High IgG: what it means

Immunoglobulins are proteins, also called antibodies, made by plasma cells in response to bacteria, viruses and other substances the body recognises as foreign. Lab Tests Online UK explains that IgG is the main class: about 70 to 80% of blood immunoglobulins. Specific IgG antibodies rise a few weeks after an infection or exposure, then fall and stabilise, leaving the body with a catalogue it can reproduce quickly if it meets the same threat again.

IgG is also the only immunoglobulin that crosses the placenta from mother to baby, and it has four subclasses (IgG1 to IgG4). A standard quantitative immunoglobulin test measures total IgG, IgA and IgM without distinguishing subclasses.

Because IgG is the dominant antibody, a high result usually means the immune system has been stimulated for some time. Lab Tests Online UK describes a polyclonal increase as the sum of immunoglobulins from many different plasma cells, and lists the associated conditions: acute and chronic infections, autoimmune disorders (rheumatoid arthritis, systemic lupus erythematosus, scleroderma), cirrhosis, chronic inflammation and inflammatory disorders, hyperimmunisation reactions and Wiskott-Aldrich syndrome. In a newborn, infection during pregnancy can also raise it.

The other pattern is a monoclonal increase, where the excess comes from the clone of one plasma cell. This is called a paraprotein. The conditions listed with it include multiple myeloma, chronic lymphocytic leukaemia, MGUS, lymphoma and, for IgM, Waldenstrom's macroglobulinaemia. In these disorders there is typically a marked rise in one immunoglobulin class and a decrease in the other two.

Doctors rarely look at IgG alone. The three classes are usually evaluated together, and an abnormal result typically indicates something affecting the immune system and may suggest the need for further testing.

The distinction below is the most useful first step in making sense of a high IgG.

If a single immunoglobulin type is raised, Lab Tests Online UK says further testing by immunofixation can show whether it comes from clones of an abnormal plasma cell. When myeloma is suspected, serum protein electrophoresis should always be done at the same time so that a paraprotein is not missed, because a paraprotein can occasionally be present but not picked up by quantitative testing. Serum free light chains and a urine sample may also be requested.

A paraprotein is not necessarily a cancer. Myeloma UK describes MGUS as a benign condition that does not usually cause symptoms and is usually diagnosed incidentally when tests are done for other problems. It says about 1 in 30 people over 50 have it, rising to about 1 in 10 people aged over 85, and that over a 10-year period about 1 in 10 people with MGUS will develop myeloma, which is why regular monitoring blood tests are used.

Symptoms that make clinicians look harder include anaemia, bone pain or fractures with minimal trauma, raised ESR and recurrent infections. Lab Tests Online UK lists these among the reasons for requesting immunoglobulin tests.

In the polyclonal pattern, the work-up follows the clinical picture. A person with joint pain and a rash might have autoimmune tests, a person with long-standing alcohol use or abnormal liver blood tests might have liver investigations, and a person with persistent infection symptoms might have microbiology tests. The immunoglobulin result itself does not pick the diagnosis.

IgG has four subclasses, and a standard test does not separate them. Lab Tests Online UK says separate subclass testing can be performed, but that it is now rarely done because subtle subclass abnormalities are rarely the cause of major clinical infection, so subclass levels are not a first-line step. A specialist condition involving one subclass, IgG4-related disease, has its own guide on this site.

A GP will usually review symptoms, medicines, infections and other blood results such as full blood count, inflammatory markers, liver function and kidney function. If a paraprotein is possible, the next step is electrophoresis and related tests, and a haematology referral may follow.

Lab Tests Online UK emphasises that a quantitative immunoglobulins test is not a routine or general screening test, and that most people never need one. If yours was requested, ask which of the possibilities it was meant to look for. Immunoglobulins mostly do not respond to lifestyle changes, so the aim is to find and treat the underlying cause, and not to change medicines without advice.