Background: The recently published Consensus on Criteria for Cure of Acromegaly (Giustina et al. JCEM, 2010) highlighted concerns regarding the quality of currently available insulin-like growth factor 1 (IGF1) immunoassays which may contribute, at least in part, to the discordance between GH and IGF1 that is observed in up to 30% of patients with acromegaly after treatment. The development of mass spectroscopy (MS)-based technology has been proposed as a potential solution to these limitations.
Methods: Here, we report the development of a stable isotope dilution ultra-performance liquid chromatography tandem MS (UPLCMS/MS)-based method for the quantitation of serum IGF1. The method employs selected reaction monitoring (SRM) of two tryptic peptides derived from IGF1, and relies on solid phase extraction for enrichment of the peptide fraction containing IGF1 rather than immunocapture, so is less susceptible to antibody interference. The UPLC separation of the peptides was performed using a C18 column prior to MS/MS analysis on a 5500 QTrap MS. The method is not affected by concentrations of IGFBP3 up to 420 nmol/l.
Results: The method showed good correlation with an IGF1 immunoassay (Siemens Immulite 2000) over a wide range of serum IGF1 concentrations (5.4261 nmol/l by immunoassay) in a cohort of 45 patients that included 25 subjects with acromegaly, assessed both before and after primary medical therapy. The Passing and Bablock regression was: LCMS/MS (nmol/l)=1.4×Immunoassay (nmol/l)+4.4. Analysis of UKNEQAS material with an immunoassay method mean of 22.2 and 45 nmol/l returned values of 22.3 and 47.6 nmol/l respectively.
Conclusions: This method relies on entirely different physicochemical principles to the ubiquitous sandwich immunoassay for IGF1, and thus provides an independent validation of suspicious immunoassay-derived results. A further advantage of the method is that, with the addition of appropriate internal standards, there is potential for extensive multiplexing of serum peptide assays.