Which chronic polyarticular diseases are most likely to be associated with low serum complement levels?
SLE and some of the vasculitis syndromes. Low serum complement levels (C3, C4, and total hemolytic complement) usually suggest the presence of an immune complex disease. In SLE, cryoglobulinemia (especially hepatitis C), and some diseases associated with vasculitis (infective endocarditis, urticarial vasculitis), immune complexes often activate the complement cascade, resulting in consumption of individual complement components. In many instances, the liver is unable to produce these components as rapidly as they are consumed, resulting in a fall in serum levels.