TB IgM, IgG, IgA

Function and Overview

The TB Serology test measures the levels of different antibodies (IgM, IgG, and IgA) produced by the immune system in response to Mycobacterium tuberculosis, the bacterium that causes tuberculosis. Unlike the T-SPOT or QuantiFERON tests which measure cell-mediated immunity, this test looks at the humoral (antibody) response.

Antibody Breakdown

• IgM: Typically appears early in an infection and suggests an acute or recent exposure.
• IgG: Represents a more mature immune response and can indicate past infection, chronic infection, or latent TB.
• IgA: Often associated with mucosal immunity and can be elevated in active pulmonary tuberculosis.

Why it is Ordered

This panel may be ordered as a supplementary diagnostic tool when healthcare providers are trying to differentiate between active and latent infection, or when other tests like sputum culture or molecular assays (GeneXpert) are inconclusive. It is also used in epidemiological studies to assess the prevalence of TB exposure in certain populations.

Associated Conditions

• Active Pulmonary Tuberculosis: Often shows elevations in IgA and IgG.
• Extrapulmonary TB: Infection in the kidneys, spine, or brain may show varying antibody profiles.
• Latent TB Infection (LTBI): May show elevated IgG but no clinical symptoms.
• Miliary TB: Disseminated infection throughout the body.

Limitations

It is important to note that the WHO does not recommend commercial serological tests for the routine diagnosis of pulmonary TB due to issues with sensitivity and specificity, particularly in HIV-positive individuals. Results must always be interpreted alongside clinical findings, chest X-rays, and culture results.