HOW MUCH INSULIN-LIKE-GROWTH-FACTOR-I (IGF-I) CIRCULATES - IMPACT OF STANDARDIZATION ON IGF-I ASSAY ACCURACY

There is a significant systematic difference between the normal range obtained from ethylenediamine tetraacetate plasma samples using the Ge nentech total insulin-like growth factor I (IGF-I) RIA and normal rang es for other total IGF-I RIAs. To determine whether the quality of the assay standard was the cause of this systematic difference, we analyz ed commercially available preparations of recombinant human IGF-I (rhI GF-I) typical of those used as IGF-I immunoassay standards along with our own well characterized rhIGF-I assay standard. For the commercial standards, high performance liquid chromatography-derived purities wer e low, and some vendor-assigned protein concentrations were inconsiste nt with values from quantitative amino acid analysis. The Genentech rh IGF-I assay standard was highly pure and quantitatively correct. Howev er, the poor quality of some commercial rhIGF-I preparations was not t he primary reason for the systematic discrepancy between the Genentech total IGF-I RIA normal range and most other normal ranges. Most assay s for total IGF-I are calibrated against the WHO International Referen ce Reagent (IRR) for IGF-I Immunoassays (87/ 518). The Genentech total IGF-I RIA is not calibrated against WHO IRR 87/518. The protein conte nt assigned to WHO IRR 87/518 was a consensus value from a multicenter collaborative study. Physicochemical analyses showed that WHO IRR 87/ 518 is Met(-1)-IGF-I of low purity (44%), and that the assigned protei n content is higher than the value determined by quantitative amino ac id analysis. Thus, assays that are calibrated against WHO IRR 87/518 w ill report total IGF-I concentrations in excess of actual values. We b elieve that calibration against WHO IRR 87/518 is the cause of the sys tematic discrepancy between the Genentech IGF-I assay normal range and most other normal ranges, and that much of the plasma IGF-I concentra tion data in the literature are of questionable accuracy.