D-dimer – laboratory test

Description

Why D-dimer is tested

D-dimers are fragments of fibrin produced when the body breaks down a clot. Their presence in the blood means that a clot is actively forming and breaking down somewhere in the blood vessels. The test is most useful for ruling out disease: a normal result in someone with a low or moderate clinical probability virtually rules out deep vein thrombosis and pulmonary embolism (risk of an event within 3 months below 1%). An elevated result does not establish a diagnosis, as levels rise in many other situations.

When to have this test

• swelling, pain, redness and warmth in one calf or thigh (suspected deep vein thrombosis),
• sudden breathlessness, chest pain, a rapid heart rate or fainting (suspected pulmonary embolism: this is an emergency; call for help and do not wait for results from a blood collection point),
• after prolonged immobility, a flight, surgery or injury, or when using hormonal contraception with risk factors,
• assessment of the risk of recurrence after completing anticoagulant treatment,
• suspected disseminated intravascular coagulation (DIC), follow-up after COVID-19 as advised by your doctor.

How to prepare

• Fast for 8–12 hours and drink only water in the morning. Have your blood sample taken in the morning.
• Avoid strenuous exercise and alcohol the day before; do not smoke on the morning of the test.
• Tell your doctor about any anticoagulants (which lower the result), hormones, recent surgery, injury or infection.
• If symptoms suggest pulmonary embolism, the test is performed in hospital, not as a routine scheduled test.

Understanding your result

Results are reported in µg/l FEU (fibrinogen equivalent units) or ng/ml FEU; some methods use DDU units, which have different thresholds, so compare your result with the reference range on the report.

In people with a high clinical probability of thrombosis, D-dimer is not measured; imaging is performed straight away instead. The specificity of the test decreases with age (to around 10% in people over 80).

What to test alongside D-dimer

Doctors often also request a full blood count (platelets), APTT, PT (INR), fibrinogen and CRP; after thrombosis with no obvious cause, tests for thrombophilia may be requested. More about thrombosis: Badania-Medyczne.pl.

Sources: ALAB laboratoria, D-dimer (preparation, 500 µg/l FEU threshold, causes of elevated levels, pregnancy); Konstantinides S.V. et al., 2019 ESC Guidelines for the diagnosis and management of acute pulmonary embolism, European Heart Journal 2020 (value in ruling out disease, age × 10 µg/l threshold, specificity in older people). This information does not replace a medical consultation.