What is the percentage of false positive D-dimer (Deep Vein Thrombosis/DVT and Pulmonary Embolism/PE) results?

False Positive Rates of D-dimer Testing in DVT and PE Diagnosis

The false positive rate of D-dimer testing for DVT and PE is approximately 40-60%, making it a useful exclusionary test but not a confirmatory one for venous thromboembolism. 1

D-dimer Test Characteristics

D-dimer is a fibrin degradation product that becomes elevated in various conditions involving clot formation and breakdown. Its primary utility in DVT/PE diagnosis is its high sensitivity but limited specificity:

• Sensitivity: 93-96% for highly sensitive assays (ELISA and turbidimetric) 1
• Specificity: Only 39-51% in outpatient settings 1
• Negative Predictive Value: ~99% in low-risk patients 1

This means that while a negative D-dimer can effectively rule out DVT/PE in low-risk patients, a positive result requires further diagnostic testing due to the high false positive rate.

False Positive Rates by Patient Population

The false positive rate varies significantly across different patient populations:

• General outpatients: 40-60% false positive rate 1
• Elderly patients: False positive rate increases with age, reaching up to 90% in patients over 80 years 1
• Cancer patients: Extremely high false positive rate with specificity as low as 18-21% 1, 2
• Hospitalized patients: Higher false positive rates than outpatients 1, 2
• Pregnant women: Increasing false positive rates with advancing gestational age 1, 2
• Trauma patients: High false positive rates, especially in the immediate post-injury period 3

Clinical Implications of False Positives

The high false positive rate has several important clinical implications:

1. Need for further testing: A positive D-dimer typically leads to additional imaging studies such as compression ultrasound for DVT or CT pulmonary angiogram for PE 1
2. Resource utilization: The number needed to test (NNT) to exclude one PE varies from 3 in emergency departments to 10 or more in specific high-risk populations 1
3. Risk stratification importance: Combining D-dimer with clinical probability assessment (like Wells score) improves diagnostic accuracy 1

Strategies to Reduce False Positives

Several approaches can reduce the impact of false positive results:

• Age-adjusted D-dimer cutoffs: For patients over 50 years, using age × 10 μg/L as the cutoff can reduce false positives while maintaining safety 2
• Clinical probability-adjusted cutoffs: Using higher cutoffs (e.g., 1000 μg/L) for patients with low clinical probability can reduce false positives by over 50% 4, 5
• Limiting D-dimer testing: Only using D-dimer in patients with low or intermediate clinical probability, as the false positive rate is highest in low-risk patients 1, 6

Pitfalls to Avoid

• Using D-dimer alone: Always combine with clinical probability assessment 1
• Relying on D-dimer in high-risk patients: The number needed to test in high clinical probability patients is 13.5, making it inefficient 6
• Ignoring clinical context: D-dimer has different performance characteristics in various clinical settings and patient populations 2
• Misinterpreting borderline results: Slightly elevated results should prompt further investigation 2

Algorithm for D-dimer Testing in DVT/PE Diagnosis

1. Assess clinical probability using validated tools (Wells score, Geneva score)
2. For low/intermediate probability patients:
   • Perform D-dimer testing
   • If negative, exclude DVT/PE (safe in ~99% of cases)
   • If positive, proceed to imaging (ultrasound for DVT, CT angiogram for PE)
3. For high probability patients:
   • Proceed directly to imaging without D-dimer testing
   • If D-dimer is performed, recognize that false negative rate may be higher (up to 5.3-40% in high-risk PE patients) 1

By understanding the high false positive rate of D-dimer testing and applying appropriate clinical strategies, clinicians can optimize its use as an exclusionary test while minimizing unnecessary additional testing.