Advances in analytical methods for regulatory complement proteins in research and clinical applications

The canonical complement system and its dysregulation are increasingly realized to be associated with pathologies like age-related macular degeneration, atypical hemolytic uremic syndrome, or dense deposit disease. Resulting drug development, clinical studies, and expanding fundamental research into the complement system and its regulatory proteins all require appropriate analytical methods with which the function of the complement system can be quantified. In clinical settings, two types of routine diagnostics are mostly used. Functional assays such as the hemolytic CH50 and AH50 tests are based on erythrocytes and typically monitor complete activation pathways. Assays that quantify certain complement proteins and activation products are typically based on turbidimetry, nephelometry, and ELISAs. However, there is the need and demand for more specific examination to also identify variants with defective functions that show no deviation from normal concentrations. New diagnostic test developments are therefore moving towards faster, simpler, and more standardized tests for application in both clinical laboratories and high-throughput facilities, with the ultimate goal of precision and personalized medicine. In recent years, significant progress has been made, particularly in functional assays for fluid-phase and in flow cytometry for membrane-bound regulatory complement proteins. Functional ELISAs and liposome assays may soon replace traditional hemolytic assays, while genetic analysis has yielded important new insights into complement-related diseases; overall, there is a strong trend toward multiplexing. This review discusses current advances in analytical methods of regulatory complement proteins, covering commercial assays from industry and newly developed assay strategies from research, and assesses their impact on the field.