Immunoaffinity LC–MS/MS Quantification of the Sepsis Biomarker Procalcitonin Using Magnetic- and Polystyrene-Bead Immobilized Polyclonal Antibodies

Procalcitonin (PCT) is a biomarker for bacterial sepsis, and accurate quantification of PCT is critical for sepsis diagnosis and treatment. Immunological PCT quantification methods are routinely used in clinical laboratories, yet there is a need for harmonization of PCT quantification protocols. An orthogonal method to clinical immunological assays, such as LC-MS/MS, is required. In this study, a highly sensitive and robust immunoaffinity LC-MRM quantitative method for detecting procalcitonin in human serum has been developed. An initial comparison of immunocapture of PCT with a polyclonal anti-PCT antibody immobilized on polystyrene nanoparticles (Latex) and magnetic beads demonstrated superior performance with magnetic beads. Three tryptic PCT peptides from the N- and C-terminal regions of PCT were selected for LC-MS/MS quantification. For PCT quantification, an LLOQ of 0.25 ng/mL of PCT in human serum was achieved using a sample volume of 1 mL. The method's trueness and precision consistently lie within the 15% margin. The parallel measurement of three PCT peptides may allow future differentiation of intact PCT vs other PCT forms originating from potential degradation, processing, or polymorphisms. An established and validated LC-MRM-based quantification of PCT will be relevant as an orthogonal method for harmonization and standardization of clinical assays for PCT.