D-dimer is a marker of blood clotting activity. It is released when a blood clot begins to break down.
Measuring D-dimer
There are many different D-dimer assays (qualitative and quantitative) available.
Check your local institution’s guidelines. Most contemporary laboratories use high sensitivity assays.
Different types of assay are not standardised and have varying sensitivities and specificities.
Different laboratories may use different cut-offs for what constitutes a positive or negative result.
When the D-dimer is indicated in emergency
- Aortic dissection
- Deep-vein thrombosis (DVT)
- Disseminated intravascular coagulation
- Pulmonary embolism (PE) is suspected
- Venom-induced consumptive coagulopathy
Other physiological states that may elevate D-dimer
Physiological states that may elevate D-dimer include, but not limited to the following.
Pathological
- Acute coronary syndromes
- Acute upper gastrointestinal haemorrhage
- Aortic dissection
- Arterial or venous thromboembolism
- Atrial fibrillation
- Consumptive coagulopathy – disseminated intravascular coagulation, venom-induced consumption coagulopathy
- Infection
- Malignancy
- Pre-eclampsia
- Sickle cell disease
- Stroke
- Superficial thrombophlebitis
- Trauma
- Venous thromboembolism
Non-pathological
- Age (healthy elderly people)
- Cigarette smoking
- Post-operatively
- Pregnancy
Using D-dimer in suspected aortic dissection
- First, assess the clinical probability of aortic dissection based on history, examination and risk factors.
- In patients with a low clinical suspicion, a D-dimer test (preferably point-of-care) may be considered to help exclude the diagnosis.
- A negative D-dimer result may support ruling out aortic dissection in this group.
- A positive D-dimer result is non-specific and does not confirm the diagnosis.
- D-dimer may also be useful in evaluating alternative differential diagnoses, such as pulmonary embolism.
- Imaging, particularly CT angiography, remains the definitive diagnostic tool in patients with a positive D-dimer or elevated clinical risk.
Using D-dimer in suspected DVT
- First, a pre-test probability for DVT should be calculated using the Wells’ score for DVT.
- In patients with a LOW pre-test probability (unlikely DVT) a D-dimer test is appropriate.
- In this cohort if the D-Dimer is negative, DVT can be excluded. If positive, then further investigation is required.
- In patients with a HIGH pre-test probability (likely DVT) a D-dimer is not appropriate. Patients should undergo further imaging to exclude DVT.
Using D-dimer in suspected PE
- First, a pre-test probability of PE should be calculated, for example Wells’ criteria or revised Geneva score.
- There are three tier models using low, intermediate and high probabilities, or two-tier models (now more accepted) using PE likely and PE unlikely.
- If using a moderately sensitive assay, then a D-dimer should ONLY be used for patients with a low pre-test probability.
- If it is a high sensitivity assay, then it can be used in both low and intermediate probability patients, and PE unlikely patients.
- In these patients if the D-dimer is negative (i.e. less than the cut-off value) a PE can be excluded. No further investigations.
- If the D-dimer test is positive further investigation is usually required such as CT pulmonary angiogram or ventilation perfusion scan.
Accessed from the Emergency Care Institute website at https://aci.health.nsw.gov.au/networks/eci/clinical/tools/d-dimer