The Tuberculin Skin Test (TST), often called the Mantoux test, screens individuals for infection with Mycobacterium tuberculosis, the bacterium that causes tuberculosis. This test involves injecting a small amount of purified protein derivative (PPD) into the skin of the forearm. Understanding the appearance of the injection site is necessary for accurate interpretation of the results, especially concerning a negative result after the required observation period.
Timing and Measurement of the Test
The accurate interpretation of the TST depends on reading the injection site within a narrow, specific time frame. Healthcare professionals must examine the site between 48 and 72 hours after the tuberculin is administered. If the reading is not performed within this 72-hour window, the test is considered invalid, and a new test must be scheduled.
The measurement focuses exclusively on the presence of induration, which is a firm, raised, and hardened swelling of the skin. This palpable, dense area results from a delayed-type hypersensitivity reaction, indicating an immune response to the PPD. Healthcare providers measure the diameter of this induration across the forearm in millimeters, perpendicular to the long axis of the arm. Redness, or erythema, that may surround the site is not measured, as it does not indicate the required immune reaction.
Visual Characteristics of a Negative Result
A truly negative TST result after 48 hours is characterized by the absence of measurable induration at the injection site. The skin in the area should feel soft and flat, essentially returning to the texture of the surrounding tissue. While there might be some residual discoloration or slight redness (erythema) remaining from the injection itself, a negative result means there is no hard, raised bump that can be felt or measured.
For a result to be recorded as negative, the healthcare professional must determine that the diameter of the induration is zero millimeters (0 mm). In some cases, a very small, non-palpable bump (less than five millimeters) may also be considered negative, depending on the person’s risk factors. The appearance of the site is often described as looking like a mosquito bite that has completely subsided, leaving behind only normal skin. The key characteristic is the lack of a defined, firm border that a professional can mark and measure with a ruler.
Interpreting the Meaning of a Negative Test
A negative TST result generally indicates that the person has not been infected with the M. tuberculosis bacteria. The immune system did not mount a delayed hypersensitivity response to the injected tuberculin protein, suggesting a lack of prior sensitization to the bacterium. This interpretation holds true for most healthy individuals who have not been recently exposed.
However, a negative result does not always guarantee the absence of infection, leading to the possibility of a “false negative” reading. One reason is the “window period,” the time between initial infection and the development of a measurable immune reaction. If the person was infected very recently (typically within the last eight to ten weeks), the body may not yet have developed the necessary immune response to trigger a positive result.
Another potential cause for a false negative is a weakened immune system, a condition known as anergy. Individuals who are immunosuppressed due to conditions like HIV, severe malnutrition, or those taking certain medications (such as high-dose corticosteroids) may fail to react to the test even if they are infected. When a false negative is suspected, the healthcare provider may recommend a repeat TST or a blood-based test for confirmation.
Understanding Positive and Indeterminate Results
In contrast to a negative result, a positive TST is defined by the presence of a significant, measurable induration at the injection site. The specific size threshold for a positive result is not universal and depends heavily on the individual’s risk factors for tuberculosis infection or progression to disease.
Positive Thresholds Based on Risk
An induration of five millimeters or more is considered positive for people with HIV, recent close contacts of infectious TB cases, and those who are immunosuppressed. For individuals with other risk factors, such as recent immigrants from high-prevalence countries, children under the age of five, or people who live or work in high-risk congregate settings, an induration of ten millimeters or more is considered positive. In people with no known risk factors, a larger induration of fifteen millimeters or more is required.
The positive result indicates the person has been exposed to the bacteria, but further testing is necessary to determine if the infection is latent or active. An indeterminate result may occur if the test was administered incorrectly (e.g., injected too deeply) or if the reading was not performed within the mandated 48-to-72-hour window.