Makindo Medical Notes

🧪 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 apart from VTE can raise levels (e.g. infection, cancer, recent surgery, trauma, pregnancy, increasing age).

🩺 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.

💡 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.