The complement system is a set of blood proteins that are circulating, acting as mediators of the body's inflammatory response. They act especially in the destruction of viruses and bacteria. The complement system does not require previous exposure to a microorganism like antibodies do. It also does not keep a memory of encounters the way antibodies do. Antibodies also need time to be generated, the complement system does not. For these reasons the complement system is a very important part of the immune system and imbalances can adversely affect the integrity and function of the immune system.
Since the complement system is used by the body to help clear immune complexes from the blood, it can be used to monitor activity in Lupus, particularly with glomerulonephritis and vasculitis.
Normal levels vary based on a number of factors including gender, age, and the lab doing the testing. Here is a basic guideline:
C3 - males: 88 to 252 mg/dl (880 to 2520 mg/L)
females: 88 to 206 mg/dl (880 to 2060 mg/L)
C4 - males: 12 to 72 mg/dl (120 to 720 mg/L)
females: 13 to 75 mg/dl (130 to 750 mg/L)
You are correct in feeling that low complement levels in Lupus usually means increased disease activity. When the body uses the complement system to clear immune complexes from the blood it results in low complement levels. Essentially, the complement proteins are not being produced as quickly as they are being used. This test is done to monitor the level of your activity in Lupus as well as to aid in diagnosing Lupus.
I hope that I have answered your question. Please let me know if you need anything further.
Peace and Blessings