title: Measurement of serum 'free' or 'easily dissociable' insulin-like growth factor-1.
abstract: BACKGROUND
  Serum insulin-like growth factor-1 (IGF-1) exists as ternary and binary complexes, and as the free hormone. An immunometric assay for 'free IGF-1' or 'easily dissociable IGF-1' has been described, although its use has not been widely assessed.
  
  
  METHODS
  The assay was assessed using the serum concentration/dilution test and by determining the relationship of the measured free hormone to the total hormone/binding protein ratio.
  
  
  RESULTS
  In six serum pools the measured free IGF-1 did not remain constant on concentration/dilution of the sample. However there was a significant correlation between the measured free IGF-1 and total IGF-1 (P < 0.001) and between total IGF-1/IGF binding protein-3 ratio (P < 0.001).
  
  
  CONCLUSIONS
  Although the assay failed the serum concentration/dilution test, there was still a significant relationship to the total hormone/binding protein ratio. It is possible that the serum dilution test is inappropriate to such a complex system or that the assay does not measure the free fraction. It is suggested that, though more cumbersome, it should be referred to as 'easily dissociable' IGF-1 as suggested by the manufacturers.
authors.affiliation: The Regional Endocrine Laboratory, University Hospital Birmingham NHS Trust, Selly Oak Hospital, Raddlebarn Road, Birmingham B29 6JD, UK.
authors.forename: M
authors.initials: M
authors.lastname: Roch
authors.forename: J
authors.initials: J
authors.lastname: Williams
authors.forename: P M
authors.initials: PM
authors.lastname: Clark
mesh_terms: D002624:Chemistry, Clinical / Q000379:methods*; D006801:Humans; D007118:Immunoassay / Q000379:methods; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000096:biosynthesis; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis*; D009994:Osmolar Concentration; D015203:Reproducibility of Results
chemical_list: D018972:Insulin-Like Growth Factor Binding Protein 3; D007334:Insulin-Like Growth Factor I