Serum lipase and amylase are the laboratory markers that support the diagnosis of acute pancreatitis, and both carry pitfalls. The diagnosis rests on a rise of at least 3-fold above the upper limit of normal in one of them, but each has reasons to mislead.
Serum amylase. Total serum amylase is made by the pancreas and the salivary glands; in normal conditions only about 45% of the total is pancreatic and the rest salivary, so a total amylase measurement is not specific for the pancreas. A pancreatic co-peptide, serum pancreatic or P isoamylase, is released in proportion to the pancreatic contribution and can be measured instead. About 25% of the serum amylase is cleared by the kidneys. In acute pancreatitis the level rises at least 2- to 3-fold above the ULN, a change with a sensitivity of 85% for the diagnosis.
The important limitation is that amylase is more a confirmatory parameter than a diagnostic one. It rises in mild acute pancreatitis, while in fatal conditions it is unchanged in most cases, and several conditions besides acute pancreatitis raise it: renal insufficiency, which can raise amylase 5- to 6-fold through reduced clearance; salivary gland disorders; fallopian tube disorders; and ovarian cysts. In mesenteric or bowel infarction, damage to the thickness of the bowel wall makes it more permeable to the luminal contents of the intestine, so serum amylase and lipase can rise in the ischaemic portion. Amylase can also attach to immunoglobulin to form macroamylase, which can lead to a falsely elevated laboratory result. In hypertriglyceridaemia-induced pancreatitis, the scatter of light used in the colorimetric method is diminished, so the measured amylase is under-estimated when triglycerides are very high. The amylase-to-creatinine clearance ratio, or ACCR, which is normally about 3%, can help in the differential diagnosis.
Serum lipase. Lipase has a similar sensitivity to amylase, but its specificity is superior, so it should replace serum amylase as the first line of laboratory diagnosis. The reason is that lipase is not made by the salivary glands and does not rise during gynaecological disorders. It too can increase in non-pancreatic conditions, mostly liver and renal failure. In some individuals the serum lipase is higher than in the normal population: in more than 20% of these people the average level is higher, and in about 2% of cases there is a 3-fold increase above the ULN. The diagnostic threshold for acute pancreatitis is considered to be around 3-fold above the ULN.
Other findings. Leukocytosis is frequent. Hyperglycaemia is frequent, due to the increase of glucagon. Cholestatic findings can increase, mainly in gallstone-induced disease: AST, ALT, bilirubin and serum alkaline phosphatase. Hypocalcaemia occurs with a low albumin level, which falls because albumin-rich exudate extravasates into the peritoneum.