Immunoassay of urinary retinol binding protein as a putative renal marker in cats

The presence of low molecular weight retinol binding protein (RBP) in urine reflects tubular damage. Therefore, RBP has been used as a renal marker in humans and dogs. Using an anti-human RBP antibody (Ab), this study first demonstrates feline urinary RBP by Western blot analysis and then evaluates its potential as a renal marker in cats by enzyme-linked immunosorbent assay (ELISA). Urine was taken by cystocentesis, centrifuged and stored at -80 degrees C until analysis. Urinary RBP levels were compared in clinically healthy cats (H), chronic renal failure patients (CRF) and cats with hyperthyroidism (HT). The detection of a band at the same position as the human RBP standard with Western blot analysis, indicated that RBP was present in the urine of CRF and HT patients but minimally present in H cats. The data obtained with ELISA were in accordance with these observations. RBP levels were expressed as RBP:creatinine (RBP:c) ratios following normalisation with urinary creatinine. The functional assay sensitivity was 1.37 microg/l RBP. Parallelism between the trend lines of the human RBP standard curve and the curves obtained from sequentially diluted urine samples indicated that feline RBP was recovered. The mean intra-assay coefficient of variance was 7% and the standardised agreement index revealed satisfactory day-to-day repeatability. The RBP:c ratio in all H cats (n=10) was below the assay sensitivity. The groups of CRF and HT patients had increased mean RBP:c ratios of 1.6+/-0.5x10(-2) microg/mg (mean+/-SEM, n=10) and 1.4+/-0.4x10(-2) microg/mg (n=13), respectively. Both groups showed a large variation in the relative RBP concentrations of individual cats. In conclusion, RBP is demonstrated for the first time in urine from most CRF and HT patients and the validated ELISA allows its evaluation as a putative renal marker in cats.