A C3 test measures certain proteins in your bloodstream that work with your immune system. There are nine of these proteins (C1-C9), but C3 and C4 are the most commonly measured. Changes in the levels of these proteins can be an early warning sign of an inflammatory process somewhere in the body.
C3 rises in conditions of chronic inflammation, including coronary artery disease and asthma, and even emotional conditions such as sustained anger, hostility and depression.
Measurement of complement proteins can be used to monitor treatment results in patients with autoimmune diseases. Lupus erythematosus can cause a C3 level below normal, while rheumatoid arthritis may cause it to rise in the blood but fall in the joint fluid.
Elevated C3 levels have been found in patients with diabetes and insulin resistance, but whether an elevated C3 means an increased risk of developing diabetes is still unknown.
A C3 test can also be used in fungal infections, generalised infections such as septicaemia ("blood poisoning") and diseases caused by parasites, such as malaria.