Serum lipase increases in activity in association with pancreatitis. It catalyzes the hydrolysis of triglycerides preferentially at the 1 and 3 positions, releasing two fatty acids and a 2′-monoglyceride. The major source of lipase is the exocrine pancreas, with a smaller amount from the intestinal epithelial cells. Small amounts of the enzyme can be located in a few other cells but are not of diagnostic significance. Lipase in serum following pancreatitis has some properties which differ from those of the exocrine pancreatic lipase, but the reason is not clear.

Serum amylase and lipase are the most commonly used diagnostic markers for acute pancreatitis. Measurement of both enzymes may improve diagnostic accuracy, at least in part because the fall in lipase is slower, so that it remains raised for longer following an acute attack. Lipase activity increases in any condition where hyperamylasaemia is due to pancreatic pathology, but not when the amylase is of non-pancreatic origin.

In particular, serum lipase activity is normal in macroamylasaemia. While lipase measurements overcome some of the non-specificity of amylase estimation in acute pancreatitis, lipase may still be raised in non-pancreatic disease. During the first 24 hours, an elevated amylase level has a sensitivity of 81–84%. By the second day, the sensitivity of amylase drops to 33%. The specificity of hyperamylasemia is low because it also may be caused by perforated peptic ulcers, bowel infarctions, salivary gland trauma, renal failure, and macroamylasemia. Serum lipase measurement is more specific than amylase and has a sensitivity of 85–100%; after 4 days it remains elevated in 90% of patients. Recent reports suggest that lipase is the more accurate test in acute pancreatitis.