All reptiles, particularly those from airid and dry regions where water conservation within the body is essential to survival, have no use for the mammalian system of using water in the process of protein metabolism. Alternately, reptiles produce uric acid as the end product of protein, nonprotien nitrogen and purine metabolism. This means that in place of urine, the uric acid is discharged as a dry powdery or pasty white matter that is excreted in place of liquid urine. This entire process requires little water and allows the reptile to conserve as much water as possible.
The most common methods for determining proper kidney function in mammals are to measure the Blood Urea Nitrogen (BUN) and Creatinine (CREA) levels. This is not possible in reptiles due to the fact that reptiles do not produce urea and do not produce creatinine in any notable amount. This makes these blood tests very ineffective in terms of evaluating the reptilian kidney.
Since BUN and CREA levels are ineffective tests in evaluation of the reptile kidney and since the most significant end product of metabolism in reptiles is Uric Acid it would stand to reason that plasma uric acid levels would be a better indicator of kidney function in reptiles, but despite this fact, plasma uric acid levels are NOT a specific test for renal disease in reptiles, but is the better of the three tests. The reason for this is that many factors can influence the amount of uric acid that is present in blood plasma. Carnivorous reptiles, for instance, will have a higher plasma uric acid following a recent meal. As a general rule plasma uric acid levels will always be higher in carnivorous reptiles than in herbivorous reptiles. Actual normal values can vary depending on the reference ranges of the referring lab. This is a problem since most veterinarians use a referring lab that deals mostly with mammals; few labs deal strictly with reptiles. In general a plasma uric acid level of over 15 mg/dl should be considered elevated and indicate that further diagnostics are required to ensure proper function of the reptile kidney.
There is still much research required to fully understand the reptile kidney, but an elevated uric acid and an inverted Calcium to Phosphorous ratio of less than 1:1 are currently the best indicators of renal function in reptiles. If these tests prove elevated or inverted than follow up diagnostics of radiographs, ultrasound, CT, MRI and radioisotope scans should be considered. The only problem with these follow up tests is the owners financial ability to have the tests performed and a hospital or institutions willingness to perform these tests on a reptile.
So, what is the bottom line? If you take your reptile to the vet and he or she asks to run a BUN or CREA to test the reptile's kidney function, say no and find another veterinarian that experienced with reptiles and not attempting to apply mammalian medicine to a reptile patient.
