The Tuberculin Skin Test (TST), also known as the Mantoux test, is a screening method used to determine if a person has been infected with Mycobacterium tuberculosis, the bacterium responsible for tuberculosis (TB). This test involves injecting a small amount of purified protein derivative (PPD) into the inner surface of the forearm. The TST primarily helps detect latent TB infection, where the bacteria are present but not actively causing disease.
Identifying and Measuring the Reaction Site
After receiving a TB skin test, you must return to a healthcare provider between 48 and 72 hours for the test to be read. At the injection site, a healthcare professional will inspect and carefully feel the area to identify induration, which is a firm, raised, and hardened swelling. It is important to distinguish induration from mere redness or bruising, as redness alone is not considered a positive reaction.
To accurately measure the induration, the healthcare worker will lightly sweep their fingertips across the area to locate the distinct edges of the raised, hardened part. A millimeter ruler is then used to measure the diameter of this indurated area. The measurement is taken across the forearm, perpendicular to its long axis. The precise measurement in millimeters is essential for interpretation.
Interpreting Results Based on Risk Factors
The interpretation of a TST result depends not only on the size of the induration but also on an individual’s specific risk factors for TB exposure and the potential for the infection to progress to active disease. Different thresholds for induration size are considered positive based on these varying risk categories.
An induration of 5 millimeters or more is considered a positive result for certain high-risk individuals. This includes people living with HIV, recent close contacts of individuals with active TB disease, and those with organ transplants or significant immunosuppression. Additionally, individuals with chest X-ray findings consistent with prior TB infection fall into this category.
For other groups, an induration of 10 millimeters or more indicates a positive result. This threshold applies to recent arrivals within the last five years from countries where TB is common, and injection drug users. Residents and employees in high-risk communal settings, such as prisons, nursing homes, hospitals, and homeless shelters, also fit this criterion. Mycobacteriology laboratory personnel and children younger than five years of age are included, along with people who have certain medical conditions that increase their susceptibility to TB.
Finally, an induration of 15 millimeters or more is considered positive for individuals who have no known risk factors for TB. If a person without these specified risk factors has an induration smaller than 15 millimeters, their result is generally considered negative.
Understanding Your Next Steps
A positive TST result indicates exposure to TB bacteria and the presence of the infection, but it does not necessarily mean active TB disease. Further medical evaluation is essential to determine if the infection is latent or active. This typically involves a chest X-ray to check for signs of active disease in the lungs.
If the chest X-ray is abnormal or if symptoms are present, sputum tests may be conducted to look for active bacteria. Blood tests, such as Interferon Gamma Release Assays (IGRAs), can also be used to confirm TB infection. Following these evaluations, a healthcare provider will discuss treatment options for latent TB infection to prevent it from progressing to active disease.
Conversely, a negative TST result generally suggests that a person is not infected with TB bacteria. However, in specific situations, such as recent exposure to someone with active TB or in cases of immunosuppression, a follow-up test might be recommended. This could involve a second TST or an IGRA. Regardless of the initial result, consulting a healthcare professional is important for accurate diagnosis, interpretation of results, and personalized medical guidance.