How do I diagnose Deep Vein Thrombosis (DVT) or Pulmonary Embolism (PE)?

Diagnostic Approach to DVT and PE

Initial Clinical Probability Assessment

Begin by stratifying patients using a validated clinical prediction rule (Wells score) to categorize them as having low, moderate, or high probability of disease before ordering any tests. 1

The Wells score can be calculated using readily available online calculators and categorizes patients into probability groups with the following DVT prevalence: 1, 2

• Low probability: 5% prevalence 2
• Moderate probability: 17% prevalence 2
• High probability: 53% prevalence 2

For PE, the Wells score similarly stratifies patients into "PE unlikely" or "PE likely" categories. 1

Diagnostic Algorithm for Suspected DVT

Low to Moderate Clinical Probability

• Order a D-dimer test first 1, 3
• If D-dimer is negative using a highly sensitive assay (ELISA-based): DVT is excluded with 99% negative predictive value—no further testing needed 1
• If using a moderately sensitive assay (SimpliRED): a negative result safely excludes DVT only in low probability patients 1
• If D-dimer is positive: proceed directly to compression duplex ultrasound 1, 3

High Clinical Probability

• Skip D-dimer testing and proceed directly to compression duplex ultrasound imaging 1, 3
• D-dimer should not be used in high probability patients as a normal result does not safely exclude DVT even with highly sensitive assays 1

Ultrasound Interpretation

• Proximal compression ultrasound examines the femoral and popliteal veins 3
• The diagnostic criterion is incomplete compressibility of the vein under gentle probe pressure 1
• If proximal ultrasound is negative in high probability patients: obtain either a highly sensitive D-dimer, whole leg ultrasound, or repeat proximal ultrasound in 1 week 1

Diagnostic Algorithm for Suspected PE

PE Unlikely (Low to Moderate Probability)

• Order a D-dimer test first 1
• If D-dimer is negative using a highly sensitive assay: PE is excluded—no further testing needed 1
• If D-dimer is positive: proceed to CT pulmonary angiography 1

PE Likely (High Probability)

• Proceed directly to imaging with CT pulmonary angiography 1
• Skip D-dimer testing as it does not safely exclude PE in this group 1

High-Risk PE (Hemodynamically Unstable)

• In patients with shock or hypotension: perform emergency CT or bedside echocardiography immediately 1
• This represents a life-threatening situation requiring immediate diagnostic confirmation 4

Alternative Imaging When CT is Contraindicated

• Ventilation-perfusion (V/Q) scan is appropriate for patients with iodine contrast allergy or renal failure 1
• V/Q scanning is diagnostic in 30-50% of cases when results show either normal or high probability 1
• A normal perfusion lung scan excludes PE 1
• Lower limb compression ultrasound showing proximal DVT confirms PE and warrants treatment without further testing 1

Critical D-Dimer Considerations and Pitfalls

D-dimer specificity decreases significantly in specific populations, limiting its utility: 1

• Patients over 80 years: specificity drops to 10% 1
• Active cancer patients: frequently elevated 1
• Hospitalized patients: commonly elevated 1
• Pregnant patients: often elevated 1
• Recent surgery or major trauma: D-dimer has little utility 1, 5

In trauma patients within 4 days of admission, D-dimer has a 24% false-negative rate for DVT—do not rely on it to exclude disease in this population. 5

The number needed to test with D-dimer to exclude one PE varies from 3 in the emergency department to 10 or more in high-risk populations. 1

Imaging Test Performance

CT pulmonary angiography (CTPA) is the most common and preferred imaging modality for PE with high sensitivity and specificity: 1

• Multidetector CT (MDCT) showing segmental or more proximal thrombus confirms PE 1
• Single-detector CT (SDCT) requires negative proximal compression ultrasound to safely exclude PE 1
• With high pre-test probability, a negative MDCT has a 3-month VTE rate of 1.5% 1

Compression ultrasound has >90% sensitivity for proximal DVT and 95% specificity 1

• Finding proximal DVT in suspected PE patients is sufficient to warrant anticoagulation without further testing 1
• CUS detects DVT in 30-50% of patients with confirmed PE 1

Echocardiography should not be used routinely for diagnosis in hemodynamically stable patients as it has limited sensitivity (60-70%) and a negative result does not exclude PE. 1