What Do Low IgG Levels Mean, and What Happens Next?

Low IgG levels mean your blood has less immunoglobulin G, the most plentiful type of antibody, than the range your laboratory uses. The medical term is hypogammaglobulinaemia (hypogammaglobulinemia in US spelling). It can be caused by something acquired, such as certain medicines or protein loss, or less often by a primary immunodeficiency that affects how the body makes antibodies.

A single low result does not tell you which of these applies, or whether it matters for your health. This guide explains the main groups of causes and the usual follow-up questions. It cannot diagnose a cause or decide whether you need treatment.

Low antibody levels can make some infections harder to fight. Do not wait for a routine appointment if an infection suddenly gets worse.

Call your local emergency number or go to an emergency department if you or someone you care for has signs that the NHS lists for possible sepsis in adults:

• Very fast breathing.
• New confusion, slurred speech or not making sense.
• Blue, pale, grey or blotchy skin, lips or tongue (on darker skin, check the palms and soles).
• A very high or very low temperature, shivering, or feeling very hot or cold.
• A rash that does not fade when pressed.
• Any symptoms you are worried might be sepsis.

MedlinePlus gives a typical adult IgG range of 700 to 1,600 mg/dL (7.0 to 16.0 g/L). Ranges differ between laboratories, and some use different units, so always compare your result with the range printed on your own report.

IgG is often measured together with IgA and IgM. Whether one type or several are low helps clinicians decide which causes are more likely. Ask for all three results, with dates, if you only have the IgG value.

How far below the range your result is, whether it has been repeated, and whether you have had unusual or repeated infections all affect what happens next. Ask your clinician whether a borderline result needs repeating, and what would lead to further tests.

Yes. A 2022 work group report from the American Academy of Allergy, Asthma and Immunology (AAAAI) describes secondary hypogammaglobulinaemia as low immunoglobulin levels caused by a medicine or another condition, rather than an inherited immune problem.

Medicine classes linked to low IgG in that report include steroid medicines used at high doses or for long periods, B-cell targeted therapies such as rituximab, some other immune-suppressing and chemotherapy medicines, and some anti-seizure medicines. The report notes that steroid-related drops tend to affect IgG more than IgA or IgM, and that vaccine responses are often preserved in that setting.

Do not stop or change a prescribed medicine because of an IgG result. Ask the prescribing team whether the medicine could be involved and whether levels were measured before treatment started. The AAAAI report notes experts recommend baseline checks before some treatments, which makes later results easier to interpret.

Antibodies are proteins, so conditions that cause the body to lose protein can lower IgG. MedlinePlus lists kidney disease, serious burns, malabsorption disorders and malnutrition among causes of low immunoglobulins. The AAAAI report describes nephrotic syndrome, where protein leaks into the urine, and protein-losing enteropathy, where protein is lost through the gut.

Some blood cancers can also affect antibody levels. MedlinePlus notes that conditions such as multiple myeloma and chronic lymphocytic leukaemia can make one antibody type very high while others are low. This is one reason clinicians look at the whole set of results, not IgG in isolation.

Sometimes. Common variable immune deficiency (CVID) is a primary immunodeficiency in which IgG and IgA, and sometimes IgM, are reduced. MedlinePlus Genetics estimates it affects about 1 in 25,000 to 50,000 people worldwide. Most people are diagnosed in their twenties or thirties, although signs can begin in childhood.

People with CVID often have repeated infections of the lungs, sinuses and ears, and some have gut infection or inflammation causing diarrhoea and weight loss. Because medicines and protein loss can produce a similar picture, the AAAAI report stresses that working out whether low antibodies are secondary or primary matters, since it changes both the tests and the treatment plan.

A level shows how much antibody is present. A vaccine response test checks whether your immune system can make new, specific antibodies. MedlinePlus lists antibody levels after vaccination among the tests used to assess antibody deficiency, usually alongside measurements of the different antibody types.

People with CVID usually cannot make an adequate response, which is why some vaccines do not protect them. The AAAAI report notes that interpreting vaccine responses is complicated and best done by clinical immunologists. Ask which vaccines are involved and what timing the specialist wants before arranging anything yourself.

Bring your full antibody results, previous results if you have them, a list of all medicines and recent treatments, and a short record of infections in the past year, including antibiotics and hospital visits.

• How low is my IgG compared with this laboratory's range, and were IgA and IgM low too?
• Could any of my current or past medicines explain this, and were my levels checked before I started?
• Do I need tests for protein loss through the kidneys or gut?
• Do I need a vaccine response test or a referral to an immunologist?
• Which infection symptoms should make me seek care sooner than usual?
• When will the test be repeated, and who will tell me the result?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with low IgG were followed up, such as which tests were repeated or when an immunologist became involved, can help you write clearer questions for your own appointment.

Other people's cases cannot tell you your cause or whether you need treatment. We suggest bringing the questions they prompt to your own clinician, who can interpret your results in context.