Immunoglobulin G (IgG) is a type of antibody that is found in blood and other body fluids. It is the most common antibody and protects against bacterial and viral infections. An IgG blood test is one of the tests that measures the levels of immunoglobulins in the blood. The test measures the amounts of IgM, IgG, and IgA in the blood to help diagnose different types of health conditions that may affect the immune system. The immune system makes antibodies to protect the body from bacteria, viruses, and allergens. IgG can take time to form after an infection or immunization.

Doctors often measure IgA, IgG, and IgM together to get a snapshot of the immune function of an individual. The results of the test can mean different things, depending on which immunoglobulins are high or low, the symptoms, and any conditions an individual may have. A low level of IgG is usually an indication that an individual may be immunocompromised. If the levels of immunoglobulins are consistently low, replacement therapy may be needed.

A blood test that measures immunoglobulin levels can diagnose IgG deficiency. It is possible to have a normal level of total IgG, so the testing of the IgG subclasses is important. Tests can also be done on saliva and cerebrospinal fluid, but a blood test is the most common. Treatment for IgG deficiency depends on how bad the symptoms and infections are.