Session: THR 457-482-Neuroendocrinology
Poster Board THR-458
Objective: IGF-1 was measured by both RIA and mass spectrometry (LC-MS/MS ) in serum samples from patients with pituitary disorders .
Methods: Serum samples prospectively obtained from 101 patients with both naïve and treated secretory and non-secretory pituitary tumors, cysts, hypophysitis,and craniopharyngioma, were evaluated by IGF-1 mass spectrometry (MS) and by Siemens Immulite 2000 IGF-1 assay (reagent lot #512; solid phase, enzyme-labeled chemiluminscent immunometric assay). After sample accrual, samples were submitted for each assay as a single batch.
Results: Although results of the two assays were highly correlated (R2=0.97), a significant positive bias was observed in the RIA compared to the MS assay, reflecting a systematic proportional difference observed between Siemens and LCMS assays, with the Siemens typically reporting higher results than the LCMS. This is reflected in the slope of the relationship observed in Deming regression y=1.57x – 54.0 and linear regression analyses y=1.55x – 48.3, . where y is the Siemens assay value . For classification into normal or abnormal values from each assay, results were interpreted according to their respective reference intervals. IGF-1 MS assay results were interpreted using published age-adjusted ranges (1) and for the Siemens assay, according to age-adjusted ranges provided in the directional insert. Twenty six of 99 (27.2%) samples for which reference ranges were available for both assays, were classified differently (in range, low, or elevated) by the Siemens and the IGF-1 LCMS assay. All 26 disagreements occurred with LCMS in the reference range: 14 were with elevated RIA and 12 with low RIA. LCMS was in the reference range significantly more frequently than RIA, 72.7% versus 46.5% of the cases, McNemar P < 0.0001.
Conclusion: While results of IGF-1 testing with the RIA and LC-MS/MS assays were highly correlated, a strong positive bias with higher results was reported using RIA compared to LC-MS/MS. Twenty seven percent of IGF-1 samples measured by the two assays were differently classified indicating a large degree of inter-assay variability. Inter-assay variability for IGF-1 results is well established and caution should be used when interpreting results of differing methodologies.The accuracy of the IGF-1 LCMS methodology, large reference ranges, and concordance of published LCMS reference ranges with a large published reference population (2 ) reinforces the clinical utility of the mass spectrometry based IGF-1 assay
Disclosure: JDC: Ad Hoc Consultant, Genentech, Inc., Ad Hoc Consultant, Ipsen, Ad Hoc Consultant, Novartis Pharmaceuticals. RER: Coinvestigator, Quest Diagnostics. ALC: Employee, Quest Diagnostics. MJM: Principal Investigator, Quest Diagnostics. MPC: Employee, Quest Diagnostics. SM: Advisory Group Member, Chiasma, Ad Hoc Consultant, ISIS, Principal Investigator, Pfizer, Inc., Ad Hoc Consultant, Novartis Pharmaceuticals, Planning Group Member, Ipsen. Nothing to Disclose: VB, OC, JM
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