Use

Complement C3c and C4c testing measures the levels of two key complement pathway proteins—C3 and C4—in the blood. Decreased levels of both C3 and C4 suggest activation of the classical complement pathway, whereas isolated C3 decreases may imply alternative pathway activation. These components are commonly used to monitor disease activity in systemic lupus erythematosus (SLE) and immune complex‑mediated vasculitis.