A complement C4 (also known as a complement component 4, or simply C4) is a test to measure the level of C4 proteins you have in your blood. These proteins play a role in how your immune system ...

Levels of C4d and C4d/C4 associate with clinical and histopathological treatment response in proliferative lupus nephritis. Complement activation markers, particularly C4d and the C4d/C4 ratio, are ...