The protein-creatinine ratio on random mid-day urine samples correlates well with the cumbersome "gold standard" (no pun intended), the 24-hour urine collection, for quantitating urinary protein loss. It is unaffected by urine volume or concentration. Determination of the urine protein-creatinine ratio assumes the following:
- Stable GFR
- Constant protein loss
- Constant glomerular filtration
- Tubular function affects proteins and creatinine similarly. In reality, tubular secretion of creatinine increases as plasma creatinine concentration increases, therefore the ratio may be decreased in azotemia.
Interpretation
- Guidelines from ACVIM consensus statement (Lees et al 2005, J Vet Intern Med; 19:377):
- Dogs: In healthy dogs, the urine protein to creatinine ratio (UP:UC) is usually < 0.5. Values between 0.5-1.0 in non-azotemic dogs are considered equivocal and continued monitoring for progression is recommended. Values >1.0 in non-azotemic dogs are abnormal and diagnostic evaluation is warranted. Glomerular proteinuria is usually associated with UP:UC ≥ 2.0. Therapeutic intervention is recommended for azotemic dogs with UP:UC ≥ 0.5. These figures are only valid for urine samples with inactive sediments.
- Cats: In healthy cats, the urine protein to creatinine ratio (UP:UC) is usually < 0.5. Values between 0.5-1.0 in non-azotemic cats are considered equivocal and continued monitoring for progression is recommended. Note that some healthy male cats can have UP:UC values within this range (up to 0.6). Values >1.0 in non-azotemic cats are abnormal and diagnostic evaluation is warranted. Glomerular proteinuria is usually associated with UP:UC ≥ 2.0. Therapeutic intervention is recommended for azotemic cats with UP:UC ≥ 0.4. These figures are only valid for urine samples with inactive sediments.
- Mild increases: Can be seen in prerenal, renal or postrenal proteinuria.
Tubular proteinuria usually results in protein-creatinine ratios of
< 2.0 (range: 1.0-5.0).
- Moderate to severe increases: Urine protein-creatinine ratios
> 2.0 (usually > 5.0) are seen with glomerular disease, e.g. glomerulonephritis or amyloidosis. The severity of proteinuria does not distinguish between causes of glomerular disease.
Factors affecting urine protein-creatinine ratios
- Hemorrhage: The protein-creatinine ratio will increase proportionally
to the degree of blood in the urine. Heavy blood contamination frequently
invalidates the ratio. Mild blood contamination from cystocentesis (i.e. 5-20 RBC/HPF
or even > 100 RBC/HPF) does not usually cause a proteinuria (on the dipstick or with urine protein to creatinine ratios).
- Infection: The protein-creatinine ratio is invalid in the presence of a urinary tract infection. Ratios as high as 40 can be seen with E. coli infections. The ratio does not correlate to the number of red or white cells/HPF in these cases.
- Inflammation: Inflammation, without infection, will increase the urine protein-creatinine ratio, but usually the ratio is < 2.0.
Therefore, urine protein-creatinine ratios should not be measured in animals with "active" urine sediments.
- Drugs: Immunosuppressive doses of corticosteroids (2 mg/kg q 12 h for 6 weeks) will increase the urine protein-creatinine ratio mildly (up to 1.3). This is attributed to mesangial cell proliferation.
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