Weekly biological variation of serum biochemistry analytes and fibroblast growth factor 23 in healthy cats and cats with chronic kidney disease

Objective The objective of the study was to determine the biological variation of select serum biochemistry analytes and fibroblast growth factor 23 in both clinically healthy cats and cats with chronic kidney disease. Methods Eleven healthy cats and seven cats with chronic kidney disease International Renal Interest Society Stages 2 and 3 were included. Sera were collected once a week for 6 weeks and frozen for batch analysis in duplicate. Blood urea nitrogen, total carbon dioxide, creatinine, magnesium, phosphate, potassium, symmetric dimethylarginine, total calcium and fibroblast growth factor 23 were measured. Restricted maximum likelihood estimations were used to determine the coefficients of variation, and the inverse indices of individuality and reference change values were calculated. Results No analytes had low individuality. Biochemistry analytes had either intermediate or high individuality in healthy cats and cats with chronic kidney disease. Fibroblast growth factor 23 had high individuality in healthy cats and cats with chronic kidney disease. The reference change values for analytes was overall similar between healthy cats and cats with chronic kidney disease, including the reference change values for creatinine (19.8% and 18.4%, respectively), symmetric dimethylarginine (35.2% and 35.5%, respectively) and fibroblast growth factor 23 (60.0% and 75.5%, respectively). Clinical Significance Biological variation estimates for each analyte were similar between healthy cats and those with chronic kidney disease. All analytes had intermediate to high individuality in cats with chronic kidney disease; thus, determining a cat’s baseline and applying the reference change values to subsequent measurements may enhance the detection of clinically relevant changes that could be missed when using population‐based reference intervals.