lack sensitivity and are not recommended. An important caveat is that imaging during the first 12 hours may be falsely reassuring as the inflammatory and necrotic process usually requires 48 hours to fully manifest. Differential diagnosis The differential diagnosis includes: Perforated peptic ulcer Biliary colic Acute cholecystitis Pneumonia Pleuritic pain Myocardial infarction Biochemical Elevated serum amylase and lipase levels, in combination with severe abdominal pain, often trigger the initial diagnosis of acute pancreatitis. However, they have no role in assessing disease severity. Serum lipase rises 4 to 8 hours from the onset of symptoms and normalizes within 7 to 14 days after treatment. Serum amylase may be normal (in 10% of cases) for cases of acute or chronic pancreatitis (depleted acinar cell mass) and hypertriglyceridemia. Reasons for false positive elevated serum amylase include salivary gland disease (elevated salivary amylase), bowel obstruction, infarction, cholecystitis, and a perforated ulcer. If the lipase level is about 2.5 to 3 times that of amylase, it is an indication of pancreatitis due to alcohol. Decreased serum calcium Glycosuria Regarding selection on these tests, two practice guidelines state: "It is usually not necessary to measure both serum amylase and lipase. Serum lipase may be preferable because it remains normal in some nonpancreatic conditions that increase serum amylase including macroamylasemia, parotitis, and some carcinomas. In general, serum lipase is thought to be more sensitive and specific than serum amylase in the diagnosis of acute pancreatitis" "Although amylase is widely available and provides acceptable accuracy of diagnosis, where lipase is available it is preferred for the diagnosis of acute pancreatitis (recommendation grade A)" Most, but not all individual studies support the superiority of the lipase. In one large study, there were no patients with pancreatitis who had an elevated amylase with a normal lipase. Another study found that the amylase could add diagnostic value to the lipase, but only if the results of the two tests were combined with a discriminant function equation. While often quoted lipase levels of 3 or more times the upper-limit of normal is diagnostic of pancreatitis, there are also other differential diagnosis to be considered relating to this rise. Computed tomography Regarding the need for computed tomography, practice guidelines state: CT is an important common initial assessment tool for acute pancreatitis. Imaging is indicated during the initial presentation if: the diagnosis of acute pancreatitis is uncertain there is abdominal distension and tenderness, fever