D-dimer test
D-dimers are determined by means of a specific antibody test. This test is not only carried out to rule out thrombosis, but also for the diagnosis and monitoring of other diseases. In clinical routine the determination of D-dimers is performed indirectly by means of specific antibodies.
These bind to a specific region of the fibrin cleavage products and lead to their agglutination (clumping). With the aid of photometric measurement, turbidity can be determined, the strength of which determines the level of the value. In the diagnosis of D-dimers, a rapid test can be used to detect D-dimers in whole blood or plasma.
These are small test cassettes to which blood is applied with a pipette. The result is read off after exactly 10 minutes. If there is another red line next to the red control line in the test field, the result is positive. In the acute situation it allows the exclusion of a disseminated intravascular coagulation disorder (DIC), deep vein thrombosis and pulmonary embolism.
D-dimer values
The D-dimer value is laboratory specific and has the same upper limit for women and men. The value is 500 micrograms per liter. The significance of a low D-dimer value is high.
This means that one can exclude with a high degree of certainty a pulmonary embolism, a disseminated intravascular coagulopathy and a deep vein thrombosis of the leg. Conversely, a high D-dimer value serves as an unspecific indication of a pathological event. The exact cause must be clarified by further diagnostics.
In women, a distinction is made between reference values with and without the use of oral contraceptives. Without, the value is 300, with 500 micrograms per liter. During pregnancy, the D-dimer value of 700 micrograms per liter increases continuously during the first three months.
In the second third of the pregnancy it is about 1200 micrograms per liter, in the last weeks before delivery it is about 2500 micrograms per liter. A high D-dimer value can be an indication of a thrombembolic event. In almost 40% of cases, however, this is a false positive result.
The value is then above the reference range, without the presence of thrombosis. The reason for this can be influencing factors such as elevated blood lipids, haemolysis and rheumatism factors, as well as other underlying diseases. Conversely, low D-dimers rule out the presence of thrombosis or pulmonary embolism with a high probability.
Changes in the D-dimer values
This question, whether there is a change in the D-dimer concentration with age, can be answered clearly with “yes”. Due to the ageing of the vascular system, general coagulation activity occurs even when the patient is completely healthy. This leads to permanently slightly increased D-dimer values.
In order to be able to roughly estimate this falsifying factor, it is suggested to adjust the limit value for a normal value to an increased value with the formula “10 x age”. This general increase in D-dimer levels makes it increasingly difficult to diagnose pulmonary embolism or thrombosis with age. During menstruation, a detachment of the mucous membrane in the uterus occurs, resulting in extensive injury.
Coagulation plays a major role in this process to protect the woman from heavy bleeding. As a result of this blood clotting, the clots dissolve again and D-dimers are formed. The woman’s menstrual period therefore leads to increased D-dimer values even in completely healthy patients.
This falsifying factor should always be taken into account when interpreting the blood results. In renal insufficiency, a loss of function occurs due to very different injuries and scarring of the fine vessels of the kidney. It is precisely these vascular changes that also lead to general coagulation activity.
As a consequence, D-dimers are continuously formed by the breakdown of the fibrin produced at the coagulation site. Renal insufficiency thus leads to permanently slightly increased D-dimer values, which should be taken into account in blood tests. In pulmonary embolism, blood clots from a thrombosis, for example of the leg, enter the lung via the bloodstream.
These block the fine blood vessels, which leads to further clots. Due to this strong clotting activity in the leg and lung and its degradation products, the D-dimer value increases considerably in most cases. In some cases, however, there is only a small increase in the values.
In order to diagnose or rule out pulmonary embolism, the D-dimer values provide important information. Due to the fact that many diseases and also healthy body changes cause the D-dimers to increase, a pulmonary embolism can only be excluded with certainty if the values are negative, i.e. very low. Positive, i.e. increased, values only make pulmonary embolism possible. Further examinations either confirm or rule out this disease.
