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Amylases are enzymes that hydrolyze complex carbohydrates. They are produced by a number of organs and tissues, predominantly the exocrine pancreas (P-type amylase) and salivary glands (S-type amylase). Plasma amylases are of relatively low molecular weight for an enzyme (55,000-60,000 daltons) and enter the urine through glomerular filtration. Conditions that cause increased entry of amylase into plasma (eg, acute pancreatitis) will thus result in increased urinary excretion of amylase. Therefore, urinary amylase is sometimes used in the diagnosis of acute pancreatitis. However, the rate of urinary amylase excretion appears to be less sensitive than plasma markers, and is not specific for the diagnosis of acute pancreatitis.
Similar to other low-molecular-weight proteins filtered by glomeruli, amylases are reabsorbed to an extent by the proximal tubule. Thus, conditions associated with increased production and glomerular filtration of other low-molecular-weight proteins that compete with tubular reabsorption of amylase or conditions of proximal tubular injury may increase urinary amylase excretion. Also, a number of disorders other than acute pancreatitis may cause increases in plasma amylase concentrations and consequent increases in urinary amylase excretion. These conditions include burns, ketoacidosis, myeloma, light-chain proteinuria, march hemoglobinuria, acute appendicitis, intestinal perforation, and following extracorporeal circulation.
Quantitation of urinary amylase excretion is also useful in monitoring for rejection following pancreas transplantation. The duodenal cuffs of donor pancreases are often surgically anastomosed to the recipient's bladder at the time of pancreas transplantation, allowing for drainage of exocrine pancreas fluid into the bladder. In pancreatic rejection, urinary amylase excretion decreases.
Assessment of acute rejection of bladder-drained pancreas transplants
As an aid in the diagnosis of acute pancreatitis
Decreases in urinary amylase excretion of greater than 30% to 50%, relative to baseline values, may be associated with acute pancreas allograft rejection. Because there is large day-to-day variability in urinary amylase excretion following pancreas transplantation, if a significant decrease is noted, it should be confirmed by a second collection. There is also large inter-individual variability in urinary amylase excretion among pancreas transplant recipients. Collecting a timed urine specimen and expressing the urinary amylase level as Units excreted/hour might reduce variability and improve test performance. However, acute rejection is usually not established solely by changes in urinary amylase excretion, but by tissue biopsy.
Urinary amylase is elevated in acute pancreatitis, but the test has poor sensitivity and specificity.
No significant cautionary statements
1. Tietz Textbook of Clinical Chemistry. 3rd edition. Edited by CA Burtis, ER Ashwood. Philadelphia, WB Saunders Co., 1999, pp 689-698
2. Munn SR, Engen DE, Barr D, et al: Differential diagnosis of hypo-amylasuria in pancreas allograft recipients with urinary exocrine drainage. Transplantation 1990;49:359-362
3. Klassen DK, Hoen-Saric EW, Weir MR, et al: Isolated pancreas rejection in combined kidney pancreas transplantation. Transplantation 1996;61:974-977
4. Benedetti E, Najaran JS, Gruessener AC, et al: Correlation between cystoscopic biopsy results and hypoamylasuria in bladder-drained pancreas transplants. Surgery 1995;118:864-872