What is the diagnostic approach for a suspected case of autoimmune pancreatitis in an adult patient presenting with abdominal pain, jaundice, and weight loss?

Diagnosis of Autoimmune Pancreatitis

Measure serum IgG4 levels as the first-line diagnostic test, with levels >280 mg/dL being the most sensitive and specific marker for type 1 AIP, followed by cross-sectional imaging (CT or MRI/MRCP) to evaluate pancreatic morphology and exclude malignancy. 1, 2

Initial Laboratory Evaluation

• Check serum IgG4 levels immediately as the primary diagnostic marker, with >280 mg/dL being highly sensitive and specific for type 1 AIP 1, 2
• Measure lipase and amylase, though these may be normal or only mildly elevated (unlike acute pancreatitis where they are markedly elevated) 1, 2
• Obtain liver function tests including bilirubin, AST, ALT, and alkaline phosphatase to assess for biliary obstruction, which commonly presents with obstructive jaundice in AIP 1, 2, 3
• Measure total serum IgG levels (elevated but less specific than IgG4) 1

Cross-Sectional Imaging

• Obtain CT or MRI with MRCP to evaluate pancreatic morphology and biliary involvement 1, 2
• Look for the classic "sausage-shaped" diffuse pancreatic enlargement with a capsule-like peripheral rim, occurring in 29-76% of cases 1, 4
• Critical caveat: Focal enlargement can mimic pancreatic malignancy, making definitive exclusion of cancer mandatory before treatment 1, 2, 5
• MRCP specifically evaluates for bile duct strictures, pancreatic duct narrowing (long-segment or multifocal), and bile duct wall thickening with visible lumen (characteristic of IgG4-related cholangitis) 2, 4

Endoscopic Evaluation When Imaging is Equivocal

• Perform EUS with brush cytology and/or endoscopic biopsy during ERCP to definitively exclude malignancy when imaging findings are ambiguous 1, 3
• EUS may reveal diffusely hypoechoic and rounded pancreatic parenchyma with distal CBD stricture 3
• Administer perioperative antibiotics to prevent cholangitis when instrumenting an obstructed duct 1
• MRCP cannot always differentiate irregular narrowing in AIP from stenosis in pancreatic carcinoma based on ductal findings alone, necessitating correlation with serum IgG4 levels and clinical response to steroids 4

Histological Confirmation

• Obtain histological confirmation when possible, showing dense lymphoplasmacytic infiltrate with fibrosis 2, 6
• The diagnosis must exclude secondary causes of sclerosing cholangitis before confirming AIP 4

Diagnostic Algorithm Summary

The diagnostic approach follows this sequence:

1. Serum IgG4 >280 mg/dL + characteristic imaging (sausage-shaped pancreas or bile duct wall thickening with visible lumen) = presumptive diagnosis 1, 2, 4
2. If imaging shows focal mass or equivocal findings: Proceed to EUS with tissue sampling to exclude malignancy 1, 3
3. If diagnosis remains uncertain: Consider diagnostic steroid trial (prednisolone 40 mg daily for 2-4 weeks) with clinical and radiological response confirming AIP 1, 3, 7

Critical Pitfalls to Avoid

• Never initiate treatment without definitively excluding pancreatic cancer, as AIP can present with identical features including jaundice, weight loss, elevated tumor markers, and pancreatic masses 1
• Be aware that AIP can start as a localized "mass-forming" pancreatitis before progressing to the diffuse form, making early diagnosis challenging 5
• Recognize that type 2 AIP (idiopathic duct-centric chronic pancreatitis) has no IgG4 elevation and is associated with inflammatory bowel disease in ~25% of cases, requiring different diagnostic criteria 6