Tuberculosis (TB) screening is a public health measure to identify individuals who have been infected with the bacterium Mycobacterium tuberculosis. The two primary methods for this screening are the Tuberculin Skin Test (TST) and the Interferon Gamma Release Assay (IGRA), a laboratory blood test. The TST, also known as the Mantoux test, requires a careful visual assessment of the injection site. Understanding what a positive TST looks like is essential, as the visible reaction is the sole indicator of a potential infection. This article will focus on the visual characteristics of the TST result.
The Tuberculin Skin Test (TST) Procedure and Initial Appearance
The TST procedure involves injecting a small, precise amount of purified protein derivative (PPD) just under the top layer of skin, a technique called an intradermal injection. This injection is typically administered on the inner surface of the forearm. The immediate outcome of a correctly performed injection is the formation of a small, pale, raised blister or bump known as a wheal. This initial wheal confirms the PPD solution has been successfully placed in the superficial layer of the skin. It is a normal, non-diagnostic result that will disappear shortly after the procedure. The true diagnostic reaction must be read by a trained professional between 48 and 72 hours following the injection.
Identifying a Negative TST Result
A negative TST result, read between 48 and 72 hours after administration, indicates that the immune system has not reacted significantly to the PPD. Visually, a negative site will appear either completely normal, or it may show only a minor, non-raised mark or slight discoloration. The key characteristic of a negative result is the absence of a firm, raised swelling, which is called induration. An individual with a negative result has a less than five-millimeter induration at the site. While minor redness (erythema) or a faint bruise may be present, these visual changes are not measured for the test result.
Interpreting a Positive TST Result
The visual characteristic that defines a positive TST result is the development of induration, a distinct, firm, and palpable area of swelling at the injection site. This induration is the result of a cellular immune response where specific T-cells and macrophages have migrated to the injection site. Simple redness or erythema around the site is not measured; only the diameter of the hard, raised area is considered. The healthcare professional uses a ruler to measure the induration across the widest part of the forearm and records the measurement in millimeters. This measurement is then interpreted in the context of the patient’s individual risk factors, as there is no single universal threshold for a positive result.
Positive TST Thresholds
A five-millimeter induration is considered positive for individuals with compromised immune systems, such as those with HIV, or for those who have had recent, close contact with a person who has active TB disease.
A ten-millimeter induration is the positive threshold for moderate-risk groups, including recent immigrants from countries with high TB prevalence, injection drug users, or residents and employees of high-risk congregate settings.
A fifteen-millimeter induration is considered positive for individuals who have no known risk factors for TB exposure.
Understanding Non-Reactive Skin Changes
Many people confuse common skin reactions following the TST with a positive result, but these changes are not considered when interpreting the test. For instance, mild bruising or slight redness (erythema) without any underlying firmness is a common, non-reactive change. Other non-reactive changes can include minor itching or localized swelling that is soft to the touch, often due to general skin irritation or a mild inflammatory response to the injection itself. These visual artifacts do not constitute a positive reading because they lack the characteristic hard, measurable induration.
When a Blood Test is Used Instead (IGRA)
The Interferon Gamma Release Assay (IGRA) is an alternative screening method that has no visual component at all, as it is a laboratory blood test. The IGRA works by measuring the release of interferon-gamma (IFN-γ), a specific immune signaling molecule, when a person’s blood is mixed with proteins unique to M. tuberculosis. This method provides a definitive answer to the screening question without relying on a visible skin reaction. An IGRA may be preferred over a TST in several situations, such as for individuals who have previously received the Bacillus Calmette-Guérin (BCG) vaccine, which can cause a false-positive TST result. It is also useful when a person may have difficulty returning for the required TST reading 48 to 72 hours later, or for those with weakened immune systems where the TST may be less reliable. While the IGRA result is a number reported by a lab, it serves the same function as the TST: determining if a person has been infected with TB bacteria.