C3 and age-related macular degeneration: The serum C3d/C3 ratio, as a measure of the systemic activity of the complement system, was a significant risk factor for AMD among sporadic patients (OR 1.84; 95% CI 1.40–2.4; p = <0.001)but did not reach significance among familial patients (OR 2.10; 95% CI 1.14–3.87; p = 0.017) after correction for multiple testing.