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Blood Test Tracker

Fibrinogen Activity

Measures how much working fibrinogen, the protein that forms the mesh of a blood clot, is in your blood.

AI-verified against cited sources. Awaiting expert reviewLOINC 3255-7

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What it is

Fibrinogen (clotting factor I) is a protein made by your liver. When you bleed, it is turned into fibrin, which forms the mesh that holds a clot together. The most common lab test, called the Clauss method, measures how well your fibrinogen forms a clot. Fibrinogen also rises when you have inflammation, so it is called an acute-phase protein.

Why it matters

Too little fibrinogen can cause bleeding that is hard to stop, which is why it is checked in bleeding problems, liver disease, DIC and before some procedures. Higher levels usually reflect inflammation or pregnancy. In large studies, people with higher fibrinogen had more heart attacks and strokes, but it is not used to predict one person's risk, and it is not known whether lowering it would help.

If it’s low

A low fibrinogen means the blood may form clots more slowly or less firmly. Levels fall when the body uses fibrinogen faster than the liver can make it, or when the liver cannot make enough. Very low levels raise the chance of bleeding.

Common causes

  • Liver disease, which reduces production
  • DIC (disseminated intravascular coagulation) or heavy bleeding, which use fibrinogen up
  • Clot-dissolving (thrombolytic) treatment
  • L-asparaginase, a cancer medicine
  • Inherited fibrinogen disorders (rare)

What can help

  • Ask your clinician about the likely cause and whether more tests, such as a repeat or a fibrinogen antigen test, are needed
  • Mention any bleeding problems or a family history of bleeding

If it’s high

A higher-than-usual fibrinogen is common and often reflects inflammation or a temporary state. By itself it does not point to a specific condition.

Common causes

  • Pregnancy
  • Infection or other inflammation, including long-term inflammatory conditions
  • Smoking
  • Recent surgery or injury

What can help

  • Ask your clinician whether an inflammation test such as CRP would help explain a high result
  • Ask whether the test should be repeated when you are well, since illness can raise fibrinogen for a while
  • Not smoking and staying active support heart health in general, although no treatment is proven to lower fibrinogen in order to prevent disease

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 200–393 mg/dL. Source: Mayo Clinic Laboratories (Extended Catalog), ARUP Laboratories, Cleveland Clinic Laboratories, Biochemia Medica, BioMed Research International, JAMA (Fibrinogen Studies Collaboration). Labs treat below 100 mg/dL as urgent.

What can affect your result

  • Pregnancy: levels rise through each trimester
  • Infection, inflammation and recent surgery or injury, which raise it
  • Smoking
  • Liver disease, DIC and clot-dissolving treatment, which lower it
  • The method (Clauss or PT-derived) and the reagent
  • Recent transfusion of fibrinogen-containing products
  • The method matters. The Clauss (functional) test is the most reliable for routine use. A fibrinogen calculated from the PT clotting curve is not recommended for routine use and can mislead in some conditions.
  • DIC and clot-dissolving (thrombolytic) treatment can make results differ between methods and reagents.
  • Cloudy (lipemic) or hemolyzed samples can interfere on some analyzers, although many labs adjust the dilution or wavelength to avoid this.
  • A recent fibrinogen concentrate, cryoprecipitate or plasma transfusion raises the level.
  • The sample needs a citrate tube filled to the line and prompt processing. Mayo accepts room-temperature plasma for 24 hours.

A snapshot of the level at the time of the draw. As an acute-phase protein it rises with inflammation and falls as the inflammation settles.

When to talk to a clinician

Get urgent medical care for bleeding that will not stop, blood in your urine or stool, black stools, vomiting blood, or a severe headache after a head injury, whatever your fibrinogen is. ARUP's list of critical results includes fibrinogen under 100 mg/dL, and Croatian national recommendations say that under 0.8 g/L (80 mg/dL) suggests a higher bleeding risk, so a lab may call you or your clinician about a result that low.

Questions you could ask

  • Which method did my lab use, and can I compare this result with earlier ones?
  • Could inflammation, an infection or pregnancy explain a high fibrinogen, and should we repeat it when I am well?
  • Would another test, such as CRP, help put this result in context?
  • What do you think explains my low fibrinogen, and do I need more clotting tests?
  • Does this change anything before a procedure or surgery?
  • Should my family history of bleeding be looked into?

Questions people ask

Why do labs give different usual ranges for fibrinogen?

Labs use different reagents and machines, and some use a slightly different method. That is why Mayo, ARUP and other labs publish somewhat different ranges. Use the range printed on your own report.

Is a high fibrinogen a heart risk?

In large studies, higher fibrinogen goes along with more heart attacks and strokes. But it also rises with inflammation and smoking, and researchers have not shown that fibrinogen itself causes the extra risk. It is not used on its own to judge your heart risk.

What is the difference between mg/dL and g/L?

They are the same measurement in different units. Multiply g/L by 100 to get mg/dL, so 3.0 g/L is 300 mg/dL.

Show the science and sources

Units

  • 1 mg/dL = 1 mg/dL
  • 1 mg/dL = 0.01 g/L

Open the Fibrinogen unit converter

Is a change real?

Fibrinogen naturally varies about 10.2% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 32% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Systematic review and meta-analysis of within-subject and between-subject biological variation data of coagulation and fibrinolytic measurands (Hollestelle MJ et al.).

For clinicians and researchers

Fibrinogen (factor I) is a liver-synthesized glycoprotein cleaved by thrombin to fibrin, and it is an acute-phase reactant. The functional Clauss assay adds a high concentration of thrombin to diluted plasma and converts the clotting time in seconds to g/L using a calibration curve traceable to an international standard. It is the most widely used and most reliable method. The PT-derived method estimates fibrinogen from the PT clotting curve and is not recommended for routine use. Immunoassays measure antigen and are used mainly with the functional result to evaluate dysfibrinogenemia (activity lower than antigen). Reagent effects are small for the Clauss assay but can appear in DIC and during thrombolytic therapy. Reference intervals are method- and reagent-dependent, and the Croatian recommendations quote about 1.8–4.0 g/L for healthy adults. Critical limits are not standardized (ARUP under 100 mg/dL; Croatian recommendations under 0.8 g/L). In the Fibrinogen Studies Collaboration (154,211 participants, 31 prospective studies) the age- and sex-adjusted hazard ratio per 1 g/L higher usual fibrinogen was 2.42 for CHD and 2.06 for stroke, about 1.8 after adjusting for other risk factors, with no threshold. The authors noted that the causal relevance of fibrinogen to disease still needs research.

LOINC 3255-7 (primary), 42568-6, 48664-7. Specimen: plasma.

Sources

  1. Fibrinogen, Plasma (test FIBTP), Mayo Clinic Laboratories (Extended Catalog)
  2. Fibrinogen, Clauss, Plasma (test CLFIB), Mayo Clinic Laboratories (Extended Catalog)
  3. Fibrinogen (ARUP test 0030130), ARUP Laboratories
  4. Critical Values List (CORP-APPEND-0104A, Rev. 46, April 2026), ARUP Laboratories (2026)
  5. Croatian Society of Medical Biochemistry and Laboratory Medicine: National recommendations for blood collection, processing, performance and reporting of results for coagulation screening assays prothrombin time, activated partial thromboplastin time, thrombin time, fibrinogen and D-dimer, Biochemia Medica (2019)
  6. Reference Values of D-Dimers and Fibrinogen in the Course of Physiological Pregnancy: the Potential Impact of Selected Risk Factors-A Pilot Study, BioMed Research International (2020)
  7. Plasma fibrinogen level and the risk of major cardiovascular diseases and nonvascular mortality: an individual participant meta-analysis, JAMA (Fibrinogen Studies Collaboration) (2005)
  8. Fibrinogen (test directory entry), Cleveland Clinic Laboratories

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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