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Inter-laboratory comparison of IGF-I concentrations measured by an automated immunoassay: Results from a multicentre study across Europe
Introduction: Measurement of Insulin-like Growth-factor I (IGF-I) is key in diagnosis and monitoring of Growth Hormone (GH) related diseases. Considerable variability between results from different assays, but also between different laboratories using the same assay was a major concern in the past. Recently, we reported age- and sex-specific reference intervals for IGF-I measured by a new automated assay (IDS-iSYS). We now evaluated the between laboratory agreement of measurements of IGF-I using this assay in laboratories across Europe.
Methods: 52 samples (serum; normal n = 20, GH deficiency/GH excess n = 20, QC samples n = 12) were aliquoted, distributed to participating laboratories (n = 13) and measured using one batch of the IDS-iSYS IGF-I assay. Results were reported in mass units (ng/mL). SD scores based on the reference intervals were calculated by the LMS formula in 6 labs.
Results: IGF-I concentrations ranged from 13 to 1056 ng/mL. Bland Altman plots show a bias between individual laboratories results (ILR) and the all participants mean (APM) between -3.44% and +4.74% (mean -0.10%) for serum samples and between -7.98% and +6.18% (mean -0.25%) for QC samples. 95% of all results (concentrations) were within +/-8.6% of the APM. The respective slopes for the correlation between ILR and APM in Passing Bablok analysis for concentrations range from 0.963 to 1.045 (serum samples) and 0.961 to 1.049 (QC samples). Corresponding SD scores ranged from -4.18 to +6.79 and were highly correlated between labs (0.974 to 1.037).
Conclusion: Our data demonstrate very good agreement of IGF-I concentrations measured by the IDS-iSYS assay in laboratories across Europe, especially in real patients' samples. SD scores calculated by the same mathematical method also exhibit strong agreement between laboratories. Harmonisation of assay reagents and statistical methods to calculate SD scores is key to reduce between laboratory differences in determination and interpretation of IGF-I data.