Immunoglobulin G (IgG) Test: Levels, Reference Ranges & Meaning

What Is Immunoglobulin G (IgG)?

Last week, a 45-year-old teacher walked into my clinic frustrated by yet another sinus infection — her fourth in six months. She wondered if something deeper was going on. I ordered an immunoglobulin panel, and her IgG came back significantly low. Stories like this are why I reach for the IgG test more often than many realise.

Immunoglobulin G (IgG) is the most abundant antibody in your bloodstream, making up about 75–80% of all antibodies. Think of it as your immune system’s long-term memory — after you’ve been exposed to a virus, bacteria, or vaccine, your body churns out IgG to protect you for months or even years.

This test measures the total amount of IgG in your blood, which gives us a window into your humoral immunity — the part of your immune system that produces antibodies.

Why Is IgG Measured?

In my clinical practice, I mainly order IgG when I suspect:

• Recurrent infections — especially sinopulmonary infections (sinusitis, bronchitis, pneumonia)
• Autoimmune conditions — where IgG levels may be chronically elevated
• Chronic liver disease — cirrhosis can alter immunoglobulin production
• Multiple myeloma — a plasma cell cancer that often spikes IgG
• Monitoring — for patients on immunoglobulin replacement therapy

How Is the Test Done?

A simple blood draw from a vein in your arm. No fasting is required. The sample is analysed using nephelometry or turbidimetry — techniques that measure how much light is scattered by IgG-antibody complexes. Results typically come back in 1–2 days.

Interpreting Your IgG Levels

Low IgG Levels (Hypogammaglobulinemia)

When I see a low IgG, I immediately think of primary immunodeficiencies like common variable immunodeficiency (CVID) or secondary causes such as:

• Protein-losing conditions (nephrotic syndrome, severe burns)
• Chronic lymphocytic leukaemia
• Long-term steroid use
• Certain anti-epileptic drugs

A low IgG means your body isn’t making enough antibodies to fight infections effectively. My patients with low IgG often describe feeling “run down” or catching every cold that goes around.

High IgG Levels (Hypergammaglobulinemia)

High IgG can be a sign of:

• Chronic infections (e.g., HIV, hepatitis C)
• Autoimmune diseases (lupus, rheumatoid arthritis)
• Multiple myeloma (especially IgG myeloma)
• Sarcoidosis

Interestingly, I’ve also seen transiently high IgG in people who have recently had a vaccine or a bad flu — the immune system is just working overtime.

IgG During Pregnancy

IgG is the only antibody that crosses the placenta. In pregnancy, maternal IgG levels typically stay within the non-pregnant range, but they can drop slightly in the third trimester due to haemodilution. That’s normal and doesn’t usually cause problems.

IgG and Autoimmune Diseases

In autoimmune conditions, IgG can be both a marker and a culprit. For example, in Graves’ disease, a specific type of IgG (thyroid-stimulating immunoglobulin) drives hyperthyroidism. In lupus, elevated IgG is often accompanied by a positive ANA test.

Reference Ranges for IgG

Ranges can vary slightly between labs. My hospital uses the following, based on international guidelines (LOINC code 2465-3):

Note: Women may have slightly lower levels than men, but the difference is not clinically significant. Always interpret results alongside other immunoglobulin levels (IgA, IgM) and clinical symptoms.

What Can Affect IgG Levels?

• Age: Infants have low IgG at birth (maternal IgG wanes), then it slowly rises.
• Hydration: Dehydration can falsely elevate IgG; overhydration can dilute it.
• Medications: Phenytoin, carbamazepine, and gold salts can lower IgG.
• Recent vaccination or infection: May cause a temporary spike.

When Should You Worry?

One abnormal IgG result is rarely a diagnosis on its own. If you’re well and have no symptoms, a mildly low or high IgG often resolves spontaneously. However, if you have recurrent fevers, chronic diarrhoea, unexplained weight loss, or swollen lymph nodes, it’s worth discussing with your GP.

In my practice, I always repeat the test after 4–6 weeks to confirm a persistent abnormality before launching into a full immunological workup.

Key Takeaways

• IgG is your immune system’s memory antibody — it protects you from past infections.
• Low IgG increases infection risk; high IgG can signal chronic inflammation or myeloma.
• Reference ranges depend on age — children’s values are lower than adults’.
• Always interpret IgG in the context of your overall health, not as a single number.