C3 and systemic lupus erythematosus: In this case, the diagnosis of SLE was based on 2019 EULAR/ACR criteria,[2] which included the following: (1) leukopenia; (2) thrombocytopenia; (3) pleural effusion; (4) proteinuria; (5) high-titer ANA; (6) positive anti-cardiolipin-IgM, anti-β2 GP1 antibodies, and lupus anticoagulant; and (7) low C3 and C4 levels.