Clinical peptide and protein quantification by mass spectrometry (MS)

MS quantitation of peptides/proteins has been slow to be adopted by clinical laboratories. The reasons are: (i) lesser perceived need for assay improvement of protein/peptide immunometric immuonassays (IAs) compared to competitive IAs, (ii) increased demands peptides/proteins place on mass accuracy/resolution, (iii) limitations of current instrumentation, (iv) stickiness of peptides/proteins, (v) large number of serum peptides/proteins and their broad range of concentrations. However, MS can (i) facilitate assay harmonization, (ii) identify clinically relevant protein-isoforms, (iii) solve the antibody interference and “hooking” artefacts of immunometric IAs, and (iv) facilitate biomarker verification/validation. Consequently, the last 10–15 years have seen steady growth in peptide/protein MS assay-development. Common protocols are emerging; most can be performed on triple quadrupole instruments. Affinity enrichment, with antibodies against analyte or proteotypic peptides, is commonly used. Assay performance characteristics are similar to IAs. The future will see standardization of assay validation/quality-control requirements and increasing use of multi-target assays.