The Tuberculin Skin Test (TST), often known as the Mantoux or PPD test, screens for infection with the Mycobacterium tuberculosis bacteria (TB). This test does not diagnose active TB disease but rather identifies individuals who have been exposed to the bacteria and developed an immune response, a condition known as latent TB infection. The procedure involves injecting a small, standardized amount of tuberculin protein derivative just under the skin of the forearm. If a person has been previously infected, their immune system’s T-cells will recognize this protein and trigger a localized, delayed-type hypersensitivity reaction. The purpose of the TST is to identify these infected individuals so they can be offered preventive treatment, stopping the latent infection from developing into a contagious and active disease later on.
The Mandatory Reading Window
The Tuberculin Skin Test must be read precisely between 48 hours and 72 hours after the injection. This window is mandatory because it aligns with the peak of the cell-mediated immune response. The reaction, a localized inflammation, takes time to develop and become fully measurable within this 48- to 72-hour period.
If the reading is performed too early, such as before 48 hours, the immune response may not be fully developed, leading to a falsely small or negative measurement. If the patient does not return within the 72-hour limit, the test is invalid because the reaction begins to wane, making the measurement unreliable. The test must be repeated to ensure a valid result.
Measuring and Identifying the Reaction
Reading the TST requires a trained professional to measure a specific localized reaction. The healthcare worker looks for induration, a firm, palpable, raised, and hardened area at the injection site. Induration is the true sign of a positive immune reaction, resulting from the accumulation of immune cells.
It is important to distinguish induration from simple redness, or erythema, which is not measured and has no significance in interpreting the result. Erythema is common but does not confirm a TB infection. To measure the induration, the reader must gently palpate the area to feel the edges of the hardened swelling, even if it is not immediately visible.
A flexible ruler marked in millimeters is used to measure the diameter of the induration across the forearm. Specifically, the measurement must be taken perpendicular to the long axis of the forearm, which is the direction across the arm rather than lengthwise. The exact measurement in millimeters is then recorded; if no induration is present, the result is recorded as zero millimeters. The final interpretation is based solely on this precise millimeter measurement of the raised, firm area.
Interpreting the Millimeter Measurements
The measured size of the induration determines if the test is positive, but the threshold is not a fixed number. The positive result depends entirely on the patient’s individual risk factors for TB exposure and developing active disease. This risk-stratified approach ensures the test remains a useful screening tool.
5 mm or More (High Risk)
Five millimeters (5 mm) or more is positive for individuals in the highest-risk categories. This group includes people with HIV, recent close contacts of active TB patients, or those with organ transplants and significant immunosuppression. Even a small reaction suggests a true infection requiring immediate attention due to their compromised ability to fight the bacteria.
10 mm or More (Moderate Risk)
Ten millimeters (10 mm) or more is the positive cutoff for individuals with moderate risk factors. This category includes healthcare workers, recent immigrants from high TB prevalence countries, injection drug users, and residents or employees of high-risk settings (e.g., nursing homes or correctional facilities). Children under the age of four are also included in this threshold.
15 mm or More (Low Risk)
For people with no known risk factors, a positive result is fifteen millimeters (15 mm) or more. A positive TST result indicates only a latent infection (dormant bacteria). A positive reading necessitates further medical evaluation, such as a chest X-ray or a blood test, to rule out active, contagious TB disease.