Complement was discovered by Jules Bordet as a heat-labile component of normal plasma that causes the opsonisation and killing of bacteria. The complement system refers to a series of >20 proteins, circulating in the blood and tissue fluids. Most of the proteins are normally inactive, but in response to the recognition of molecular components of microorganisms they become sequentially activated in an enzyme cascade – the activation of one protein enzymatically cleaves and activates the next protein in the cascade.

The complement system plays a critical role in inflammation and defence against some bacterial infections. Complement may also be activated during reactions against incompatible blood transfusions, and during the damaging immune responses that accompany autoimmune disease. Deficiencies of individual complement components or inhibitors of the system can lead to a variety of diseases, which gives some indication of their role in protection against disease.

The Complement Total, or CH50 Blood Test, assesses the overall activity of the complement system, and mainly evaluates the classic complement activation pathway. A CH50 Blood Test is often ordered to evaluate complement component deficiency and evaluate complement activity in cases of immune complex disease, glomerulonephritis (inflammation of the kidneys filters), rheumatoid arthritis and cryoglobulinemia (abnormal proteins in the blood that thicken in cold temperatures). It can also be used to evaluate a persons response to systemic lupus erythematosus (SLE) therapy and predict disease flares.

This test is usually ordered for people with a family history of complement deficiency and those who have symptoms of:

- RA

- kidney disease

- lupus, which is a multisystem autoimmune disease

- myasthenia gravis

- an infectious disease, such as meningitis

- cryoglobulinemia, which is the presence of abnormal proteins in the blood