• 530.541.3420 | 2170 South Avenue, S. Lake Tahoe, CA

Factor I

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 called fibrinogen in your blood. 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 the blood tests PT and PTT that suggest a clotting problem.

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

  • You are being screened for liver disease.

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

  • You have a family history of a bleeding or clotting disorder. 

What other tests might I have along with this test?

Your doctor may order other tests to evaluate blood clotting. You may also take other kinds of fibrinogen tests.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

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 associated with:

  • Infections and inflammation

  • Cancer

  • Rheumatoid arthritis

  • Heart attack

  • Stroke

  • Pregnancy 

Low fibrinogen values may be associated with:

  • Liver disease

  • DIC

  • Malnutrition

  • Inherited or congenital blood clotting disorders

  • Frequent blood transfusions

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm. 

Does this test pose any risks?

Taking a blood sample with a needle carries some small risks. These include bleeding, infection, and bruising. You may also feel lightheaded. When the needle pricks your arm, you may feel a slight pain or sting. Afterward, the site may be sore.

What might affect my test results?

If you are taking the drug heparin, it could interfere with your test results. If you are taking oral contraceptive drugs or estrogen, it could increase your fibrinogen level. Vigorous exercise may cause a temporary rise in fibrinogen. Drugs that may lower fibrinogen levels include streptokinase, valproic acid, phenobarbital, and anabolic steroids.

Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.

How do I get ready for this test?

You may be asked to avoid vigorous muscular exercise for a few days before this test.