Tuberculosis (TB) is a serious global health concern caused by the bacterium Mycobacterium tuberculosis. While it primarily affects the lungs, it can also impact other organs like the kidney, spine, or brain. Testing for TB is a crucial public health measure used to identify infected individuals, even if they are not currently sick, allowing for preventive treatment and control of the disease’s spread. Screening tests detect the body’s immune response to the bacteria, signaling that the organism has entered the body.
Understanding the Two Main TB Screening Tests
Screening for TB infection involves two main diagnostic tools: the Tuberculin Skin Test (TST) or the Interferon-Gamma Release Assay (IGRA), a blood test. Both tests detect the immune system’s reaction to specific TB proteins, indicating the presence of the bacteria in the body.
The TST, or Mantoux test, involves injecting a small amount of purified protein derivative (PPD) beneath the skin, usually on the forearm. The patient must return 48 to 72 hours later for a reading. The IGRA requires a single blood draw sent to a specialized lab for analysis. The blood sample is exposed to TB-specific antigens to check if white blood cells release a specific immune molecule.
The TST requires two visits, while the IGRA is completed in one visit. The IGRA is often preferred for individuals who have received the Bacillus Calmette-Guérin (BCG) vaccine, as the vaccine can cause a false positive result on the TST. Neither screening test can differentiate between latent infection and active disease; they only confirm the bacteria’s presence.
Interpreting the Tuberculin Skin Test Result
A healthcare professional must examine the injection site 48 to 72 hours after the TST fluid is administered. They measure the diameter of the induration, which is the firm, raised swelling at the injection site, not any redness. The size of this induration, measured in millimeters (mm), is interpreted based on the patient’s individual risk factors for TB exposure.
The definition of a positive result is stratified into three categories based on induration size:
5 mm or More
This size is considered positive for individuals at the highest risk. This includes those with HIV, recent close contacts of a person with infectious TB, or people with compromised immune systems.
10 mm or More
This size indicates a positive result for people who have recently immigrated from high TB rate countries, intravenous drug users, or residents of high-risk settings like correctional facilities or nursing homes.
15 mm or More
For individuals with no known risk factors for TB, a positive TST requires an induration of 15 mm or greater.
The test must be read within the 72-hour window, as results become unreliable afterward.
Interpreting the Blood Test Result and What Positivity Means
The Interferon-Gamma Release Assay (IGRA) is a laboratory-based test that avoids the subjective measurement of the skin test. The lab reports the IGRA result as positive, negative, or indeterminate. A positive IGRA result suggests that the person’s immune cells have been sensitized by Mycobacterium tuberculosis and released the measured interferon-gamma molecule.
An indeterminate result means the test could not provide a clear answer, often due to technical issues or a weakened immune system. If this happens, a repeat IGRA or a TST may be recommended to clarify the infection status.
A positive result from either screening test signifies Latent TB Infection (LTBI). In LTBI, the bacteria are present but inactive; they are not causing symptoms and cannot be spread to others. This contrasts with Active TB Disease, where the bacteria multiply, cause illness, and are contagious. A positive test indicates the potential for the infection to progress to active disease if the immune system weakens.
Next Steps After a Positive Screening Result
A positive screening test requires immediate medical follow-up to determine if the person has Latent TB Infection (LTBI) or Active TB Disease. The first step is usually a chest X-ray to look for signs of active disease in the lungs, such as infiltrates or cavities. The provider will also perform a clinical evaluation for active TB symptoms, including a prolonged cough, unexplained weight loss, fever, or night sweats.
If the chest X-ray is normal and the person has no symptoms, LTBI is typically diagnosed. Treatment for LTBI is often recommended to prevent the infection from progressing to active disease.
If the chest X-ray shows abnormalities or if the patient has symptoms, further testing is needed to confirm active disease. Confirmatory tests involve collecting sputum samples for microscopic examination and culture to look for live bacteria. If Active TB Disease is confirmed, a multi-drug regimen is started immediately. If active disease is ruled out, preventive treatment for LTBI significantly lowers the risk of developing the full illness.