Tuberculosis (TB) is a global health challenge. Accurate and timely testing is crucial for identifying individuals infected with Mycobacterium tuberculosis, the bacteria that causes TB. This enables appropriate treatment and helps prevent further spread, benefiting both individual health and public health.
Blood Tests for Tuberculosis
Tuberculosis can be tested through a blood draw using Interferon-Gamma Release Assays (IGRAs). IGRAs detect TB infection by measuring the immune system’s response to specific TB-related proteins. When blood from an infected person is mixed with these proteins in a laboratory, certain white blood cells release interferon-gamma (IFN-γ).
The amount of IFN-γ released indicates exposure to TB bacteria. Common FDA-approved IGRA tests include QuantiFERON®-TB Gold Plus (QFT-Plus) and T-SPOT®.TB. IGRAs are not affected by prior BCG (Bacille Calmette-Guérin) vaccination, which can cause false positive results in traditional skin tests. These blood tests require only a single patient visit for sample collection.
Traditional Tuberculosis Skin Test
The traditional method for detecting TB infection is the Tuberculin Skin Test (TST), also known as the Mantoux test. This test involves injecting a small amount of purified protein derivative (PPD), derived from Mycobacterium tuberculosis, just beneath the skin on the inner forearm. If a person has been exposed to TB bacteria, their immune system will react to the PPD.
A healthcare provider examines the injection site 48 to 72 hours later to measure the size of any firm, raised bump (induration). This reaction indicates a delayed-type hypersensitivity response. The TST has been a standard screening tool since the 1930s and is widely used in TB control.
Selecting the Appropriate Test
Choosing between a blood test (IGRA) and a skin test (TST) for TB depends on several factors. IGRAs are preferred for individuals who have received the BCG vaccine, as it can lead to false-positive TST results. IGRAs offer higher specificity because they use antigens not found in the BCG vaccine.
IGRAs are also beneficial for people who might not return for a second visit, as the blood test requires only one appointment. For children younger than 5 years, the TST is often the recommended method. Both tests can have limitations for immunocompromised individuals due to a weakened immune response.
Understanding Test Outcomes
A positive result from either a blood or skin test indicates exposure to TB bacteria. It signifies the presence of TB bacteria in the body, but it does not mean the person has active tuberculosis disease. Most individuals with a positive test have latent TB infection, where the bacteria are present but inactive and not causing symptoms.
Further evaluation, such as a chest X-ray and sputum tests, is often required to determine if active TB disease is present. A negative test result means TB infection is unlikely. However, false negatives can occur in situations like very recent exposure, a severely weakened immune system, or in very young children. It is important to consult a healthcare professional for interpretation of test results and to discuss next steps.