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Labcorp
Broadest draw-site network nationwide.
Quest Diagnostics
Same-day appointments in most metros.
Complement C4 is a blood protein that is part of the complement system, which supports the immune response by helping to fight infections and remove damaged cells. A C4 test is often used to help diagnose or monitor autoimmune conditions like systemic lupus erythematosus (SLE) and hereditary angioedema.
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