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Mass spectrometry-based assays have been increasingly implemented in various disciplines in clinical diagnostic laboratories for their combined advantages in multiplexing capacity and high analytical specificity and sensitivity. It is now routinely used in areas including reference methods development, therapeutic drug monitoring, toxicology, endocrinology, paediatrics, immunology and microbiology to identify and quantify biomolecules in a variety of biological specimens. As new ionisation methods, instrumentation and techniques are continuously being improved and developed, novel mass spectrometry-based clinical applications will emerge for areas such as proteomics, metabolomics, haematology and anatomical pathology. This review will summarise the general principles of mass spectrometry and specifically highlight current and future clinical applications in anatomical pathology.

Immune-checkpoint blockade (ICB) uses antibody targeting of specific inhibitory receptors and ligands. The major limitations of ICB, such as high cost, limited success rate, and immune-related adverse events (irAE), highlight the need for predictive biomarkers. We analyzed preimmunotherapy and post-immunotherapy serum samples of 24 patients treated with pembrolizumab for changes in PD-1 and over 1,000 additional protein markers using a multiplex proximity extension assay (PEA) to identify potential predictive biomarkers of response and/or toxicity. Candidates were selected based on the criteria that at least 2 patients within any of 3 patient groups (responders without irAEs, responders with irAEs, or nonresponders with irAEs) had either a !4-fold increase or 4-fold decrease in expression post-immunotherapy. Female and male control samples were used as technical duplicates. A patient group with no response and no irAEs was used to exclude candidates. Following treatment with pembrolizumab, there was a relative increase of PD-1 in the serum of all patients, compared with controls (average 4.4-fold). We identified 7 additional serum proteins that met our candidate selection criteria. These candidate markers did not have any significant association with response or toxicity to pembrolizumab. Overall, we show that serum PD-1 increases post-therapy with pembrolizumab treatment but has no predictive value for response or toxicity in this small set of patients.