Does this test have other names?

Factor I (fibrinogen), serum fibrinogen, functional fibrinogen

What is this test?

This test measures the level of a protein in the blood called fibrinogen. It helps find out whether you have a bleeding or blood clotting disorder. Fibrinogen is an important protein made by your liver. If you have bleeding anywhere in your body, fibrinogen is released from your liver and travels to the site of bleeding to help form a blood clot. Fibrinogen is also called coagulation factor I. 

Why do I need this test?

You may need this test if you bleed too much or for too long. You may also need this test if you are forming blood clots abnormally. Too little fibrinogen can cause prolonged bleeding. But too much fibrinogen can cause you to form clots inside blood vessels. These clots could break loose and travel to your brain or your lungs, putting your life in danger. Other reasons for ordering this test may include:

• You have abnormal results on other tests for blood clotting, such as prothrombin time (PT), partial thromboplastin time (PTT), or thrombin clotting time (TCT), that suggest a clotting problem.

• You have symptoms of a disease called disseminated intravascular coagulation (DIC) in which the clotting proteins are too active.

• You are being screened for liver disease.

• You need to be watched while you are taking medicine to prevent blood clots.

• You have a family history of a bleeding or clotting disorder such as factor I deficiency. 

What other tests might I have along with this test?

Your healthcare provider may order other tests to evaluate blood clotting. You may also have other kinds of fibrinogen tests.

What do my test results mean?

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

Fibrinogen is measured in milligrams per deciliter (mg/dL):

• A normal value for fibrinogen is between 200 and 400 mg/dL.

• A fibrinogen value of less than 50 mg/dL may mean you're in danger of bleeding after surgery.

• A fibrinogen value of more than 700 mg/dL may mean you're in danger of forming clots that could harm your heart or brain. 

High fibrinogen values may also be linked with:

• Infections and inflammation

• Cancer

• Rheumatoid arthritis

• Nephrotic syndrome

• Heart attack

• Stroke

• Pregnancy 

Low fibrinogen values may be linked with:

• Liver disease

• DIC

• Cancer

• Malnutrition

• Inherited or congenital blood clotting disorders

• Frequent blood transfusions

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand.

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

What might affect my test results?

If you are taking the medicine heparin, it could interfere with your test results. If you are taking birth control pills or estrogen, it could increase your fibrinogen level. Vigorous exercise may cause a temporary rise in fibrinogen. Medicines that may lower fibrinogen levels include streptokinase, valproic acid, phenobarbital, and anabolic steroids.

How do I get ready for this test?

You may be asked to not do vigorous muscular exercise for a few days before this test. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.