A complement test uses a blood sample to detect a group of proteins that help the body attack foreign substances.

When there are a lot of foreign substances in the body, such as bacteria or viruses, a low level of complement means the body is trying to get rid of the foreign substances. If the body is attacking its own tissues instead of foreign substances (as in autoimmune diseases such as lupus), a low level of complement may mean that the body is attacking and damaging tissues such as the kidneys.

The complement test can be repeated at regular intervals to monitor disease progression.