Serum Cystatin C and the Urinary NAG Index Identify an Altered Renal Biochemical Phenotype in Clinically Healthy Geriatric Dogs with Normal Serum Creatinine: A Three-Tier Cross-Sectional Study.

Serum creatinine (sCr), blood urea nitrogen (BUN) and symmetric dimethylarginine (SDMA) reflect glomerular filtration only and are subject to non-renal confounders including lean body mass, limiting early detection of renal dysfunction in geriatric dogs. We evaluated serum cystatin C (sCysC) and the urinary N-acetyl-β-D-glucosaminidase-to-creatinine ratio (NAG index) as early renal markers in healthy geriatric dogs. Sixty-one neutered geriatric dogs, free of clinical disease and of extrarenal laboratory abnormalities, were stratified into three groups: controls (n = 36), subclinical (normal sCr, ≥1 elevated biomarker; n = 17) and azotaemic chronic kidney disease (CKD; sCr > 1.5 mg/dL; n = 8). sCysC was measured by a canine-specific ELISA and the NAG index by colorimetric assay. Of the 61 dogs, 17 (27.9%) had at least one elevated biomarker with normal sCr, and all 17 also had normal BUN, indicating pre-azotaemic changes identified predominantly by sCysC. The NAG index was elevated in 6/17 (35.3%) subclinical dogs and SDMA in none (0/17). Because sCysC status partly informed group allocation and no independent measure of glomerular filtration rate was obtained, these findings characterise an altered renal biochemical phenotype rather than establish diagnostic accuracy, and require prospective validation.