Variability of serum concentrations of cystatin C and urinary retinol-binding protein, neutrophil gelatinase-associated lipocalin, immunoglobulin G, and C-reactive protein in dogs

Background: Markers of kidney dysfunction and damage have potential to detect chronic kidney disease (CKD) in early stages. However, data on long-term variation of these markers in healthy dogs is lacking and is crucial for the interpretation of results. Hypothesis/Objectives: To determine temporal variations of serum cystatin C (sCysC) and urinary retinol-binding protein (uRBP), neutrophil gelatinase-associated lipocalin (uNGAL), immunoglobulin G (uIgG), and C-reactive protein (uCRP) in healthy dogs. Animals: Eight clinically healthy adult Beagles were evaluated. Methods: Longitudinal observational study. Serum cystatin C was determined by particle-enhanced nephelometric immunoassay. Urinary retinol-binding protein, uNGAL, uIgG and uCRP were determined by ELISA and concentrations were indexed to urinary creatinine. Within- and between-dog variance components (VC) and within-dog coefficients of variation (CV) were determined from blood and urine collected at eight time points over 1.5 years. Results: Urinary C-reactive protein (uCRP) concentrations were consistently below the detection limit (5.28 ng/mL). Mean within-dog standard deviation for sCysC, uRBP/c, uNGAL/c and uIgG/c was 0.15 +/- 0.01 mg/L, 0.09 +/- 0.03 mg/g, 2.32 +/- 2.03 g/g and 12.47 +/- 10.98 mg/g, respectively. Within-dog CV for sCysC, uRBP/c, uNGAL/c and uIgG/c was 8.1%, 33.7%, 87.2% and 88.1%, respectively. Conclusions and clinical importance: Serum cystatin C, uRBP/c, uNGAL/c and uIgG/c exhibit a wide range of long-term within-dog variability. Researchers and veterinarians might need to take this into account when interpreting their results. To assess their diagnostic and predictive ability, future studies need to establish reference ranges for healthy dogs and dogs with CKD.