Chapter 2 Clinical Evaluation of the Urinary Tract
Introduction and Terminology
Clinical Approach
History
Urination
Water Intake
Physical Examination
Laboratory Evaluation of Renal Function
Glomerular Function (values for glomerular function tests in the dog and cat are presented in Table 2-2)
TABLE 2-2 Normal Values for Clinical Glomerular Function Tests
Test (units) | Dog | Cat |
---|---|---|
Blood urea nitrogen (mg/dL) | 8-25 | 15-35 |
Serum creatinine (mg/dL) | 0.3-1.3 | 0.8-1.8 |
Serum cystatin C (mg/dL) | 0.5-1.5 | NA |
Endogenous creatinine clearance (mL/min/kg) | 2-5 | 2-5 |
Exogenous creatinine clearance (mL/min/kg) | 3-5 | 2-4 |
Iohexol clearance (mL/min/kg) | 1.7-4.1 | 1.3-4.2 |
24-hour urine protein excretion (mg/kg/d) | <30 | <20 |
UPr/UCr | <0.4 | <0.4 |
Microalbuminuria (mg/dL) | <1 | <1 |
NA, not available.
TABLE 2-3 Discordant Results Between Blood Urea Nitrogen (BUN) and Serum Creatinine (SCr) Concentration
Disproportionate ↑ BUN Relative to SCr | Disproportionate ↑ SCr Relative to BUN |
---|---|
Severe dehydration and volume depletion (common) | Liver disease |
Gastrointestinal hemorrhage | Anorexia or low protein diet |
Emaciated animal | Massive muscle injury (acute) |
Young animal | Well-muscled individual |
Increases in serum creatinine concentration over time may indicate progressive loss of renal mass even if results still are within normal limits.

FIGURE 2-1 Steady-state relationship of GFR to BUN and serum creatinine. Note the exponential relationship between loss of functioning renal mass and changes in renal function assessed by BUN or serum creatinine. At least 75% loss of excretory renal function must occur before the BUN or serum creatinine leave the normal range. With more than 75% loss of excretory renal function there are correspondingly larger increases in BUN or serum creatinine while on the rise of the exponential curve. Consequently, small increases or decreases in GFR along this region of the curve can create large numerical change in BUN or creatinine. When two thirds of renal mass has been lost most dogs elaborate urine that is considered isosthenuric (1.007-1.018 USG); many cats with this degree of renal mass loss continue to be able to elaborate urine that is at least moderately concentrated to more than 1.030 specific gravity. The area up to 50% loss of renal mass is referred to as that of decreased renal reserve, from 50% to 75% loss that of renal insufficiency, and 75% loss that of renal failure associated with azotemia. Area A represents diminished renal reserve with advancing chronic renal disease (International Renal Interest Society [IRIS] Stage 1). Area B represents renal insufficiency (late IRIS Stage 1 and early IRIS Stage 2). Area C represents azotemic renal failure (IRIS Stages 3 and 4).
Renal Clearance
Single Injection Methods for Estimation of Glomerular Filtration Rate
Effects of Sedatives and Anesthetics on Glomerular Filtration Rate
Radioisotopes
Serum Phosphorus Concentration
Urine Protein

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