Immunoglobulin G (IgG) Subclass 4 in Serum or Plasma

This blood test measures the level of immunoglobulin G4 (IgG4), a specific subtype of the IgG antibody, in your blood. Measuring IgG4 helps your doctor evaluate and monitor conditions involving the immune system, such as IgG4-related disease.

IgG4 is one of five main subclasses of Immunoglobulin G (IgG), the most common antibody in your blood. IgG4 antibodies play a role in immune defense but can become abnormally high in some diseases, including certain autoimmune conditions or lymphoproliferative disorders. This test specifically focuses on the IgG4 subclass, which may be elevated in conditions like IgG4-related disease (IgG4-RD), where inflammation occurs in multiple organs.

Your doctor may order this test if you have symptoms such as swelling of organs or tissues, unexplained weight loss, or abnormal imaging results. It is also used to help diagnose, monitor, or evaluate treatment for IgG4-related disease and other conditions that affect IgG4 levels.

Your healthcare provider may order this test if you have symptoms or a history of conditions linked to elevated IgG4 levels, such as swelling in organs (e.g., pancreas, liver, lungs), chronic sinusitis, or other autoimmune diseases. It may also be used to monitor treatment progress in patients already diagnosed with IgG4-related disease.

Before the test, tell your doctor about all medicines, vitamins, and supplements you take, because certain drugs can affect IgG4 levels. Fasting is usually not required, but follow any specific instructions your healthcare team gives you and mention any recent infections or illnesses.

No special preparation is usually needed. You may be asked to fast for 8–12 hours before the test if blood is drawn from a vein (venipuncture). If a blood sample is taken from your finger, no fasting is required. Always follow your healthcare provider’s instructions.

During the test, a healthcare professional will clean your skin and draw a small sample of blood from a vein in your arm using a thin needle. The procedure usually takes only a few minutes, and you may feel a quick pinch or slight discomfort. The sample is then sent to a laboratory for analysis.

A healthcare professional will draw blood from a vein in your arm using a small needle. The process is quick and typically painless. You may feel a brief pinch, but most people tolerate it well. The blood sample is then sent to a lab for analysis using an immunoassay method to measure IgG4 levels.