The diagnosis is ultimately based on cutaneous histopathology, but suggested laboratory studies include a complete blood count, serum creatinine, C-reactive protein (C-RP), erythrocyte sedimentation rate (ESR), urinalysis, complement studies (C1q, C3, C4), anti-C1q antibody assays and tests for underlying connective tissue disease or viral infection. Here, C3 is linked to viral infectious disease.