Tuesday, June 15, 2010

TOPIC DISCUSSION: Amylase in the urine?? Hmm!!

Has anyone ever used urinary amylase levels or amylase creatinine ratio as a marker of pancreatic function. Urinary amylase has been of historical interest for the diagnosis of pancreatitis and even in some cases of pancreatic transplant rejection.  Some might claim that the amylase creatinine clearance ratio (ACCR) is a more sensitive index of pancreatic malfunction, then serum amylase levels. 

The renal clearance of amylase is 1-3.2 ml/min and the amylase filtered through the glomerulus is not reabsorbed by the tubular epithelium. It easily passes through the glomerular filter. Ratios of urinary amylase to urinary creatinine have proved to be sensitive for pancreatitis but increases also occur in patients hospitalized for other diseases.Hence it is not as specific as the ACCR. The amylase creatinine clearance ratio obviates variability in renal function and is useful when the patient has more than one cause for a raised serum amylase. 
In 1975, Warshaw and Fuller found that the renal clearance ratios of the various isoenzymes of amylase were the same and the urinary amylase molecules in patients of pancreatitis were of the same molecular weight; as the serum amylase from normal individuals, leaving only an increased renal permeability, due to enzymatic action, kinins or other vasoactive substances to explain the rise in ACCR.
While it has been claimed to rise specifically in pancreatitis, reports of its rise in febrile illnesses, diabetic ketoacidosis and in patients with burns and increase alcohol intake have been reported.
In pancreatic transplants, usually serum lipase followed by amylase is used as an early marker of rejection.  A rise in blood glucose is a late indicator of pancreas allograft rejection, and signifies that more than 90% of the islet cell mass may already have been destroyed. In allografts draining into the urinary bladder, a decrease in urinary amylase excretion is perhaps the earliest metabolic marker for rejection. But, hypoamylasuria does not prove that the injury to the pancreas is immunologically mediated, so that a tissue diagnosis is still needed to clinch the diagnosis of rejection. ACCR to my knowledge has not be studied in pancreatic transplants.
Nevertheless despite a few false positives, a rise in ACCR remains a good indicator of pancreatic damage. Wherever the urine can help, we can help ;)
Caution especially of using such ratios if the renal function is abnormal!
Image source :  http://www.polyenzyme.com/images/a-Amylase.jpg


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