C3 and systemic lupus erythematosus: 15/19 (79%) had a history of LN and experienced a “lupus flare”, defined as a sustained increase in urinary protein excretion by more than 25% of the baseline value, associated with a significant decrease in serum C3 levels and/or increase in the serum anti-dsDNA antibody titer, and/or other signs of active SLE.