TB IgG

Function

The TB IgG test detects Immunoglobulin G antibodies produced by the immune system in response to Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). IgG is a 'long-term' antibody that typically appears weeks after infection and may persist for years. Unlike the skin test (Mantoux) or IGRA (QuantiFERON), which measure cellular immunity, this test looks for a humoral (antibody) response.

Why it is Ordered

While not the gold standard for diagnosing active TB (due to sensitivity and specificity issues), the TB IgG test is sometimes used as a supplementary tool in regions where other diagnostic methods are unavailable. It can help identify individuals who have been exposed to the bacteria in the past, aiding in the diagnosis of latent TB or assessing exposure in high-risk populations.

Associated Conditions

• Latent Tuberculosis Infection (LTBI): The patient carries the bacteria but is not symptomatic or contagious.
• Active Tuberculosis: In some cases of active disease, IgG levels will be significantly elevated.
• Extrapulmonary TB: IgG testing is occasionally explored when TB is suspected outside the lungs and sputum samples are difficult to obtain.
• Past Exposure: Reflects a prior infection that the immune system has successfully contained.