Complement & Cryoglobulins: The Overlooked Labs That Connect Lupus, HCV, and Vasculitis
Complement & Cryoglobulins: The Overlooked Labs That Connect Lupus, HCV, and Vasculitis Low C4 isn't always lupus. A false-negative cryoglobulin is almost always a specimen-handling error. Here's how to get these right. Complement levels and cryoglobulin testing sit at the intersection of rheumatology, hepatology, hematology, and nephrology. They're ordered less frequently than ANA or RF, but when you need them, getting them right is critical—and getting the cryoglobulin specimen wrong means the test is useless before it even reaches the lab. Part 1: Complement (C3 and C4) What Are C3 and C4? C3 and C4 are proteins in the complement cascade—part of the innate immune system that helps clear pathogens and immune complexes. When complement is "consumed" by active immune complex formation, serum levels drop. This consumption is a hallmark of active autoimmune disease, particularly SLE. C3 — Central to both the classical and alternative complement pathways. Low ...