I searched through the forum, and see most posts regarding this talk about people having low levels. Mine were literally a point or four from being over-range. I was being tested for all things autoimmune. What does that mean in the grand scheme of things?
Here's a study you might find interesting. Usually complement C3 and C4 are measured as biomarkers for lupus, if they are low. This study finds a link between C3 and C4 levels and metabolic syndrome, associated with obesity, hypertension, high triglycerides and risk of diabetes. Not to say this is the only possible reason for elevated C3 and C4. Other diseases known to be associated with elevated C3 and C4 are cancer and ulcerative colitis. However, the most likely answer for labs in the normal range is that they are normal!