Insulin‐like growth factor I in cats: validation of an enzyme‐linked immunosorbent assay and determination of biologic variation

Background Insulin‐like growth factor I ( IGF ‐I) measurements are used in veterinary medicine for diagnosing growth hormone disorders. IGF ‐I assays are subject to interference by IGF ‐binding proteins ( IGFBP ) which may not be efficiently removed by standard extraction methods. Adding excess IGF ‐ II during analysis may improve accuracy. Objectives The purpose of the study was to validate a commercial human IGF ‐I ELISA which uses excess IGF ‐ II for feline samples and to evaluate biologic variation. Methods Precision was determined by calculating the coefficient of variation ( CV ). Accuracy was determined by recovery after removal of IGFBP , addition of IGF ‐I, and linear dilution after the addition of IGFBP . Biologic variation was determined by repeated sampling in 7 cats. Results There was interference by IGFBP in the high measuring range, resulting in falsely low IGF ‐I concentrations. This was overcome by the addition of high concentrations of IGF ‐ II . Untreated serum had a measured/expected ratio of 98–115% compared to serum where IGFBP had been removed. Recovery after the addition of IGF ‐I was 83–112%. Inter‐ and intra‐assay CV s ranged from 2.4% to 5.0% which is within the minimum acceptance criteria based on biologic variation. The reference interval of IGF ‐I was wide (90–1207 ng/mL) and there was a significant association between body weight and ln  IGF ‐I ( P  <   .1). Conclusions This human ELISA is suitable for feline samples, but interfering IGFBP can cause falsely low concentrations. It is recommended to dilute samples such that IGF ‐I is < 28 ng/mL on the standard curve to grant for sufficient IGF ‐ II for binding of interferent IGFBP .