What is the diagnostic approach for pulmonary embolism?

Diagnostic Approach for Pulmonary Embolism

Diagnose pulmonary embolism using a structured algorithmic approach that begins with clinical probability assessment (Wells score or gestalt), followed by D-dimer testing in low-risk patients, and CT pulmonary angiography (CTPA) as the definitive imaging modality when indicated. 1

Step 1: Clinical Probability Assessment

Assess pre-test probability using either a validated prediction rule or clinical gestalt before ordering any diagnostic tests. 2, 1

Wells Score (Structured Approach)

The Wells score assigns points for specific clinical features to stratify patients into low, intermediate, or high probability categories 2:

• Low risk (Wells <2): 3% PE prevalence 2
• Intermediate risk (Wells 2-6): 13% PE prevalence 2
• High risk (Wells >6): 36% PE prevalence 2

Clinical Gestalt (Experienced Clinician Assessment)

Gestalt assessment performs comparably to Wells score, with PE rates of 3%, 10%, and 33% in low (<15%), intermediate (15-40%), and high (>40%) probability groups respectively 2. Both approaches are validated and acceptable 2.

Key Clinical Features to Assess

Look for these specific predictors when calculating probability 2:

• Symptoms: Sudden-onset dyspnea (strongest predictor), chest pain, syncope, hemoptysis
• Risk factors: Immobilization, prior DVT, age >57 years, male sex
• Signs: Unilateral leg swelling, tachycardia
• ECG findings: Acute cor pulmonale pattern (S1Q3T3, right precordial T-wave inversions, right bundle branch block)
• Negative predictors: Fever ≥38°C, wheezes, crackles, pre-existing cardiovascular or pulmonary disease

Critical caveat: Clinical features alone cannot definitively diagnose or exclude PE—objective testing is mandatory 3, 4.

Step 2: Apply PERC Rule (Low-Risk Patients Only)

For patients with low clinical probability, apply the 8-item Pulmonary Embolism Rule-Out Criteria (PERC); if all criteria are met, no further testing is needed. 1

This step prevents unnecessary testing and radiation exposure in truly low-risk patients 1.

Step 3: D-Dimer Testing

Order D-dimer testing for low and intermediate probability patients who do not meet PERC criteria. 2, 1

• D-dimer <500 ng/mL: Safely excludes PE without imaging 2, 1
• D-dimer ≥500 ng/mL: Proceed to imaging 2, 1

Important limitation: D-dimer has minimal utility in hospitalized patients due to high false-positive rates from infection, cancer, inflammation, and postoperative states 2. In these patients, D-dimer rules out PE in less than 10% of cases 2.

Step 4: Imaging Studies

CT Pulmonary Angiography (First-Line)

CTPA is the preferred imaging modality when available and not contraindicated. 1

CTPA has replaced ventilation-perfusion (V/Q) scanning as the primary diagnostic test in most centers due to superior visualization of pulmonary vasculature 1.

V/Q Scanning (Alternative)

Consider V/Q scanning in specific situations 2, 1:

• Pregnancy (to reduce fetal radiation exposure if chest X-ray is normal) 1
• Renal failure (to avoid contrast-induced nephropathy) 1
• Contrast allergy

V/Q scan interpretation 2:

• Normal perfusion scan: Excludes PE (no further testing needed) 2
• High-probability scan with high clinical probability: Confirms PE 2
• Non-diagnostic scan (most common result): Requires further testing with either lower extremity ultrasound or pulmonary angiography 2

The PIOPED study demonstrated that 16% of patients with "low probability" V/Q scans actually had PE, highlighting the limitations of this modality 2.

Step 5: Lower Extremity Venous Ultrasound

If imaging is non-diagnostic or unavailable, perform compression ultrasound of lower extremities to detect deep vein thrombosis. 2

• Positive DVT: Treat as PE (70% of PE patients have proximal DVT) 2
• Negative DVT: Does not exclude PE; proceed to pulmonary angiography if clinical suspicion remains high 2

Special Scenario: Suspected Massive PE (Hemodynamically Unstable)

For patients presenting with shock or hypotension, perform immediate bedside echocardiography as the initial test. 2, 1

Echocardiography Findings

Look for indirect signs of acute pulmonary hypertension and right ventricular overload 2, 1:

• RV dilation
• RV hypokinesis
• Interventricular septal flattening
• Tricuspid regurgitation

In highly unstable patients, thrombolytic therapy may be initiated based on compatible echocardiographic findings alone. 2 If the patient stabilizes, confirm diagnosis with CTPA, V/Q scan, or bedside transesophageal echocardiography 2.

Critical point: A normal lung scan or CTPA in a patient with shock should prompt immediate search for alternative diagnoses (cardiogenic shock, tamponade, aortic dissection) 2.

Diagnostic Algorithm Summary

For hemodynamically stable patients (non-massive PE) 2, 1:

1. Assess clinical probability (36% of patients)
2. If low probability: Apply PERC or check D-dimer
3. If D-dimer <500: Stop (no PE)
4. If D-dimer ≥500: Perform lower extremity ultrasound (64% of patients)
5. If DVT present: Treat (11% of patients)
6. If no DVT: Perform CTPA or V/Q scan (53% of patients)
7. If normal scan: Stop (8% of patients)
8. If high-probability scan: Treat (10% of patients)
9. If non-diagnostic scan: Pulmonary angiography (11% of patients)

Common Pitfalls to Avoid

Overdiagnosis risk: Increased use of CT imaging has led to detection of subsegmental PEs of unclear clinical significance, exposing patients to unnecessary anticoagulation risks, radiation, and contrast complications 1.

Underdiagnosis risk: Failure to follow evidence-based diagnostic strategies when withholding anticoagulation despite clinical suspicion results in increased VTE episodes and sudden death 2. The European Heart Journal found that 70% of fatal PEs were not suspected clinically before death 2.

Hospitalized patients: Do not rely on D-dimer in postoperative, infected, or cancer patients—proceed directly to imaging if clinical suspicion exists 2.

Pregnancy: Prioritize V/Q scanning over CTPA when chest X-ray is normal to minimize fetal radiation exposure 1.