Glomerular filtration rate by iohexol clearance in cats and dogs

What it shows

Iohexol is an X-ray contrast agent that the kidneys filter freely and do not take back. The vet injects it into a vein and takes several blood samples at recorded times; the laboratory measures how fast iohexol disappears from the blood and turns that into a clearance — the volume of plasma cleared per minute. Because the substance leaves only through filtration, the result is a measured GFR rather than an estimate.

Creatinine, urea and SDMA are indirect markers. In cats, urea and creatinine together explained less than half of the differences in GFR between animals, and creatinine also depends on muscle mass. A large drop in filtration can happen before creatinine rises noticeably, so a measured GFR is the most sensitive way to see reduced kidney function early. Iohexol plasma clearance studies in cats reported no side effects from the agent.

Higher than reference — what it may be a sign of

• Hyperfiltration — the remaining kidney tissue can work harder and keep GFR within the normal range despite damage, which is why a normal GFR does not prove healthy kidneys.
• Hyperthyroidism in cats — an overactive thyroid raises GFR and can hide chronic kidney disease; GFR measured before therapy helps predict whether kidney values will rise afterwards.
• A protein-rich meal before the test may increase GFR, which is why the animal is usually fasted overnight before the test.

Lower than reference

After causes outside the kidneys are excluded — dehydration, low blood pressure, heart disease — a lower GFR generally means kidney disease. Even mild, unnoticed dehydration lowers the result, so the animal is kept well hydrated for the test. In dogs with naturally occurring hypothyroidism, GFR was below 2 mL/min/kg in all 14 dogs of one study and improved once thyroid function was corrected. A low GFR is a finding to interpret, not a stage or a diagnosis by itself.

Look at together with

• Creatinine and SDMA — the usual kidney markers; measured GFR is what they try to estimate.
• Urine specific gravity — shows the ability to concentrate urine, which is not the same as filtration.
• Urine protein : creatinine — protein loss through the kidneys, a separate question.
• Total T4 — in cats with hyperthyroidism, read before and after thyroid therapy.

Units and conversion

Clearance is a volume of plasma per minute, divided by body weight: mL/min/kg (some sources write mL/kg/min — the same thing). This is the only unit for this test, so there is nothing to convert. Laboratories apply a correction formula to the raw calculation; methods can differ from each other by 10–20%, so repeat measurements are compared only when the same technique was used.

Reference range

The reference range is on your lab’s form. Published values depend on species, body size, breed and method. In dogs, a value below about 1.5 mL/min/kg is generally considered decreased, but IRIS notes that a single cut-off is not appropriate, because GFR per kilogram is lower in large dogs than in small ones. Day-to-day variation in the same animal is generally under 20% (coefficient of variation), so a small change is not necessarily meaningful.

Cats and dogs

Sources

1. International Renal Interest Society. Glomerular filtration rate in dogs and cats, iris-kidney.com.
2. eClinPath (Cornell University College of Veterinary Medicine). GFR — chemistry / kidney.
3. Finch N. Measurement of glomerular filtration rate in cats: methods and advantages over routine markers of renal function. J Feline Med Surg 2014;16(9):736–748.