Levels of plasma NFL, plasma CK, serum ferritin and complement C3 protein (C3) were elevated in first-visit samples of ALS patients compared with healthy (NFL ALS 216.70 pg/ml, healthy control 50.2 pg/ml, P < 0.0001; CK ALS 191.3 IU/l, healthy control 101.60 IU/l, P < 0.001; ferritin ALS 121.96 μg/l, healthy control 79.7 μg/l, P < 0.001; C3 ALS 1.3 g/l, healthy control 1.2 g/l, P = 0.021) and disease controls (NFL 65.9 pg/ml, P < 0.0001; CK 125.8 IU/l, P < 0.001; ferritin 91.1 μg/l, P = 0.015; C3 1.2 g/l, P = 0.038). Here, C3 is linked to amyotrophic lateral sclerosis.