What is D-Dimer?
It is a marker of blood clotting activity.
How is the D-Dimer measured?
There are many different D-Dimer assays (qualitative and quantitative) available.
Different types of assay are not standardised and have varying sensitivities and specificities. Furthermore, different laboratories may use different ‘cut-offs’ for what constitutes a positive or negative result. Check your local institutions guidelines. Most contemporary laboratories use high sensitivity assays.
D-dimer can be reported as fibrinogen equivalent units (FEU) or d-dimer units (DDU). One DDU = 2 FEU. The reporting units used will affect the cut-off. The standard cut-off for venous thromboembolism is 0.5mg/L FEU or 0.25mg/L DDU.
When can the D-Dimer be positive?
|Acute coronary syndromes||Age (healthy elderly people)|
|Acute upper gastrointestinal haemorrhage||Cigarette smoking|
|Arterial or venous thromboembolism||Pregnancy|
|Consumptive coagulopathy – DIC, VICC|
|Sickle cell disease|
When is the D-Dimer indicated in Emergency?
1. As part of a probability assessment when DVT or PE is suspected
2. To help diagnose fibrinolytic disorders such as venom-induced consumptive coagulopathy (VICC) and disseminated intravascular coagulation (DIC)
How can I use it in suspected PE?
First a pre-test probability of PE should be calculated. For example Wells criteria / 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 CTPA or VQ scan.
Click here for a link to the British Thoracic Society for use of D-Dimer.
How do I use it 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 a 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 a DVT.
- Clinicians should use age-adjusted D-dimer thresholds in patients older than 50 years of age to determine whether imaging is required.
- Positive D-dimer result:
- : D-dimer >0.5mg/L
- >50 years: D-dimer >(age x 0.01mg/L)
- For example 60 years: D-dimer >60 x 0.01mg/L = 0.6mg/L