title: Characteristics and reference ranges of Insulin-Like Growth Factor-I measured with a commercially available immunoassay in 724 healthy adult Caucasians.
abstract: OBJECTIVE
  Measurements of Insulin-Like Growth Factor-I (IGF-I) play a pivotal role in the evaluation of the growth hormone-IGF-I axis. Due to assay variation IGF-I reference ranges are assay specific. We provide serum IGF-I reference ranges for adult men and women obtained by a commercially available assay.
  
  
  METHODS
  IGF-I was measured by an enzyme-linked immunosorbent assay (R&D Systems). Assay precision was evaluated in low, medium and high IGF-I pools and in single samples from outpatients. The reference ranges were obtained in 724 healthy Caucasians, mean age 48 years (range 19-91).
  
  
  RESULTS
  IGF-I was measured higher in serum compared to plasma samples (p<0.001, R2=0.96). To convert EDTA plasma values to serum values a factor of 1.22 was calculated. The intraassay coefficients of variation (CV) for the low: 56.4+/-2.7 ng/mL, medium: 179.7+/-5.9 ng/mL and high pool: 445.61+/-3.2 ng/mL (mean 1 SD) were 5, 3 and 3%. Interassays CVs for the low, medium and high pool varied between 7-10, 5-7, and 6-9%. Reproducibility between 4 different lots showed a intraclass CV of 0.99 (95%CI 0.98-0.96). Logarithmically transformed IGF-I levels were linearly associated with age with a 13% reduction in IGF-I per decade in females and 11% in males.
  
  
  CONCLUSIONS
  We have provided IGF-I reference ranges obtained by an assay that showed variations and reproducibility that was considered of sufficient high quality for clinical and scientific use.
authors.affiliation: Department of Endocrinology, Herlev Hospital, University of Copenhagen, Denmark. andreassenmikkel@hotmail.com
authors.forename: Mikkel
authors.initials: M
authors.lastname: Andreassen
authors.forename: Kaspar
authors.initials: K
authors.lastname: Nielsen
authors.forename: Ilan
authors.initials: I
authors.lastname: Raymond
authors.forename: Lars Østergaard
authors.initials: LØ
authors.lastname: Kristensen
authors.forename: Jens
authors.initials: J
authors.lastname: Faber
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001800:Blood Specimen Collection; D005260:Female; D006262:Health; D006801:Humans; D007118:Immunoassay / Q000379:methods*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D011933:Reagent Kits, Diagnostic; D012015:Reference Standards; D012016:Reference Values; D012727:Sex Characteristics; D044465:White People; D055815:Young Adult
chemical_list: D011933:Reagent Kits, Diagnostic; D007334:Insulin-Like Growth Factor I