The D-dimers come from the degradation of fibrin, a protein involved in blood clotting.

When the blood clots, for example in the event of an injury, some of its constituents attach themselves to each other, in particular with the help of the fibrin.

When there is insufficient blood clotting, it can cause spontaneous bleeding (hemorrhages). On the contrary, when it is excessive, it can be associated with the formation of blood clots which may have harmful consequences (deep vein thrombosis, pulmonary embolism). In this case, a protective mechanism is put in place to degrade the excess fibrin and reduce it to fragments, some of them being D-dimers. Their presence can therefore testify to the formation of a blood clot.

The doctor will prescribe a D-dimer test if he suspects the presence of blood clots. These can cause serious problems, such as:

• a deep vein thrombosis (also called deep phlebitis, it results from the formation of a clot in the venous network of the lower limbs)

• pulmonary embolism (presence of a clot without the pulmonary artery)

• or a stroke

The dosage of D-dimers is carried out by a venous blood sample, generally carried out at the level of the fold of the elbow. They are most often detected by immunological methods (use of antibodies).

No special preparation is required.

The concentration of D-dimer in the blood is normally less than 500 µg / l (micrograms per liter).

The D-dimer assay has a high negative predictive value. In other words, a normal result allows the exclusion of the diagnosis of deep vein thrombosis and pulmonary embolism. On the other hand, if the level of D-dimer is found to be high, there is a suspicion of the presence of a clot indicating a possible deep vein thrombosis or pulmonary embolism. This result must be confirmed by other examinations (in particular by imaging): the analysis must therefore be interpreted with caution.

There are indeed cases of an increase in the level of D-dimers unrelated to the presence of deep vein thrombosis and pulmonary embolism. Let us quote:

• pregnancy

• liver disease

• blood loss

• resorption of a hematoma,
• recent surgery

• inflammatory disease (such as rheumatoid arthritis)

• or simply being old (over 80)

Note that the determination of D-dimers is a relatively recent procedure (since the end of the 90s), and that the standards are still the subject of questioning. So much so that in France, the level is established as having to be less than 500 µg / l, whereas in the United States this threshold is lowered to 250 µg / l.