OTHER TESTS OF GFR
The ideal substance for measuring GFR should be a) exclusively cleared by glomerular filtration, b) neither secreted nor reabsorbed
by the kidney tubules, and c) easily measured.32 Unfortunately, a substance meeting all these criteria is not available as a simple or routine test on most commercially
available veterinary biochemical profiles. The crudest estimates of GFR on a serum chemistry profile are BUN and creatinine
concentrations; however, as discussed above, measuring and interpreting these values have their limitations.
Cystatin C has been investigated as a marker of GFR in dogs. This cysteine protease inhibitor is freely filtered by the glomerulus
and unaffected by nonrenal factors such as inflammation and sex. It correlates well with GFR measurements and may be a reasonable
alternative to creatinine concentration measurement.33 The main limitation of cystatin C measurement in people is intraindividual variation. It has lower sensitivity than serum
creatinine concentration in detecting changes in the same individual, even though cystatin C may be a better indicator of
early decreases in GFR.34 Cystatin C measurement is not available for veterinary patients outside a research setting.
Multiple tests are typically reserved for a laboratory or university setting that provides accurate measurements of GFR. Many
of these methods have several disadvantages, including high cost, labor intensiveness, risks caused by anesthesia, or need
for specialized equipment or licensing. The gold standard is the urinary clearance of inulin. Other tests include nuclear
scintigraphy, clearance tests of endogenous and exogenous creatinine, clearance of chromium-51 EDTA, and the iohexol clearance
test.35 Of the aforementioned tests, the iohexol clearance test is probably the most practical and easiest to perform (see the Related Link "Case study: Acute kidney failure from hypotension in a dog" below). It does not require any specialized equipment or urine collection and can be completed in four hours.36,37 (Iohexol measurement for the iohexol clearance test is available at the Diagnostic Center for Population and Animal Health,
Michigan State University, 4125 Beaumont Road, Lansing, MI 48910-8104; phone: (517) 353-1683; FAX: (517) 353-5096;
http://www.animalhealth.msu.edu/.)
SUMMARY
A combination of routine blood and urine tests, along with more specialized laboratory tests, can provide a wealth of information
about the presence of kidney disease and clues about its cause. In addition, these tests may help detect complications associated
with kidney disease.
Michael Geist, DVM, DACVIM
VCA Animal Specialty Group
5610 Kearny Mesa Road, Suite B
San Diego, CA 92111
Cathy Langston, DVM, DACVIM
The Animal Medical Center
510 East 62nd St.
New York, NY 10065
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