The IgG4 test measures the IgG4 subclass of immunoglobulin G in blood. It helps identify and monitor IgG4‑related disease — a systemic fibroinflammatory condition — and supports diagnosis alongside clinical assessment, imaging, and tissue biopsy. Elevated IgG4 can also occur with allergies, parasitic infections, or other immune disorders; results must be interpreted with symptoms and specialist input.

Low IgG4 levels can result from primary immune disorders (selective IgG subclass deficiency, common variable immunodeficiency), B‑cell depletion from drugs (rituximab, other immunosuppressants), protein loss (nephrotic syndrome, protein‑losing enteropathy), hematologic malignancies, chronic infections, malnutrition, or recent infections/vaccination. Levels can be age‑related in children; clinical significance depends on recurrent infections and the full immunoglobulin profile, so clinician evaluation is advised.

Red flags for IgG4‑related disease: painless, progressive swelling or mass (pancreas, salivary/lacrimal glands, orbit), unexplained obstructive jaundice, retroperitoneal fibrosis or hydronephrosis, multi‑organ involvement, elevated serum IgG4 (often >135 mg/dL) but not always, and histology showing dense lymphoplasmacytic infiltrate rich in IgG4+ plasma cells with storiform fibrosis and obliterative phlebitis. Referral for specialist assessment and biopsy is recommended.

Lowering elevated IgG4 is achieved by treating IgG4‑related disease under specialist care. First‑line therapy is systemic corticosteroids to induce remission; steroid‑sparing immunosuppressants (azathioprine, mycophenolate, methotrexate) help maintenance. Rituximab is effective for relapsing or refractory cases. Regular monitoring of symptoms, organ function, and IgG4 levels guides therapy duration. Lifestyle measures alone do not reliably reduce IgG4.

An isolated high IgG4 can suggest IgG4‑related disease — a systemic fibroinflammatory condition affecting pancreas, salivary/lacrimal glands, bile ducts or retroperitoneum — but may also occur with allergies, chronic infections, other autoimmune disorders or malignancy. Elevated IgG4 alone isn’t diagnostic: clinicians correlate symptoms, imaging and often biopsy, and may order further blood tests. See a specialist; treatment often includes corticosteroids.

Early symptoms of IgG4-related disease are often vague and organ-specific. Common early signs include painless swelling or lumps (salivary or lacrimal glands), persistent eye swelling or dry eyes, unexplained enlarged lymph nodes, abdominal pain or jaundice from pancreatic involvement, sinus or nasal congestion, fatigue, and mild weight loss. Symptoms worsen as affected organs become inflamed or fibrotic.