Laboratory Tests to Rule Out Autoimmune Pancreatitis
Serum IgG4 level is the single most important laboratory test to rule out autoimmune pancreatitis, with a sensitivity of 90%, specificity of 98%, and accuracy of 95% in differentiating AIP from pancreatic cancer. 1
Essential Laboratory Tests
Primary Diagnostic Marker
- Serum IgG4 level is the most sensitive and specific laboratory indicator for AIP, and should be obtained in all patients with suspected autoimmune pancreatitis 2, 3, 1
- An elevated serum IgG4 level (typically >140 mg/dL, though cutoffs vary by laboratory) strongly supports the diagnosis of type 1 AIP 3, 4
- Normal IgG4 levels do not exclude AIP type 2, which is not IgG4-related 5
Supporting Laboratory Tests
- Serum IgG levels (total immunoglobulin G) may be elevated and support the diagnosis 3
- Liver function tests (bilirubin, AST, ALT, alkaline phosphatase) to assess for obstructive jaundice, which is the most common presenting symptom 2, 1
- Serum amylase and lipase to document pancreatic inflammation, though these may be normal or only mildly elevated in AIP 2
- CA 19-9 tumor marker may be elevated in AIP and must be measured to help differentiate from pancreatic cancer, though elevation is non-specific 2
Autoantibody Panel
- Antinuclear antibody (ANA) is positive in approximately 40% of AIP patients, though it is not disease-specific 1
- Other autoantibodies are rarely found and have limited diagnostic utility 1
- The presence of concomitant autoimmune diseases should be documented in the history, as AIP type 1 is often associated with other IgG4-related conditions 2, 5
Additional Metabolic Tests
- Serum calcium level to exclude hypercalcemia as an alternative cause of pancreatitis 2
- Fasting triglyceride level to exclude hypertriglyceridemia as an etiology 2
- Assessment of exocrine and endocrine pancreatic function, as approximately half of AIP patients demonstrate dysfunction 1
Critical Diagnostic Caveat
The diagnosis of AIP cannot be made on laboratory tests alone and requires integration with imaging findings (diffuse or focal pancreatic enlargement with ductal narrowing), histopathology (periductal lymphoplasmacytic inflammation, obliterative phlebitis, abundant IgG4-positive plasma cells), and response to steroid therapy. 3, 4, 5
Tissue Diagnosis When Needed
- IgG4 immunostaining of biopsy specimens from the major duodenal papilla shows significant infiltration (≥10 IgG4-positive plasma cells per high-power field) in AIP patients with pancreatic head involvement 6
- EUS-guided fine-needle aspiration with IgG4 immunostaining can provide tissue diagnosis when non-invasive criteria are insufficient 2
- Histologic confirmation is particularly important before initiating steroid therapy if there is any concern for malignancy 2, 3