Serum Insulin-like Growth Factor I Quantitation by Mass Spectrometry: Insights for Protein Quantitation with this Technology

Liquid chromatography mass spectrometry (LC-MS) is a widely used technique in the clinical laboratory, especially for small molecule quantitation in biological specimens, for example, steroid hormones and therapeutic drugs. Analysis of circulating macromolecules, including proteins and peptides, is largely dominated by traditional enzymatic, spectrophotometric, or immunological assays in clinical laboratories. However, these methodologies are known to be subjected to interfering substances, for example heterophilic antibodies, as well as subjected to non-specificity issues. In recent years, there has been a growing interest in using LC-MS platforms for protein analysis in the clinical setting, due to the superior specificity compared to immunoassay, and the possibility of simultaneous quantitation of multiple proteins. Different analytical approaches are possible using LC-MS-based methodology, including accurate mass measurement of intact molecules, protein digestion followed by detection of proteolytic peptides, and in combination with immunoaffinity purification. Proteins with different complexity, isoforms, variants, or chemical alteration can be simultaneously analysed by LC-MS, either by targeted or non-targeted approaches. While the LC-MS platform offers a more specific determination of proteins, there remain issues of LC-MS assay harmonization, correlation with current existing platforms, and the potential impact in making clinical decision. In this review, the clinical utility, historical aspect, and challenges in using LC-MS for protein analysis in the clinical setting will be discussed, using insulin-like growth factor (IGF) as an example.