Gregory F. Grauer, DVM, MS, DACVIM
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Practitioners have long interpreted proteinuria clinically in conjunction with their canine and feline patients' urine specific
gravity and sediment examination results. For example, a urine dipstick colorimetric or sulfosalicylic acid turbidimetric
test reading of trace or 1+ protein in urine that has a high specific gravity is often attributed to urine concentration rather
than to pathologic proteinuria. Or a dipstick result positive for protein in a patient with microscopic hematuria or pyuria
is usually attributed to urinary tract hemorrhage or inflammation.
However, in both cases, these interpretations may be incorrect because these semi-quantitative screening tests have several
limitations.1,2
CAN I RELY ON URINE DIPSTICK PROTEIN RESULTS?
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Albumin is the primary protein found in urine in both healthy dogs and cats as well as in dogs and cats with renal disease.
The urine dipstick test is the most commonly used screening test, but it has relatively low sensitivity (many false negative
results) and low specificity (many false positive results) for albuminuria.1 Although the sulfosalicylic acid test has greater sensitivity than the dipstick test, its specificity for albuminuria is
also relatively poor.1 In addition, grading the turbidity of the sulfosalicylic acid test and the dipstick color change is subjective, and results
can differ among laboratories and individuals.
Two questions need to be answered when proteinuria/albuminuria is detected on screening tests. One: Is the proteinuria persistent
(vs. transient)? Two: What is the cause or origin of the proteinuria? Serial testing along with urine sediment examination
will help answer these two questions.
HOW IS PERSISTENT PROTEINURIA DEFINED?
Persistent proteinuria is proteinuria found on three separate screening tests separated by seven to 10 days.2
WHAT ARE THE POSSIBLE CAUSES OF PROTEINURIA?
Proteinuria can be caused by physiologic or pathologic conditions.
Physiologic proteinuria may result from fever, exposure to extreme cold or heat, strenuous exercise, stress, or seizures;
it is usually transient and resolves after the underlying cause is corrected.
Pathologic proteinuria can be caused by nonurinary system problems that are prerenal (e.g. Bence-Jones proteins produced by neoplastic plasma cells) or postrenal (e.g. prostatitis or metritis) in origin. To reduce the possibility of contaminating the urine with protein from the lower urinary
tract, obtain samples by cystocentesis.
Pathologic proteinuria can also result from urinary system problems and may be nonrenal (lower urinary tract inflammation
or hemorrhage) or renal (glomerular or tubulointerstitial conditions) in origin. Renal proteinuria from glomerular and tubular
disease is usually persistent and most often accompanied by inactive urine sediment findings, with the exception of hyaline
casts. However, renal proteinuria can also result from inflammatory (e.g. pyelonephritis) or infiltrative (e.g. neoplasia) kidney disorders; thus, the urine sediment findings are often active (e.g. bacteriuria, pyuria, hematuria, neoplastic cells). Persistent proteinuria arising from glomerular or tubular lesions is a
marker of chronic kidney disease that is likely to be progressive.
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