Barrak M. Pressler, DVM, PhD,     Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, 601 Vernon Tharp Street, Columbus, OH 43210, USA. Email: pressler.21@osu.edu

Serum creatinine concentration is insensitive for detecting kidney injury and does not assist in differentiation between glomerular versus tubular damage. Advanced renal function tests, including glomerular filtration rate testing, determining fractional excretion of electrolytes, and assay of urine biomarkers, may allow earlier detection of reduced renal function mass, differentiation of renal from non-renal causes of azotemia, and assist with localization of damage. This article reviews the principles, indications, and limitations of these tests and describes their use in sample clinical scenarios.

Keywords

Biomarkers

Fractional excretion

Glomerular filtration rate

γ-Glutamyl transpeptidase

Microalbuminuria

Urine

Key points

Serum biochemical analysis and urinalysis are the mainstay diagnostic tests for initial detection and estimation of severity of kidney disease in dogs and cats. Increased serum creatinine concentration and impaired urine concentrating ability, however, are relatively insensitive for detecting early kidney injury and do not assist in differentiation between glomerular versus proximal or distal tubular damage. Advanced renal function tests, including GFR testing, determining fractional excretion (FE) of electrolytes, and assay of urine biomarkers, may allow earlier detection of reduced renal functional mass and differentiation of various renal and nonrenal differential diagnoses and assist with localization of damage. This article reviews the principles, indications, and limitations of these tests and describes their use in sample clinical scenarios.

Glomerular filtration rate

Serum creatinine concentration is insensitive for detecting kidney injury. Increases in serum creatinine concentration are mild and often remain within reference range, until approximately 60% to 75% of all nephrons are no longer functional. In contrast, measurement of GFR allows for precise quantitative assessment of remaining filtration and excretion ability by the kidneys. Example situations when GFR measurement may provide critical information regarding remaining kidney function beyond serum creatinine concentration alone include diagnostic evaluation of dogs and cats with unexplained polyuria and polydipsia, to avoid overdosing of medications that are excreted by the kidneys or that have potential nephrotoxic effects, and to predict risk of overt renal failure after nephrectomy in dogs or cats with unilateral kidney disease, such as tumors or pyonephrosis.

Several methods for determination of GFR have been validated in dogs and cats, all of which report the volume of plasma, which has been cleared over a given interval of time, per kilogram of patient body weight. After injecting a substance (the marker) that is eliminated solely via filtration through the glomeruli and which then passes into the urine without being reabsorbed or further secreted by the tubules, the rate at which the concentration of marker decreases in successive blood samples allows calculation of the plasma clearance and GFR.1 Assays that measure the rate of marker appearance in urine are more accurate than those that assay marker disappearance in plasma (because few markers are solely excreted via glomerular filtration without any tubular reuptake or secretion); however, urine assays that allow calculation of renal clearance (vs plasma clearance) are more cumbersome to perform because they require collection of all urine produced in a 24-hour period. Fortunately, plasma clearance assays using blood sampling techniques are sufficiently close to renal clearance, such that in the clinical setting urine collection is not required.1–4

Several markers have been validated for measurement of GFR in dogs and cats, including creatinine, cystatin C (CysC), iohexol, and radiolabeled molecules. In people, GFR is most commonly estimated (rather than measured) using serum creatinine concentration, body weight, and correction factors based on a patient’s gender and race. Unfortunately, formulae for estimating GFR from serum creatinine have not proved accurate in dogs and cats due to greater individual, gender, and breed variation than occurs in people.5,6 Intravenous administration of a sterile creatinine bolus is safe and cost effective; however, comparison of various markers suggests that exogenous creatinine GFR assays underestimate true GFR, likely due to some excretion into the gastrointestinal tract and perhaps tubular reuptake.2,7,8 CysC is an endogenous protein produced by all nucleated cells at a constant rate that undergoes glomerular filtration without tubular secretion; however, commercial assays are limited, and comparative studies in dogs have suggested lower specificity for detection of reduced kidney function than exogenous creatinine GFR.9–11

Iohexol GFR measurement uses a marker that can be easily obtained by veterinarians and has been well validated for use in dogs and cats, and a commercial assay is available at a reasonable cost to owners. After intravenous bolus injection of the same iodinated contrast agent used in diagnostic imaging studies, plasma samples are collected at predetermined times (usually 2, 3, and 4 hours after injection); the volume of injection is based on concentration of elemental iodine within the iohexol.12 Iodinated compounds are stable for long periods, and plasma samples can be frozen for extended periods of time and assayed later if indicated.13 Intravenous iohexol can induce acute kidney injury and renal failure in people, particularly in patients with preexisting kidney damage; however, this idiosyncratic drug reaction is rare in dogs and cats.14,15 Iohexol GFR assays have been safely used to study renal function in healthy dogs and cats,16–21 in dogs with gentamicin-induced acute kidney injury,22 in dogs and cats administered various nonsteroidal anti-inflammatory drugs (NSAIDs) after anesthesia,20,23 and in cats with untreated or post-treated naturally occurring hyperthyroidism.24–26 The most commonly used commercial assay for iohexol concentrations is offered by the Michigan State University Diagnostic Center for Population and Animal Health (http://www.animalhealth.msu.edu/); this diagnostic laboratory reports the calculated GFR after assaying serial plasma iohexol concentrations.

Radiolabeled markers validated for measurement of GFR currently used in clinical patients include chromium-51 ethylenediaminetetraacetic acid (EDTA) and technetium-99m diethylenetriamine pentaacetic acid (DTPA). These radionucleotides undergo glomerular filtration without tubular reabsorption or excretion and are stable in dog and cat blood samples but have short half-lives in vivo; this permits storage and shipping of samples to outside laboratories for assay of plasma clearance while patients are cleared of radioactivity and able to be released to owners within 24 to 48 hours.27 Use of radiolabeled markers is limited, however, to specialty practices that are appropriately licensed to perform nuclear medicine–based testing. Radionucleotide GFR assays have been safely used to study renal function in anesthetized dogs,28 in cats with solid tumors administered nephrotoxic chemotherapeutic agents,29 and in dogs and cats with naturally occurring (cats with polycystic kidney disease30 or azotemic chronic kidney disease31) or induced (cats with rejection of transplanted kidneys32 or dogs that had previously undergone renal biopsy33)...