What Is a Fibrinogen Blood Test?

Medically Reviewed by Dan Brennan, MD on May 16, 2021

The fibrinogen blood test is used to assess how well the protein called fibrinogen — also called coagulation factor I — performs in the blood and to measure its levels in your blood. The test is also called the factor I (fibrinogen), serum fibrinogen, and functional fibrinogen test.

Fibrinogen is produced by the liver and is important for blood clotting. It is a protein that helps stop bleeding and support wound healing by forming clots at the site of bleeding wherever it is on your body. 

There are two kinds of fibrinogen blood tests:

  1. Fibrinogen activity test: ‌This test looks at how well your fibrinogen functions by looking at how long it takes for a blood clot to form. If it takes too long, it could mean that your fibrinogen is not working well or that its levels are lower than they should be
  2. Fibrinogen antigen test: ‌This test is used to measure the level of fibrinogen in your blood. 

When Do You Need to Take a Fibrinogen Blood Test?

Your doctor will usually recommend a fibrinogen blood test if they think you have a bleeding disorder or a blood clot in your blood vessel preventing blood from freely flowing.

Sometimes, it is also used to find out whether you have a chance of getting heart disease. This is because a high fibrinogen level can show an increased chance of having heart disease and stroke.

Generally, if you have the following problems, your doctor will ask you to get this test done:

  • ‌You show signs of excessive bleeding, for example, you bruise easily or you have frequent nosebleeds or bleeding gums
  • ‌You have a family history of bleeding disorders or clotting disorders
  • ‌You are being tested for liver disease
  • ‌You are showing signs of a condition called disseminated intravascular coagulation (DIC)
  • ‌You are showing abnormal results on other blood clotting tests
  • ‌You need someone to keep an eye on you in case you develop blood clots after taking medication

Your chances of getting blood clots are higher if you: 

  • Had surgery recently and are recovering
  • Have been sitting in one position for long periods like on a plane
  • Have diabetes or cholesterol
  • Are overweight or obese
  • Are above 60 years of age

Test Procedure

For the test, you’ll be asked to provide a sample of your blood. Your health care practitioner will inform you if you need to follow any specific instructions such as stopping a medicine before you appear for the test.

Talk to your practitioner about all the medication, vitamins, and supplements you are taking. If you’re taking any herbs or other natural supplements, these need to be discussed also. 

During the test, blood will be drawn from a vein in your arm using a needle attached to a syringe. You will feel a pricking sensation when the needle goes in but it won’t be too painful and won’t last for long. The entire process is quick, usually getting over in just a few minutes. 

If you feel uncomfortable about drawing blood or needles and syringes in general, talk to your health care practitioner so they can help you feel more comfortable doing the test. 

Once the test is over, you’ll be informed when you can expect the results and how you’ll get them.

Possible Side Effects of the Test

A fibrinogen blood test is simple and quick, and it usually has no major side effects. The amount of blood taken from you is also very small. 

You may sometimes feel dizzy or lightheaded afterward. You may also have minor pain or slight bruising at the site — both of which are harmless. These symptoms usually go away within a few days.

What Do the Test Results Mean?

The normal fibrinogen levels for an adult range between 200 and 400 milligrams/deciliter. ‌If they are more than 700 milligrams/deciliter, you may have a higher chance of getting blood clots that can travel to your brain, lungs, or heart, ultimately causing damage. Additionally, you may have an infection, inflammation, cancer, arthritis, kidney disease, heart attack, or stroke or you may even be pregnant.

If your fibrinogen levels are less than 50 milligrams/deciliter, you may have a higher chance of bleeding excessively after surgery. You may even have liver disease, cancer, malnutrition, DIC, blood clotting disorders either inherited or congenital (present at birth), and frequent blood transfusions.     

Show Sources


Medscape: "Fibrinogen." 

MSD Manual Consumer Version: "Fibrinogen."

National Library of Medicine: " Fibrinogen as a risk factor for coronary heart disease." 

NHS: "Blood tests." 

University of Rochester Medical Center: "Factor I." 

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