Laboratory Diagnosis of Complement-Mediated Diseases: A Practical Overview

Background Complement is not a niche test area. An expanding group of rare diseases, including hereditary angioedema, atypical hemolytic uremic syndrome, C3 glomerulopathy, and paroxysmal nocturnal hemoglobinuria, now has approved targeted therapies, and the laboratory is central to both diagnosis and treatment monitoring. Yet complement testing remains poorly standardized across institutions, and its preanalytical fragility makes it uniquely susceptible to errors that other immune assays tolerate. Content This review offers a working framework for complement diagnostics in the clinical laboratory. We describe the 3 activation pathways and their key regulators and then outline a tiered diagnostic panel: functional assays of classical pathway activity (CH50), alternative pathway activity, protein quantitation (C3, C4, C1q, complement factor H, complement factor I, and C1 inhibitor antigen and function), and activation products (C3d, C4d, Bb, and sC5b-9), and disease-specific autoantibodies. Diagnostic algorithms are presented for the main complement-mediated conditions, with emphasis on the serological features that distinguish 1 disease from another. Preanalytical requirements receive dedicated attention as tube selection, centrifugation temperature, processing delay, and freeze conditions all materially affect results. A practical approach to laboratory-developed test validation is outlined, and we address the interpretive challenges introduced by approved complement inhibitors (eculizumab, ravulizumab, pozelimab, zilucoplan, danicopan, iptacopan, pegcetacoplan, and sutimlimab), whose effects on routine complement assays are not always straightforward. Future directions, including multiplex platforms and artificial intelligence–assisted interpretation, are briefly considered. Summary A well-structured complement laboratory, based on rigorous preanalytical practices and validated assays, provides diagnostic information not available from other tests. This review could serve as a practical reference for laboratory clinical biochemists building or optimizing complement diagnostics in their setting.