D Dimer

🧪 D-dimer is a fibrin degradation product (FDP), a small protein fragment released into the blood after a blood clot is broken down by fibrinolysis. It is widely used in the investigation of suspected venous thromboembolism (VTE) such as DVT or PE, and in disseminated intravascular coagulation (DIC).

📖 About

• 🧩 D-dimer is a fragment of cross-linked fibrin produced during clot breakdown.
• 🔎 Its presence reflects recent or ongoing activation of coagulation and fibrinolysis.
• 🚑 Clinically used to investigate suspected DVT or PE.

⚙️ Background

• In normal physiology, circulating D-dimer fragments should be absent or minimal.
• 🔬 The D-dimer assay is highly sensitive and detects very small amounts of fibrin breakdown products.
• However, it is non-specific - many conditions see below.

🩺 Diagnostic Use

• ✅ Excellent rule-out test: A negative D-dimer in a low–intermediate risk patient (based on Wells score) can safely exclude DVT/PE without imaging.
• ⚠️ Poor specificity: A positive result does not confirm clot - it necessitates further imaging (e.g. CTPA, V/Q scan, or leg Doppler).
• 📈 Typically very high in DIC due to widespread fibrin deposition and breakdown.

📊 Age-Adjusted Cut-Off

• D-dimer levels naturally rise with age, reducing test specificity in older patients.
• Most labs report results in Fibrinogen Equivalent Units (FEU):
  • Standard cut-off: 500 µg/L FEU.
  • If reported as D-dimer Units (DDU), cut-off ≈ 230 µg/L (note: 2 FEU = 1 DDU).
• 🧮 Formula for age-adjusted threshold:
  • For patients >50 years: Age × 10 µg/L FEU.
  • Example: Age 75 → cut-off = 750 µg/L FEU.
  • If DDU assay used: Age × 5 µg/L.
• Age adjustment improves specificity without losing sensitivity in older adults.

🧪 Causes of False-Positive D-dimer

D-dimer reflects fibrin formation and breakdown. It is highly sensitive for VTE but poorly specific - many conditions activate coagulation or fibrinolysis and elevate D-dimer in the absence of PE/DVT.

🔴 Physiological Causes

• Increasing age (baseline rises with age - use age-adjusted threshold >50 years)
• Pregnancy (especially 2nd/3rd trimester) and postpartum period
• Recent strenuous exercise

🟠 Inflammatory & Infective States

• Sepsis
• Pneumonia
• COVID-19 and other viral illnesses
• Autoimmune disease (e.g. SLE, vasculitis)
• Systemic inflammatory response

🟡 Malignancy

• Active solid tumours
• Haematological malignancy
• Metastatic disease

🟢 Recent Tissue Injury or Surgery

• Recent surgery (particularly major or orthopaedic)
• Trauma or fractures
• Burns
• Post-operative state

🔵 Cardiovascular & Systemic Conditions

• Acute coronary syndrome
• Heart failure
• Aortic dissection
• Stroke
• Disseminated intravascular coagulation (DIC)

🟣 Liver & Renal Disease

• Chronic liver disease (reduced clearance)
• Renal impairment

⚠️ Other Considerations

• Hospitalisation itself (acute illness)
• Recent bleeding
• Thrombolysis
• Heparin-induced thrombocytopenia

🧠 Clinical Interpretation Pearls

• D-dimer is a rule-out test, not a rule-in test.
• Only use in low or intermediate pre-test probability patients.
• Use age-adjusted thresholds in patients >50 years (Age × 10 micrograms/L FEU).
• Do not use to exclude PE in pregnancy.
• In high clinical probability → proceed directly to imaging.

💡 Clinical Pearls

• Always interpret D-dimer in context of pre-test probability (e.g. Wells score).
• A negative D-dimer is only reliable in low to moderate risk patients - not in high-risk or inpatient settings.
• Conditions that commonly cause false positives: infection, inflammation, trauma, surgery, malignancy, pregnancy, liver disease, ageing.
• Persistently elevated D-dimer after VTE treatment may predict recurrence risk.