Practical Matters: Practical ways to measure GFR in your patients
As an alternative, measuring the GFR can be useful in numerous situations commonly encountered in practice, such as
In this test, 300 mg/kg of iohexol, a commonly used radiographic contrast agent, is administered intravenously, and blood samples are typically drawn two, three, and four hours later.1,2 The timing of sample collection is flexible, but the exact time the samples were drawn needs to be accurately recorded so the precise time elapsed since injection can be calculated. The most commonly used veterinary diagnostic laboratory that offers measurement of iohexol concentrations and calculated GFR is Michigan State University's Diagnostic Center for Population & Animal Health (animalhealth.msu.edu/Submittal_Forms/AD.ADM.FORM.017.pdf), and results are generally available within a week. Other radiographic contrast agents such as iothalamate cannot be substituted for iohexol because the assay specifically measures iohexol and does not detect iothalamate. Although iodine allergic reaction or development of mild reversible acute kidney failure has been reported in people undergoing this type of test, the risk in animals appears to be low.3
Case examples: Iohexol clearance
The next four brief case examples reveal how measuring GFR with the iohexol clearance test was a valuable tool in patient management.
Her GFR was measured by iohexol clearance, with a result of 1.635 ml/kg/min, which is a 70% reduction from the expected mean of 5.48 ml/kg/min, confirming IRIS Stage I chronic kidney disease (iris-kidney.com/pdf/IRIS2009_Staging_CKD.pdf). Inadequate urine concentration tends to occur with over 66% reduction in GFR, whereas azotemia occurs with over 75% reduction.
A renal diet was initiated. One month later, hypertension was diagnosed, so benazepril and amlodipine were prescribed. Nine months later, Sheba remains clinically stable.
A senior dog with PU/PD and cystic calculi. Belle, an 11-year-old spayed female mixed-breed dog, was presented for evaluation of polyuria and polydipsia. Her BUN concentration was 12 mg/dl, creatinine concentration was 1 mg/dl, and urine specific gravity was 1.018. ACTH stimulation test results were normal, as were fasting and postprandial bile acid concentrations. An abdominal ultrasonographic examination revealed cystic calculi. The owner was unsure if there had been a partial improvement in the polydipsia after a course of antibiotics.