Fibrinogen

Fibrinogen (also called clotting factor I) is a large protein your liver makes that turns into fibrin, the mesh that forms a blood clot. It is also an 'acute-phase reactant', meaning it rises with inflammation. Labs measure it with the Clauss clot-based test or an immunoassay, and the usual range is 200 to 400 mg/dL (2.0 to 4.0 g/L). Beyond clotting, your fibrinogen level is an independent heart-risk marker. A large pooled analysis of 154,211 adults (the Fibrinogen Studies Collaboration, JAMA 2005) found that every 1 g/L higher fibrinogen carried about a 2.4-fold higher risk of coronary heart disease (95% CI 2.3 to 2.6), with similar links to stroke and non-vascular death. Watch the confounders, though: infection or injury push it up within a day or two, as do smoking, pregnancy, oral estrogen, cancer, and chronic inflammation. It drops in conditions like widespread clotting (DIC), severe liver disease, clot-busting therapy, and the inherited low-fibrinogen disorder.

Last reviewed: June 22, 2026

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