Tuberculosis (TB) is a bacterial infection caused by Mycobacterium tuberculosis, which primarily affects the lungs but can attack any part of the body. Screening for this infection is used to identify the presence of the bacteria, even in individuals who show no outward signs of illness. The goal is to detect the infection early so that treatment can be started to prevent progression to active disease. Understanding the results of a TB test involves recognizing what a positive finding looks like, which differs based on the specific testing method used.
The Two Main Types of TB Screening Tests
Screening for TB infection relies on two distinct testing approaches that detect the body’s immune response to the bacteria. The first method is the Tuberculin Skin Test (TST), often known as the PPD test, administered directly into the skin. This test involves a healthcare professional injecting a small amount of purified protein derivative (PPD) solution. The individual must then return to the clinic for a follow-up visit to have the reaction measured.
The second primary method is a blood test called the Interferon Gamma Release Assay (IGRA), which is commercially available under names like QuantiFERON-TB Gold or T-SPOT.TB. Unlike the skin test, the IGRA requires only a single visit for a blood draw. The collected blood sample is then sent to a laboratory for analysis, which eliminates the need for a second appointment to read a physical reaction.
Visual Interpretation of a Positive Skin Test
For the Tuberculin Skin Test (TST), a positive reaction is defined by the presence of induration, which is a raised, firm, and hardened area of the skin that develops within 48 to 72 hours after the injection. Redness or bruising at the injection site is not considered positive. Healthcare providers must measure the diameter of this induration in millimeters, as redness around the site is not considered in the interpretation. The presence of induration confirms that the immune system has previously recognized the TB bacteria.
The size of the induration required for a positive result operates on a sliding scale based on the individual’s specific risk factors.
Induration Thresholds
- 15 millimeters or larger: For people with no known risk factors for the infection.
- 10 millimeters or greater: For individuals with certain risk factors, such as recent arrivals from high-prevalence countries, injection drug users, and residents of high-risk settings like nursing homes.
- 5 millimeters or greater: The lowest threshold, applying to the highest-risk groups, including people living with HIV, recent close contacts of a person with active TB disease, and individuals with a compromised immune system (e.g., those on immunosuppressive therapy or who have had an organ transplant).
The provider determines the result by taking the precise induration measurement and applying the criteria relevant to the person’s health history.
Interpreting a Positive Blood Test Result
The Interferon Gamma Release Assay (IGRA) provides a positive result delivered through a laboratory report. This blood test operates by mixing a person’s white blood cells with specific TB-related proteins. If the person has been infected with the TB bacteria, their white blood cells will recognize these proteins and release a specific substance called interferon-gamma.
The lab report quantifies this immune response, and the result is typically communicated as a “positive” or “negative” finding. A positive IGRA result indicates that the body’s immune cells have reacted to the TB proteins, suggesting the presence of the infection. Unlike the TST, the IGRA is not affected by prior vaccination with the Bacille Calmette-Guérin (BCG) vaccine, which can sometimes cause a false-positive result on the skin test. A positive blood test confirms an infection has occurred, prompting the need for further clinical evaluation.
Moving from Infection to Disease: What a Positive Test Confirms
A positive result from either the TST or the IGRA usually confirms the presence of Latent TB Infection (LTBI). This means the Mycobacterium tuberculosis bacteria are present in the body, but they are inactive and are not causing symptoms or illness. Crucially, a person with LTBI cannot spread the bacteria to others.
Since a positive test cannot distinguish between latent infection and active, contagious disease, follow-up steps are necessary to rule out active TB. The next step involves a medical examination, a review of symptoms, and typically a chest X-ray. If the chest X-ray is normal and the person is asymptomatic, the diagnosis is confirmed as LTBI.
The goal of identifying LTBI is to prevent its progression to active TB disease, which is symptomatic and infectious. Without treatment, about 5 to 10 percent of people with latent infection will develop active disease at some point in their lives. Therefore, a positive test result provides a window to begin a course of medication to eliminate the dormant bacteria.