LC–MS/MS for protein and peptide quantification in clinical chemistry

The LC-triple quadrupole mass spectrometer (LC–MS/MS) is an increasingly common tool in the clinical laboratory. Established applications include routine assays for detecting inborn errors of metabolism, and for monitoring therapeutic drugs and steroids. Peptides and proteins in biological matrices have traditionally been quantified by immunological methods such as RIA or ELISA. These methods have the drawback of being insufficiently selective, often not allowing differentiation between the peptide and its derivatives or degradation fragments. The improved robustness and sensitivity of LC–MS-based techniques provide reliable alternatives for peptide quantification. Mass spectrometry does not require specific antibody reagents and is a powerful tool for the study of posttranslational modifications (PTM). In addition, several studies have demonstrated the utility of selected reaction monitoring (SRM) assays using stable-isotope-labelled (tryptic) peptides for quantifying proteins in human serum. Peptide-based MS/MS is a relatively new development in the measurement of clinically significant proteins, offering cost effectiveness, high throughput, multiplexed analysis and quantification, with the potential for combining the measurement of small molecules, peptides and proteins on a single technology platform. Quantitative analysis of proteins and peptides by LC–MS/MS is becoming a practical technique for clinical laboratories. To move from the laboratories of highly skilled analysts to routine clinical diagnostic laboratories requires that a number of technical hurdles be overcome in regard to sensitivity, imprecision, accuracy and the sample handling necessary for clinical use.