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Updated: Sep 26, 2026
Measuring Relative Insulin Secretion using a Co-Secreted Luciferase Surrogate
Published on: June 25, 2019
Measurement of serum 'free' or 'easily dissociable' insulin-like growth factor-1
M Roch1, J Williams, P M Clark
1The Regional Endocrine Laboratory, University Hospital Birmingham NHS Trust, Selly Oak Hospital, Raddlebarn Road, Birmingham B29 6JD, UK.
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).
Discussion:
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.
