Background Aim of the study was the investigation and comparison of components of variance of endogenous markers of glomerular filtration rate (GFR) in healthy dogs and their impact on the interpretation of results.
Hypothesis: Is Cystatin C (CysC) in the dog superior to Creatinine (crea) and Urea (urea) in detecting decreased renal function due to a high index of individuality (IoI) like in man.
Animals/Method - Variance components of cysC, crea (two methods: creaE, creaJ), urea and inorganic phosphate (Pi) in plasma were determined in a longitudinal study over 6 month in 24 healthy dogs (6 german shorthair, 18 beagles). Index of Individuality (IoI) and critical differences (CD) were calculated as well as the numbers of measurements necessary to determine the individuals homeostatic set-point.
Results - Mean concentration of cysC, creaJ, creaE, urea and Pi was 0.93 ± 0.19 mg/L (range 0.55 - 2.14 mg/L), 0.94 ± 0.17 mg/dl (range 0.6 - 1.8 mg/dl), 0.76 ± 0.18 mg/dl (range 0.40 - 1.60 mg/dl), 35.34 ± 9.08 mg/dL (range 17.30 66.40 mg/dL) and 3.74 ± 0.68 mg/dL (range 1.86 3.10 mg/dL), respectively. The IoI for cysC, creaJ, creaE, urea and Pi was 0.96, 0.89, 0.80, 0.90 and 1.16, respectively. The CD for cysC, creaJ, creaE, urea and Pi was 0.37mg/L, 0.26 mg/dL, 0.27 mg/dL, 16.94 mg/dL and 1.45 mg/dL, respectively.
Conclusion In dogs, unlike man, components of biological variance of CysC and crea are of the same range. Analytical precision requirements were fulfilled by creaboth, urea and Pi. All parameters showed an intermediate IoI, allowing the application of population based reference limits. Application of the CD for creatinine or CysC might be useful in detecting decrease of glomerular filtration rate, when sequential measurements in an individual show an increase exceeding the critical difference but not the upper reference limit. For these patients further investigation of GFR by other means e.g. exogenous creatinine clearance might be of benefit to detect low grade glomerular functional deficits.
Canine / Biological Variation / Index of Individuality / Critical difference / Creatinine / Urea / Cystatin C / Glomerular filtration rate