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 CVs 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