What are D-dimers?
D-dimers are cleavage products of the fibrous protein fibrin, which plays a major role in blood clotting:
When fibrin and blood platelets (thrombocytes) clump together, a blood clot forms – both in healthy blood clotting (wound healing) and in the pathological formation of blood clots (thrombi) within intact vessels. Such thrombi can block a vessel at the site of their formation (thrombosis) or be carried along with the bloodstream and cause a vessel occlusion elsewhere (embolism).
When a clot or blood clot is dissolved (fibrinolysis), the cross-linking strands in the fibrin are split. This produces smaller fibrin fragments, including the D-dimers.
When are D-dimers determined?
The physician determines the D-dimers from a blood sample if a thromboembolism (such as leg vein thrombosis, pulmonary embolism) or excessive blood clotting (disseminated intravascular coagulation) is suspected.
An indication of a vascular occlusion in the legs is given, for example, by the following complaints in the affected extremity:
- Swelling
- dull pain
- bluish coloration (cyanosis) due to the undersupply of oxygen
- Overheating
- feeling of heaviness or tension
- clear protruding of the veins
D-dimer as exclusion method
On the other hand, it also means that the test is very sensitive to vascular occlusion: In patients with a normal D-dimer value, a vascular occlusion is very unlikely. Therefore, the laboratory value is well suited to clarify the suspicion of a vascular occlusion.
D-dimers: normal values
The D-dimer value in the blood in adults is normally between 20 and 400 micrograms per liter (µg/l).
Caution: In women, the D-dimer level rises naturally to a certain extent towards the end of pregnancy, without this having any clinical significance. The physician must take this into account when assessing the laboratory value.
When are the D-dimer levels lowered?
If the D-dimer values are below the so-called cut-off, this has no significance.
When are the D-dimers elevated?
The cause of elevated D-dimers is usually a thromboembolic event, i.e. a blood clot that blocks a vessel. Thus, the D-dimer level is elevated in almost all patients with deep vein thrombosis and drops back into the normal range within a few days to weeks.
In addition to deep vein thrombosis, possible causes of elevated D-dimers include the following conditions or situations:
- Other thromboses or embolisms (such as pulmonary embolism, myocardial infarction, etc.)
- Disseminated intravascular coagulation (DIC, consumption coagulopathy)
- Operations
- Liver cirrhosis
- Hemolytic uremic syndrome (HUS)
- Cancer
- Inflammations such as “blood poisoning” (sepsis)
D-dimers: Pregnancy
During pregnancy, there is a natural increase in D-dimers. At the same time, pregnancy carries an increased risk of blood clots. The upper limit at which the physician must think of a thromboembolism in a pregnant patient is therefore higher than in other patients (see above: D-dimer values: Table for pregnant women).
What to do if D-dimers are altered?
If D-dimers are elevated, vascular occlusion is possible. The physician will clarify this suspicion promptly in order to be able to take any necessary countermeasures immediately.
If a pulmonary embolism is suspected, for example, a CT angiography – a computer tomographic examination of the blood vessels using a contrast medium – is useful. In addition, the physician can perform an ultrasound examination of the heart (echocardiography): As a result of the pulmonary embolism, there is a strain on the right ventricle of the heart, which is then noticeable in the examination, for example, by a ventricular dilatation or a weakness of the relevant heart valves.
If a patient’s symptoms and risk factors make vascular occlusion highly likely, the physician will perform an imaging study such as CT angiography even if the D-dimer readings are normal.
