Agreement between circulating IGF-I, IGFBP-1 and IGFBP-3 levels measured by current assays versus unavailable assays previously used in epidemiological studies

- IGF assays used in epidemiological research do not always produce perfectly comparable levels of IGF-I, IGFBP-1 and IGFBP-3.
- There is substantial agreement between measures from different assays (kw; IGF-I: 0.78, IGFBP-1: 0.69, IGFBP-3: 0.76).
- Regression equations can be used to obtain comparable IGF measures for the assays mentioned in this study.
- Such conversions should only be used when absolutely necessary in epidemiological studies.
- Conversions should not be used when assay measures are used to diagnose or monitor treatment in the clinical setting.

ObjectiveLevels of insulin-like growth factor (IGF) proteins are associated with the risk of cancer and mortality. IGF assays produced by Diagnostic Systems Laboratories (DSL) were widely used in epidemiological studies, were not calibrated against recommended standards and are no longer commercially available.DesignIn a split sample study among 1471 adults participating in the Cardiovascular Health Study, we compared values obtained using DSL assays with alternative assays for serum IGF-I (Immunodiagnostic Systems, IDS), IGFBP-1 (American Laboratory Products Company, ALPCO) and IGFBP-3 (IDS).ResultsResults were compared using kernel density estimation plots, quartile analysis with weighted kappa statistics and linear regression models to assess the concordance of data from the different assays. Participants had a mean age of 77 years. Results between alternative assays were strongly correlated (IGF-I, r = 0.93 for DSL versus IDS; log-IGFBP-1, r = 0.90 for DSL versus ALPCO; IGFBP-3, r = 0.92 for DSL versus IDS). Cross tabulations showed that participants were usually in the same quartile categories regardless of the assay used (overall agreement, 74% for IGF-I, 64% for IGFBP-1, 71% for IGFBP-3). Weighted kappa also showed substantial agreement between assays (kw, 0.78 for IGF-I, 0.69 for IGFBP-1, 0.76 for IGFBP-3). Regressions of levels obtained with DSL assays (denoted X) to alternative assays were, IGF-I: 0.52X + 15.2 ng/ml, log-IGFBP-1: 1.01X − 1.73 ng/ml IGFBP-3: 0.87X + 791.1 ng/ml. Serum values of IGF-I, IGFBP-1 and IGFBP-3 measured using alternative assays are moderately correlated.ConclusionsCare is needed in the interpretation of data sets involving IGF analytes if assay methodologies are not uniform.