C3 and C4 are measured at the same time since this gives an indication of the complement pathway (classical or alternative) which is being activated and thus the cause of this activation.
C3 alone is often decreased in infectious disease (septicaemia, endocarditis), so not v interesting if isolated, unless concern about immunodeficiency (see below).
C4 alone is characteristically decreased in hereditary angioedema, can also be immune complex diseases particularly vasculitis, and in cryoglobulinaemia and cold agglutinin disease. Immune complex diseases can lead to consumption of both C3 and C4, with low levels.
Measurement of serum complement is useful in the monitoring of specific immune complex diseases eg SLE, post streptococcal disease, subacute bacterial endocarditis. Consumption of one or both components may also be useful prognostically eg nephritis in lupus.
Complement deficiency is a type of immunodeficiency. Though genetic C3 deficiency is v rare, deficiencies in other components (which are more common, though still very rare) can result in low C3. CH50 or CH100 are better tests of whole pathway.
Genetic deficiencies in C4 are rarely detected.