IgG test

The IgG test measures immunoglobulin G, the most abundant antibody in your blood and tissues. IgG helps your immune system recognize and remember germs and responds to vaccines. Depending on what your clinician orders, the laboratory may measure total IgG or IgG directed at a specific virus, bacterium, or other target.

Most testing is done on a blood sample from a vein. In certain situations, other body fluids may be tested, or urine may be checked to assess whether IgG is being lost. Your report will list the specific method and unit used, since different assays can use different scales.

Clinicians use this test to assess how well you make and maintain antibodies, to look for evidence of past infection or vaccination, and to monitor immune or inflammatory conditions. Low IgG can be seen with inherited or acquired immune deficiency, protein loss, or medicines that reduce antibody production. High IgG can occur with chronic infection, autoimmune disease, liver disease, or plasma cell disorders.

For infection-specific testing, IgG usually reflects prior exposure or vaccine response rather than a new infection. It is often paired with other tests, such as IgM, nucleic acid tests, cultures, or functional antibody assays, depending on the clinical question.

Your result should be interpreted using the reference interval on your report, together with your symptoms and medical history. Different laboratories may use different methods and units, so results from two places may not be directly comparable. If you have received antibody-containing products, such as intravenous immunoglobulin, those antibodies can appear in your blood and influence the result.

If total IgG is low, your clinician may repeat the test, check IgG subclasses, review vaccine antibody responses, or evaluate for protein loss. If total IgG is higher than expected, they may look for inflammation, chronic infection, or a monoclonal protein. For infection-specific IgG, a positive result usually indicates immune memory; if testing occurred soon after exposure, repeat testing or a different assay may be recommended to clarify timing.