Biological Variation of Serum Phosphorus and Fibroblast Growth Factor-23 in Cats with Chronic Kidney Disease

Abstract: Background – Biological variation parameters are useful for choosing a subject- or population-based reference interval and to assess the clinical relevance of changes in serial analytical measurements. Phosphorus and fibroblast growth factor-23 (FGF-23) are used in the diagnosis and management of phosphorus overload in cats with chronic kidney disease (CKD). Biological variation of FGF-23 concentrations is unknown in cats. Objective – Determine the biological variation of phosphorus and FGF-23 in healthy cats and cats with CKD.Animals – 11 healthy client-owned cats and 8 client-owned cats with stable CKD Stage 1-3 Methods – Serum samples were collected once weekly for 6 weeks and frozen for batch analysis in duplicate at a commercial laboratory. Restricted maximum likelihood estimations were used to determine intraindividual (CVI), between individual (CVG), and analytical coefficients of variation (CVA) and then used to calculate the inverse index of individuality (II) and bidirectional reference change value (RCV) for serum phosphorus and FGF-23 concentrations. Results – CVA for serum phosphorus concentrations was 0.9% and for serum FGF-23 concentrations was 12.4%. Conclusions and clinical importance – Overall, biological variation estimates for each analyte were similar between healthy cats and those with CKD. Phosphorus has low to intermediate individuality, indicating that a population-based reference interval is appropriate. FGF-23 had high individuality, indicating that a subject-based reference interval is preferred to determine the clinical relevance of concentrations over time in an individual cat with stable health status.