Which test is used to assess TB infection, such as latent TB infection?

• A. IGRA
• B. Sputum culture
• C. Chest X-Ray
• D. Sputum smear

Answer: (A)

The main idea is how we detect TB infection versus active disease. For latent TB infection, the test looks at the body's immune memory of TB exposure rather than finding bacteria in the lungs.

Interferon-Gamma Release Assays measure how strongly a person’s T cells release interferon-gamma in response to TB-specific antigens. Because these antigens are specific to Mycobacterium tuberculosis and not shared with most BCG vaccines, IGRA has good specificity in people who have been vaccinated. It’s done with a blood sample and can indicate TB infection even when there are no symptoms or chest findings. However, a positive IGRA doesn’t tell you whether the infection is latent or active; further workup is needed to evaluate for active disease (symptoms, imaging, and microbiology).

Other tests you might hear about—sputum culture and sputum smear—are designed to find bacteria in the lungs. They indicate active TB when bacteria are present in the sputum, not latent infection, and would often be negative in someone with latent TB. A chest X-ray can show changes consistent with TB but cannot by itself confirm TB infection or distinguish latent infection from active disease.