Clinical use of amylase clearance and isoamylase measurements

Isoamylase determinations and measurements of the ratio of the renal clearance of amylase relative to creatinine (C(Am)/C(Cr) were employed in an attempt to improve the diagnostic accuracy of the standard amylase measurement. An elevated C(Am)/C(Cr) reflects defective proximal tubular reabsorption of amylase which occurs in virtually all patients with clear-cut acute pancreatitis. However, other conditions that apparently are associated with acute defective tubular function, such as burns and diabetic acidosis, may cause an elevated ratio. Thus, elevations of C(Am)/C(Cr) cannot be considered to be specific for acute pancreatitis. Pancreatic isoamylase represents, on the average, about 33% of the normal serum amylase activity, whereas about 66% is salivary-type isoamylase. Isoamylase measurements are useful in determining whether an elevated value for serum amylase activity is of pancreatic origin. However, this measurement is not useful for determining whether patients with normal serum amylase activity have pancreatitis.