PATHCHAT Edition No. 21
Please contact your local Ampath pathologist for more information.
What Are D-Dimers?
🔹 D-Dimer is a marker of fibrinolysis, formed during the breakdown of fibrin clots.
- Coagulation results in the formation of fibrin clots.
- Fibrinolysis is the breakdown of these clots.
- D-Dimer is a specific product of cross-linked fibrin degradation by plasmin.
- Elevated D-Dimer levels indicate increased clot formation and breakdown.
Clinical Applications of D-Dimer Testing
✅ 1. Exclusion of Venous Thromboembolism (VTE) in Specific Patients
- Deep vein thrombosis (DVT).
- Pulmonary embolism (PE).
✅ 2. Prediction of Recurrent VTE
- Helps assess the risk of VTE recurrence.
✅ 3. Diagnosis and Monitoring of Disseminated Intravascular Coagulation (DIC)
- D-Dimer levels are elevated in DIC, aiding in diagnosis and disease progression monitoring.
D-Dimer Testing at Ampath
🔹 Ampath uses an ELISA-based D-Dimer assay, which provides:
- High sensitivity (excellent negative predictive value).
- Acceptable specificity.
- Rapid turnaround time (<1 hour).
- Quantitative results available 24/7.
Patient Preparation & Sample Collection
🔹 To ensure accurate results:
✅ Avoid excessive trauma or prolonged stasis during venesection.
✅ Collect blood in a citrate tube (blue top), ensuring a minimum fill of 4 mL.
✅ Samples remain stable for up to 24 hours at room temperature.
D-Dimer Reference Range
✔ Normal D-Dimer levels: <0.50 mg/L.
D-Dimer as a Diagnostic Tool
🔹 High Negative Predictive Value for VTE Diagnosis:
✅ In low-risk patients, a normal D-Dimer level effectively rules out thrombosis.
✅ Most useful in young patients without co-morbidities and recent symptoms.
✅ Eliminates unnecessary imaging tests (e.g., venograms, CT pulmonary angiograms).
🔹 Limitations – A Positive D-Dimer Does NOT Confirm Thrombosis:
🚨 D-Dimer levels can be elevated due to other conditions (false positives).
Common Causes of False-Positive & False-Negative D-Dimer Results
✅ False Positives (Elevated D-Dimer Without VTE):
- Recent surgery.
- Haemorrhage.
- Trauma.
- Malignancy.
- Sepsis or severe infection.
- Advanced age with co-morbidities.
- Pregnancy.
- Liver disease.
🚨 False Negatives (Normal D-Dimer Despite VTE):
- Symptoms older than 7–10 days.
- Patient already on anticoagulation (heparin or warfarin).
- Children (lower baseline D-Dimer levels).
- Small or distal DVT.
- Upper extremity DVT.
- Hypofibrinolysis (poor clot breakdown).
D-Dimer & Disseminated Intravascular Coagulation (DIC)
✅ DIC is a serious condition caused by widespread clotting and fibrinolysis, leading to:
- Consumption of clotting factors & platelets.
- Uncontrolled bleeding.
🔹 Diagnosis Requires Both Clinical & Laboratory Findings:
- D-Dimer is significantly elevated in DIC.
- The International Society for Thrombosis and Haemostasis (ISTH) scoring system helps diagnose DIC.
✅ ISTH DIC Scoring System:
- Determine if the patient has a known underlying disorder associated with DIC.
- If YES, order coagulation tests and assign scores based on the following:
✔ Platelet Count:
- >100 × 10⁹/L → Score 0.
- 50–100 × 10⁹/L → Score 1.
- <50 × 10⁹/L → Score 2.
✔ Fibrin Marker (D-Dimer Level):
- No increase → Score 0.
- Moderate increase → Score 2.
- Strong increase → Score 3.
✔ Prolonged Prothrombin Time (PT):
- <3 seconds → Score 0.
- 3–5.9 seconds → Score 1.
- >6 seconds → Score 2.
✔ Fibrinogen Level:
- >1 g/dL → Score 0.
- <1 g/dL → Score 1.
🔹 Scoring Interpretation:
- Score ≥5: Compatible with overt DIC – Repeat scoring daily.
- Score <5: Suggests non-overt DIC – Repeat in 1–2 days.
D-Dimer & Venous Thromboembolism (VTE) Risk Stratification
✅ All patients should undergo a pre-test probability assessment using clinical prediction rules.
- The Wells Score is commonly used to estimate the likelihood of DVT or PE.
Wells Prediction Rule for Diagnosing DVT
✔ Clinical Features & Scoring:
- Malignancy → Score 1.
- Paralysis, paresis, or recent limb immobilization → Score 1.
- Bedridden >3 days or major surgery within 12 weeks → Score 1.
- Local tenderness along the deep vein system → Score 1.
- Entire leg swollen → Score 1.
- Calf swelling >3 cm compared to asymptomatic leg → Score 1.
- Pitting oedema in symptomatic leg → Score 1.
- Collateral superficial veins → Score 1.
- Alternative diagnosis as likely as DVT → Score -2.
✔ Interpreting the Total Score:
- Low probability (<1 point).
- Intermediate probability (1–2 points).
- High probability (>2 points).
Wells Prediction Rule for Diagnosing PE
✔ Clinical Features & Scoring:
- Signs & symptoms of DVT → Score 3.
- No alternative diagnosis more likely than PE → Score 3.
- Heart rate >100 bpm → Score 1.5.
- Recent surgery or immobilization → Score 1.5.
- Previous DVT or PE → Score 1.5.
- Haemoptysis → Score 1.
- Malignancy → Score 1.
✔ Interpreting the Total Score:
- Low probability (<2 points).
- Intermediate probability (2–6 points).
- High probability (>6 points).
Key Takeaways for Clinicians
✅ D-Dimer has high negative predictive value – a normal result effectively rules out VTE in low-risk patients.
✅ D-Dimer should not be used alone to diagnose thrombosis – pre-test probability assessment is essential.
✅ False positives and false negatives are common – consider clinical context before interpreting results.
✅ D-Dimer levels can help assess VTE recurrence risk and should be monitored after anticoagulation therapy.