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

• Published in: Veterinary clinical pathology Vol. 44; no. 4; pp. 542 - 551
• Main Authors: Strage, Emma M., Theodorsson, Elvar, Ström Holst, Bodil, Lilliehöök, Inger, Lewitt, Moira S.
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 01.12.2015; Wiley Subscription Services, Inc
• Subjects: Acromegaly - blood; Acromegaly - metabolism; Acromegaly - veterinary; Animals; Annan veterinärmedicin; Cat Diseases - blood; Cat Diseases - metabolism; Cats; Diabetes Mellitus - blood; Diabetes Mellitus - metabolism; Diabetes Mellitus - veterinary; Enzyme-Linked Immunosorbent Assay - methods; Enzyme-Linked Immunosorbent Assay - veterinary; Growth hormone disorders; Humans; Insulin-Like Growth Factor Binding Proteins - blood; Insulin-Like Growth Factor Binding Proteins - metabolism; Insulin-Like Growth Factor I - metabolism; insulin-like growth factor-binding proteins; Other Veterinary Science; Reference Values; Reproducibility of Results; size-exclusion chromatography; size-exclusion chromatography; Growth hormone disorders; insulin-like growth factor-binding proteins
• ISSN: 0275-6382; 1939-165X; 1939-165X
• DOI: 10.1111/vcp.12289

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 < .000001). 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.