W. Braselton, PhD
W. Rumbeiha, DVM, PhD, DABVT
Toxicology Section
517-355-0281
J. Kruger, DVM, PhD, DACVIM
Small Animal Clinical Sciences
517-353-5420

Determination of glomerular filtration rate (GFR) is an important aspect of the evaluation of kidney function in the diagnostic approach to patients with suspected renal disease. Screening tests such as BUN and serum creatinine do not begin to register abnormal until about 75% of the kidney's nephrons are nonfunctional, whereas GFR is directly related to the functional renal mass. Measurement of renal clearance of inulin has been the standard for determination of GFR but is seldom used in clinical practice because it is difficult and time consuming to perform. Other methods that give acceptable determination of GFR include measurement of renal clearance of creatinine and plasma clearance of radioactive substances such as 51CR-EDTA or 99mTc-DTPA, but these have the attendant drawbacks of extended urine collection or the need for facilities to handle radiopharmaceuticals. Iohexol is an iodinated radiographic contrast medium developed for use in a variety of diagnostic procedures such as myelography, urography, angiocardiography, visceral and cerebral arteriography, and in contrast enhancement for computerized tomography. Because it is not protein bound and is totally excreted by the kidney through the process of glomerular filtration, it is gaining widespread recognition as the new standard for use in the estimation of GFR in humans and domestic animals. Several analytical methods have been developed for determination of iohexol concentrations in serum or plasma to allow estimation of GFR through measurement of plasma clearance following a single injection of iohexol. Since these methods have also presented drawbacks, we have investigated utilization of our existing inductively coupled plasma-atomic emission spectrometer (ICP-AES) for estimation of GFR through determination of iohexol plasma clearance by the measurement of serum iodine. The excellent sensitivity, accuracy, precision and linearity achieved with the ICP-AES method in the range of iodine useful in clinical determination of GFR in small animals overcomes the limitations of other published methods.

We are now making the test available as a glomerular filtration rate to be performed on canine or feline serum. The cost of the test is $65. To perform the test, accurately administer a single dose of iohexola at 300 mg iodine/kg I.V. and record the time to the nearest minute. Draw a 3-4 ml blood sample at 2 hr, 3 hr, and 4 hr after administration, labeling each tube with the owner's name, animal i.d., and time of sampling to the nearest min. Allow the blood to clot and transfer each serum sample (1.2 ml or greater is needed) to a plastic vial labeled with the exact sampling time. Ship chilled in an insulated container. Enclose a completed AHDL submittal form requesting the GFR test. Be sure to include information about the animal (id., species, breed, age, sex), a patient history, exact dose (in mg iodine/kg body weight), exact time of administration, and exact sampling times. We plan to run the analysis on Thursdays. Please schedule for samples to arrive at the AHDL by no later than Wednesday afternoon. Our report will indicate the GFR calculated from the plasma clearance of iohexol determined with the 3 timed samples and will include the expected normal range of GFR for the species.