Five commercially available insulin-like growth factor I (IGF-I) assays in comparison to the former Nichols Advantage IGF-I in a growth hormone treated population

BACKGROUND: The serum insulin-like growth factor I (IGF-I) level is accepted to diagnose the growth hormone (GH) status. Here, we evaluated the DRG IGF-I 600 ELISA, DSL IGF-I ELISA, IDS OCTEIA IGF-I, Mediagnost IGF-I-ELISA, and the Siemens Immulite 2500 IGF-I in comparison to the former Nichols Advantage IGF-I assay. METHODS: Imprecision was determined by use of a serum pool and commercial control materials. Accuracy was evaluated by means of a method comparison to Nichols in 173 serum samples of GH deficient patients. RESULTS: The Siemens and the IDS IGF-I assays showed the lowest imprecision with coefficients of variation up to 3.6% and 6.9%, respectively. Both correlated best to Nichols (Siemens: y=0.667X+8.8 microg/L, r=0.950; IDS: y=0.527 X+4.6 microg/L, r=0.927) with the lowest dispersion of residuals from a linear equation. The DSL assay had the highest comparability to Nichols (y=1.000 X+35.5 microg/L, r=0.864), but with a considerable scattering. CONCLUSIONS: To yield IGF-I determination comparable to the former Nichols IGF-I, either the Siemens or the IDS assay should be applied, and the results should be converted by a linear method transformation. Where a conversion factor is not desired, the DSL assay should be selected.