title: Insulin-like growth factor I in cats: validation of an enzyme-linked immunosorbent assay and determination of biologic variation.
abstract: 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.
  
  
  OBJECTIVE
  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.
authors.affiliation: Department of Clinical Sciences, University Animal Hospital, Swedish University of Agricultural Sciences, Uppsala, Sweden.|Department of Clinical Pathology Laboratory, University Animal Hospital, Swedish University of Agricultural Sciences, Uppsala, Sweden.
authors.forename: Emma M
authors.initials: EM
authors.lastname: Strage
authors.affiliation: Department of Clinical Chemistry and Department of Clinical and Experimental Medicine, Linköping University, Linköping, Sweden.
authors.forename: Elvar
authors.initials: E
authors.lastname: Theodorsson
authors.affiliation: Department of Clinical Sciences, University Animal Hospital, Swedish University of Agricultural Sciences, Uppsala, Sweden.
authors.forename: Bodil
authors.initials: B
authors.lastname: Ström Holst
authors.affiliation: Department of Clinical Pathology Laboratory, University Animal Hospital, Swedish University of Agricultural Sciences, Uppsala, Sweden.
authors.forename: Inger
authors.initials: I
authors.lastname: Lilliehöök
authors.affiliation: School of Health, Nursing and Midwifery, University of the West of Scotland, Paisley, UK.
authors.forename: Moira S
authors.initials: MS
authors.lastname: Lewitt
mesh_terms: D000172:Acromegaly / Q000097:blood / Q000378:metabolism / Q000662:veterinary; D000818:Animals; D002371:Cat Diseases / Q000097:blood / Q000378:metabolism; D002415:Cats; D003920:Diabetes Mellitus / Q000097:blood / Q000378:metabolism / Q000662:veterinary; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods / Q000662:veterinary*; D006801:Humans; D018969:Insulin-Like Growth Factor Binding Proteins / Q000097:blood / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D012016:Reference Values; D015203:Reproducibility of Results
chemical_list: D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I
keywords: Growth hormone disorders; insulin-like growth factor-binding proteins; size-exclusion chromatography