements. These thresholds reflect the likelihood of true infection and the risk of progression to active disease, ensuring vulnerable patients receive appropriate follow-up.
An induration of five millimeters or more (≥ 5 mm) is considered a positive result for individuals in the highest risk category. This includes people living with HIV, recent close contacts of a person with infectious tuberculosis disease, and organ transplant recipients. This low threshold also applies to highly immunocompromised individuals, such as patients receiving prolonged immunosuppressive therapy.
The next threshold, ten millimeters or more (≥ 10 mm), is considered positive for a broader, moderate-risk group. This group includes recent immigrants (arrived within the last five years) from countries where TB is common. It also encompasses people who inject drugs, residents and employees of high-risk congregate settings (like prisons or nursing homes), and children younger than five years old.
Finally, an induration of fifteen millimeters or more (≥ 15 mm) is considered positive for all persons with no known risk factors for TB exposure or infection. This is the highest threshold because, without underlying conditions or exposure history, a smaller reaction is less likely to represent a true infection. A measurement below the defined threshold for any risk category is considered a negative result.
Confirmatory Testing and Follow-Up Actions
A positive TST result only indicates that the person has been exposed to the M. tuberculosis bacterium and developed an immune response, but it does not differentiate between latent infection and active disease. Therefore, a medical evaluation is required following any positive skin test. This evaluation begins with a thorough medical history and physical examination to check for symptoms of active disease, such as a persistent cough, unexplained weight loss, or fever.
The primary confirmatory test is a chest radiograph, used to look for characteristic abnormalities in the lungs that suggest active tuberculosis disease. If the chest X-ray is clear and the person has no symptoms, the diagnosis is typically Latent TB Infection (LTBI). In LTBI, the bacteria are present but dormant and non-contagious. If the radiograph shows signs of disease, further specialized testing is needed to confirm active TB.
Another follow-up tool is a blood test known as an Interferon Gamma Release Assay (IGRA), such as the QuantiFERON-TB Gold test. Unlike the TST, the IGRA is not affected by prior BCG vaccination. It measures the release of interferon-gamma from immune cells when exposed to specific TB proteins. A positive TST, especially one confirmed by an IGRA, necessitates consultation with a healthcare provider to discuss treatment to prevent the latent infection from progressing to active disease.