Immunoglobulin G Test - IgG

The immunoglobulin G (IgG) test measures the level of IgG antibodies in the blood (and sometimes in other fluids such as saliva or cerebrospinal fluid). Immunoglobulins are proteins made by the immune system in response to infective agents and other foreign substances (antigens). When an infection occurs, the immune system produces specific antibodies against it, and remembers the infective agents so it can respond faster if the same infection returns.

There are five types of immunoglobulins: IgM, IgG, IgA, IgD, and IgE. IgG is the most abundant, making up roughly 70 to 80% of all the antibodies in the body. It is found in all body fluids and provides long-term protection against infection by remembering past invaders. IgG is also the only antibody that crosses the placenta from mother to baby during pregnancy, giving the newborn protection in the first months of life. It has four subclasses: IgG1, IgG2, IgG3, and IgG4 - which can be measured separately when needed.

Doctors order an IgG test to assess how well the immune system is working. A low level can mean weaker immunity and a greater tendency to infections, while a high level can reflect an overactive immune response, chronic infection, autoimmune disease, or certain cancers, such as multiple myeloma. The IgG test is also widely used to check for antibodies after recovering from an infection or after vaccination - for example, it was used during the COVID-19 pandemic to confirm an antibody response. Importantly, the IgG test is not used to diagnose an active, current infection.

Your doctor may recommend an IgG test when there are signs of a problem with immunity, recurrent infections, or to confirm antibodies after an infection or vaccination.

Symptoms or Situations That May Prompt the Test

• Frequent or recurrent infections, including of the lungs, sinuses, stomach, or intestines.
• Frequent fever, persistent (chronic) diarrhoea, or unexplained weight loss.
• Skin rashes, allergies, or sinus infections.
• A family history of immunodeficiency.
• To check antibody status after recovering from an infection or after vaccination.

Conditions for Which an IgG Test May Be Ordered

• Suspected immune deficiency or chronic illness.
• Multiple myeloma, leukaemia, or other conditions affecting antibody production.
• Autoimmune disorders such as rheumatoid arthritis.
• HIV/AIDS, pneumonia, or infections in newborns.

The IgG test is suitable for both males and females of all ages. It is often ordered together with IgA and IgM as part of a complete immunoglobulin profile.

Your doctor decides how often to repeat the IgG test based on your health status. When investigating an immune problem, it is usually done alongside other tests. Once treatment begins, the IgG test may be repeated at regular intervals to assess your antibody levels and immune response.

By measuring IgG, the test helps assess immune function and supports the diagnosis and monitoring of several conditions:

The IgG test needs no special preparation, which makes it simple and convenient.

What to Expect Before the Test

• No fasting is usually required for a standalone IgG test.
• If it is combined with another test that needs preparation, your doctor may advise fasting for a specific time.
• Tell your doctor about all medicines, recent vaccinations (within 6 months), and treatments such as radiation or chemotherapy, as these can affect the result.
• Always speak with your doctor before the test for guidance.

What to Expect During the Blood Collection

A phlebotomist cleans the skin with an antiseptic, then places an elastic band around your upper arm to make the veins easier to access. A small needle is inserted into the vein; you may feel a brief pinch. Blood is collected into a labelled tube in under a minute.

What to Expect After the Test

A small cotton swab and bandage are placed over the site to stop any minor bleeding. There may be slight soreness at the needle site or mild light-headedness. If you feel dizzy, sit and rest for a few minutes.

The IgG Test measures a single parameter: the level of immunoglobulin G in your blood, reported in g/L or mg/dL. The result reflects the strength of your antibody-based immunity.

• Immunoglobulin G (IgG): the most abundant antibody, providing long-term immune memory. A low value suggests weaker immunity and more risk of infection, while a high value suggests an overactive immune response or an underlying condition.

For a complete picture, IgG is often measured together with IgA and IgM. Separate testing can measure the four IgG subclasses (IgG1 to IgG4) when a specific subclass deficiency is suspected.

IgG levels change with age. The table below shows reference ranges in g/L, including the standard adult range. Note that the values for older children and teenagers fall between these - refer to your own report for the exact age band.

Your IgG result is interpreted with your age, symptoms, and other tests. The general patterns are:

A high or low IgG is a clue, not a diagnosis. Your doctor will combine it with the other immunoglobulins and specific tests to identify the underlying cause and plan management.

The IgG test is a routine blood draw that is generally very safe, with minor procedural risks and some limitations.

Potential Risks and Complications

Most people feel only a brief pinch. Consult your doctor if you notice:

• Persistent bleeding at the puncture site that does not stop with gentle pressure.
• Redness, swelling, or pain at the needle insertion site.
• Light-headedness that continues after the blood draw. Sit and rest before leaving.

Understanding Immunoglobulin G IgG Test Limitations

• The test shows the level of immunity, not a specific disease, and does not diagnose an active infection.
• Results can be affected by alcohol, certain medicines, recent vaccination, radiation, and chemotherapy.
• IgG is best interpreted alongside IgA and IgM as part of a full immunoglobulin profile.
• Reference ranges differ between labs, so results may not be directly comparable.

If your IgG result is abnormal, your doctor may recommend additional tests to find the cause.