Fibrinogen

Also written as: factor I, fibrinogen activity, fibrinogen level

The clotting protein that forms the mesh of a blood clot; it also rises with inflammation.

What it measures

The amount or activity of fibrinogen, a protein made by the liver that is converted into fibrin, the mesh that holds a blood clot together. It is also an acute phase protein, meaning the liver makes more of it during inflammation.

Why a doctor orders it

Ordered when unusual bleeding is being investigated, during major bleeding events to guide care, and when other clotting tests give puzzling results. It is sometimes measured as part of assessing inflammation or long-term cardiovascular risk.

What is generally associated with a higher result

Broadly associated with inflammation, infection, recent surgery or injury, pregnancy, and smoking, because fibrinogen rises as part of the body's stress response. Persistently high levels have been studied as a marker of cardiovascular risk.

What is generally associated with a lower result

Broadly associated with the liver making less of it, with rare inherited fibrinogen disorders, and with situations where clotting proteins are used up faster than they are replaced.

Everyday things that move this number

Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:

Any current or recent infection or inflammation
Pregnancy and estrogen-containing medicines, which raise it
Smoking
Recent surgery or injury
Sample handling problems, including a difficult draw or an underfilled tube

Questions to take to your appointment

Could a recent illness or ongoing inflammation be raising this?
Should liver tests be checked alongside it?
Do my other clotting results fit together with this one?

Build a printable questionnaire with your own results →

Where to read more

MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .

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