Complement (C3/C4) are proteins that are part of the immune system. Measuring complement involved a simple blood test that measures the levels of C3 and C4 in the blood.
Measuring complement levels is typically done in autoimmune diseases that affect the levels of complement. For example, in patients with lupus the complement levels can be low when the disease is active.
Science Behind this Test
C3 and C4 Levels
Complement are small proteins that are produced by the liver. The complement system is part of the immune system. It's normal job is to enhance (or complement) the ability of the immune system to clear foreign invaders.
When the immune system becomes activated (with autoimmune diseases) the complement system may become involved in the attack. This can result in a reduction in the levels of complement as it is consumed in the autoimmune attack.