C4 determinations are indicated when assessing the course of hypocomplement conditions. C4 deficiency is a hallmark for the following conditions: C1 esterase inhibitor deficiency (which controls the classical pathway) known as Hereditary Angioedema (further investigation into C1IN is needed); cryoglobulaemia. (cryoglobulin testing and hepatitis serology may be indicated); and C4 shares an inverse correlation with anti-dsDNA antibodies in SLE.
