Generality

Fibrin is a plasma protein that participates in the formation of blood clots . Its blood concentrations increase in all those circumstances, specific or non-specific, associated with or characterized by fibrin formation and fibrinolysis.

The evaluation of fibrin and related parameters (D-dimer, prothrombin time, fibrinogen plasma concentration, etc.) is particularly useful to exclude or confirm the presence of pathologies of excessive or inappropriate coagulation .

What's this

Fibrin and blood coagulation

Fibrin (factor Ia) is an insoluble fibrillar protein, present in the blood in an inactive precursor form, fibrinogen (factor I), which is instead soluble in water.

The genesis of fibrin from fibrinogen (which is produced at the hepatic level), due to the intervention of the thrombin enzyme in the presence of calcium ions, represents the final step in a series of chain reactions, which on the whole are called the "coagulation cascade ” .

Just like fibrin, most of the substances that take part in this process are normally present in the plasma in an inactive form.

Also the same thrombin (factor IIa), involved in fibrinogen activation of fibrin, derives from a precursor molecule called pro-thrombin (or coagulation factor II). This conversion is mediated by the activated factors number Va (activated by thrombin) and X, common products of two routes originally considered distinct, but still interconnected:

  • the intrinsic or the blood (slower and activated when the blood comes into contact with the extracellular matrix, in particular with the collagen macromolecules);
  • the extrinsic or tissue one (more rapid and activated when a lesion of a blood vessel produces the release, from damaged cells, of phospholipids and of a protein complex called tissue factor or tissue thromboplastin).

Once activated, thrombin, in addition to catalyzing the formation of fibrin monomers starting from fibrinogen, also favors its polymerization in stable and soluble aggregates.

The formation of these fibrin clots, stabilized by the XIIIa factor in a braided weave in which plasmines and blood cells remain embedded, is essential - in synergy with vasoconstriction and platelet aggregation - for adequate haemostasis (arrest of hemorrhage in case of lesion of a blood vessel). This process, however, must necessarily be self-limited, in order to avoid the formation of very dangerous anomalous clots (thrombi), which can grow to such an extent that they obstruct the circulation or fragment, leading to embolic phenomena.

The dissolution of fibrin clots is entrusted to the so-called fibrinolytic system, in which a protein - plasmin - plays a leading role. Even this system, however, is subject to a homeostatic control mechanism, so that it does not excessively consume the wealth of pro-coagulant factors (risk of bleeding syndromes).

Why do you measure

To evaluate the coagulation capacity of the blood, the doctors resort to different analyzes, such as the thrombin time and the fibrinogen plasma concentration . The first test measures the time needed for the formation of fibrin after the addition of thrombin to the blood sample.

Also through a common blood test it is also possible to evaluate the concentrations of antithrombin, a plasma glycoprotein with an anticoagulant action that represents the most important physiological inhibitor of thrombin (IIa).

As for fibrin, this is not generally measured, but sometimes the haematic concentration of some substances deriving from its degradation (FDPs), such as D-dimer, is sought. The dosage of these fibrin dissolution products is carried out to investigate the organism's fibrinolytic activity, but also in the presence of doubts about diseases such as disseminated intravascular coagulation, deep vein thrombosis and pulmonary embolism . Normally. the physiological concentration of these fibrin degradation products is less than 10 mcg / mL, although the reference values ​​may vary slightly from laboratory to laboratory.

Normal values

In physiological conditions, fibrin is present in a state of balance between formation and lysis.

For this reason, in the blood of healthy subjects, typically low concentrations of D-dimer (a degradation product of fibrin) are found; the reference interval (normal range) for this parameter is equal to 0-500 ng / ml.

Fibrin High - Causes

Fibrin can increase in various physiological and pathological conditions, which include:

  • Old age;
  • Neonatal period;
  • Physiological and pathological pregnancy (including the puerperium);
  • Patients hospitalized and / or with functional disabilities;
  • Infections;
  • Neoplasms;
  • Surgical interventions;
  • Trauma;
  • Burns;
  • Disseminated intravascular coagulation (CID);
  • Venous thrombo-embolism;
  • Ischemic heart disease;
  • Peripheral arteriopathy of the lower limbs;
  • Aneurysms;
  • Congestive heart failure;
  • Acute respiratory distress syndrome (ARDS);
  • Subarachnoid hemorrhages and subdural hematomas;
  • Liver disease and nephropathy;
  • Inflammatory bowel diseases;
  • Chronic inflammatory diseases (eg SLE, rheumatoid arthritis etc.)
  • Thrombolytic therapy.

Low Fibrin - Causes

Normally, normal or low values ​​of fibrin dissolution products do not indicate the presence of a problem.

How to measure it

The tests that aim to assess blood coagulation are performed by taking a blood sample from a vein in the arm.

Preparation

In some cases, to perform the determination of the fibrin dissolution products or other related parameters, the doctor may indicate a fast of at least 8 hours, during which time you can drink water. Before taking it, you must be in an upright position for at least 30 minutes.

Interpretation of Results

The dosage of fibrin dissolution products can be used to determine if further investigations are needed for the diagnosis of diseases that can lead to excessive coagulation or inappropriate clot formation

This test is carried out to investigate the fibrinolytic activity of the organism in the presence of doubts about diseases such as:

  • Deep venous thrombosis;
  • Pulmonary embolism.

D-dimer levels can also be used to support the diagnosis of disseminated intravascular coagulation (CID) and to monitor therapeutic treatment.