D-Dimer

• Suspected Deep Vein Thrombosis (DVT) or Pulmonary Embolism (PE) in patients with a LOW or MODERATE pre-test probability (via Wells Score).
• Diagnosis and monitoring of Disseminated Intravascular Coagulation (DIC).

• High pre-test probability of PE: Skip the D-Dimer and go straight to CT Pulmonary Angiography (CTPA).
• Post-operative patients, pregnancy, active malignancy, or severe trauma: D-Dimer will be physiologically elevated, rendering the test useless.

• Sensitivity: ~95% (Excellent for Ruling OUT disease).
• Specificity: ~40% (Terrible for Ruling IN disease).
• Negative Predictive Value (NPV): > 99% in low-risk patients.

• If < 500 ng/mL: DVT/PE safely ruled out. Stop workup.
• If > 500 ng/mL: DVT/PE NOT ruled out. Proceed to Ultrasound (for DVT) or CTPA (for PE).

D-Dimer fragments have a half-life of roughly 8 hours. Levels decline over days following the cessation of thrombosis or initiation of anticoagulation.

D-Dimer naturally increases throughout pregnancy. Standard cutoffs are invalid. Typical median values:
1st Trimester: < 700 ng/mL
2nd Trimester: < 1000 ng/mL
3rd Trimester: < 1700 ng/mL

D-Dimer is a specific fibrin degradation product (FDP). It is a small protein fragment present in the blood only after a blood clot has formed and is actively being degraded by plasmin (fibrinolysis).

1. Deep Vein Thrombosis (DVT) & Pulmonary Embolism (PE)
2. Disseminated Intravascular Coagulation (DIC)
3. Recent Surgery or Major Trauma
4. Active Malignancy
5. Advanced Age & Pregnancy

N/A – Low or undetectable levels represent a normal physiologic state (absence of active thrombosis).

No fasting or special preparation required.

Centrifuge within 1 hour to separate plasma. Plasma should be tested within 4 hours if kept at room temperature.

• Underfilled tubes: Alters citrate ratio, causing false results.
• Clotted specimens: In-vitro clotting consumes fibrinogen.
• Severe Hemolysis.

High levels of Rheumatoid Factor (RF) can cause false-positive D-Dimer results in some latex agglutination assays.

Direct Oral Anticoagulants (DOACs) and Warfarin do not directly interfere with the D-Dimer ELISA assay itself, though active anticoagulation will naturally lower D-Dimer levels over time by preventing new clot formation.

💡 Pearl: The Age-Adjusted Cutoff
In patients over 50, use the formula (Age × 10). An 80-year-old patient with a D-Dimer of 750 ng/mL has a NORMAL D-Dimer, and PE is ruled out.

• The SNOUT Rule: Highly SeNsitive tests rule OUT disease. Boards love testing that D-Dimer is a rule-out test only.
• PERC Rule: Before ordering a D-Dimer, always apply the Pulmonary Embolism Rule-out Criteria (PERC). If PERC is 0, do not order the D-Dimer.