IgG blood test (immunoglobulin G)

Immunoglobulin G (IgG) is the most abundant antibody type in the blood — there is more of it than of any other immunoglobulin type. IgG antibodies are produced in the later phase of the immune response, after IgM, and they stay in the body for a long time — sometimes for life — which is why they are responsible for long-term immune memory after an infection or a vaccination. IgG is also the only immunoglobulin type that can cross the placenta, so in a newborn's first months protection comes from the IgG antibodies received from the mother. In blood, IgG is measured by quantitative nephelometry together with IgA and IgM — these three markers are called the immunoglobulin (protein) panel. The test is useful for assessing whether the body is able to produce antibodies properly, and whether there is chronic inflammation, liver disease, a blood cell cancer (multiple myeloma, for example) or a congenital or acquired immunodeficiency. An IgG result is always assessed together with the clinical picture and other tests, rather than in isolation.

• When frequent or severe infections keep recurring — where an immunodeficiency is suspected
• To assess whether the body responds properly to past infections or to vaccinations (immune memory)
• Where chronic inflammation, an autoimmune disease or liver disease is suspected
• Where a blood cell cancer, such as multiple myeloma, is suspected
• To monitor a known immunodeficiency, or the effect of treatment that acts on the immune system

IgG is considered raised under MedlinePlus guidance when it is above 16.0 g/l. The most common causes are chronic inflammation or infection, liver disease, rheumatoid arthritis, or an excessive increase in antibody production. Less often the cause is IgG-type multiple myeloma. The result should be discussed with a doctor together with other tests.

• Chronic inflammation or infection
• Liver disease
• Rheumatoid arthritis
• Hyperimmunisation (an increased amount of specific antibodies)
• IgG-type multiple myeloma (a bone marrow cancer)

Alongside a raised IgG, these are the things usually assessed:

• Whether there are signs of chronic inflammation or infection — assessed together with CRP and a full blood count
• Liver markers, if liver disease is suspected
• Protein fractions and immunofixation, if myeloma or another plasma cell disease is suspected
• When to repeat the test, so that the change over time is visible

IgG is considered low under MedlinePlus guidance when it is below 7.0 g/l. This may mean a rare congenital immunodeficiency (agammaglobulinaemia), leukaemia, treatment with certain chemotherapy medicines, or pre-eclampsia during pregnancy. A low IgG can explain recurring infections, and it is important to discuss it with a doctor.

• Agammaglobulinaemia — a very rare congenital condition in which immunoglobulin levels are extremely low
• Leukaemia
• Some chemotherapy medicines
• Pre-eclampsia (raised blood pressure during pregnancy)
• Multiple myeloma (of other types, where IgG production is suppressed)

Alongside a low IgG, the questions are different:

• Whether frequent or severe infections keep recurring — that can point to an immunodeficiency
• IgA and IgM levels, so the overall immunoglobulin picture is visible
• In pregnancy — assessment of blood pressure and of signs of pre-eclampsia
• When to repeat the test to assess the change over time