Practical Matters: Practical ways to measure GFR in your patients - Veterinary Medicine
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Practical Matters: Practical ways to measure GFR in your patients


GFR was measured by iohexol clearance, with a result of 3.1 ml/kg/min (reference range = 2.89 to 8.07 ml/kg/min). Based on these normal results, a water deprivation test was performed. Belle's urine specific gravity increased to 1.031 within 10 hours, and psychogenic polydipsia was diagnosed. The owner was instructed on behavior modification techniques to decrease stress and advised that some water restriction would be safe. The owners declined therapy for the cystic calculi. Two years later, Belle presented for an unrelated problem and was noted to have a urine specific gravity of 1.050.

A boxer with chronic PU/PD. Ali, a 10-year-old spayed female boxer, presented for evaluation of polyuria and polydipsia of 12 months' duration that had gotten progressively worse over the previous three months. Her BUN concentration was 11 mg/dl, creatinine concentration was 0.8 mg/dl, and urine specific gravity was 1.004.

ACTH stimulation test results and fasting and postprandial bile acid concentrations were normal. No abnormalities were seen on an abdominal ultrasonographic examination. During a water deprivation test, when Ali had lost 5% of her body weight, her urine specific gravity was still 1.020. Urine concentration did not increase after administration of antidiuretic hormone.

GFR was measured by iohexol clearance to exclude renal insufficiency. The result was 4.932 ml/kg/min, well within the reference range. Nephrogenic diabetes insipidus was diagnosed, and chlorothiazide therapy was initiated.

A Westie with a history of acute renal failure. Nico, a 6-year-old castrated male West Highland white terrier, was presented for a follow-up evaluation. He had a history of acute renal failure associated with pyuria two years previously. His azotemia had resolved completely. He remained polyuric since that episode, with urine specific gravity measurements ranging between 1.010 and 1.027.

On a routine recheck, microalbuminuria was noted, and the owner requested testing to determine if Nico had chronic kidney disease. The benefits of renal biopsy to determine if structural changes were present were discussed with the owner as well as GFR measurement to determine whether functional changes were present. The result of GFR measurement by iohexol clearance was 6.95 ml/kg/min and indicated that renal function had returned to normal after the pyelonephritis episode. No renal therapy was prescribed. Biannual urinalysis, urine protein concentration determination, and serum chemistry profile testing and annual GFR measurement were recommended to monitor Nico for renal disease progression.


While the exogenous creatinine clearance test is generally less accurate than iohexol clearance in animals with reduced GFR or with fewer than six sampling points, it has the advantage of providing results rapidly after completion of the test.2 In this test, exogenous creatinine (60 mg/kg) is administered intravenously. Many different protocols exist for the timing of obtaining the blood samples after injection, but, in general, at least three samples are needed, and the last sample should be at least six hours after the injection. The protocol I use includes blood sampling immediately before creatinine administration and 10, 60, 240, 360, and 600 minutes after administration. The serum creatinine concentration results, which can be obtained through standard laboratories, are used to calculate the GFR.

Creatinine is safe to administer intravenously and does not induce signs of uremia. Injectable-grade creatinine is not readily available commercially but can be obtained through some specialty pharmacies that will custom-formulate preparations from chemical-grade creatinine. Royal Canin has developed a software program to calculate the GFR for creatinine clearance.4,5 (For a copy, e-mail the author at

Case example: Exogenous creatinine clearance in a young Mastiff with renal dysplasia

Table 1: Annie's Exogenous Creatinine Clearance Results
Exogenous creatinine clearance was performed in Annie, a 1-year-old female Mastiff with renal dysplasia. Because the 10-hour sample had such a high creatinine concentration (Table 1), Annie was presented again the next day for a 24-hour sample. Creatinine clearance was 0.6 ml/kg/min, which is markedly below the reference range of 2 to 4.5 ml/kg/min. By the following week, the serum creatinine concentration had decreased to the baseline at 7.2 mg/dl. Annie was enrolled in a drug study at that time. Three months later, her baseline serum creatinine concentration was 9.9 mg/dl and her GFR measured by creatinine clearance had dropped to half of the initial value—0.3 ml/kg/min. She was euthanized two months later because of progressive signs of uremia.


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