What laboratory tests should be ordered in the initial work‑up for suspected pulmonary embolism?

Laboratory Tests for Suspected Pulmonary Embolism

The initial laboratory work-up for suspected PE should be D-dimer measurement using a high-sensitivity assay, but ONLY after clinical probability assessment—and D-dimer should NOT be ordered in high-risk patients. 1

Risk-Stratified Laboratory Approach

Step 1: Clinical Probability Assessment FIRST

Before ordering ANY labs, use a validated clinical prediction rule (Wells score or revised Geneva score) to stratify pretest probability 1. This determines which labs are appropriate.

Step 2: D-Dimer Testing (Risk-Dependent)

Low or Intermediate Clinical Probability:

• Order high-sensitivity D-dimer (ELISA assay preferred) 1
• Use age-adjusted cutoff: age × 10 ng/mL for patients >50 years (NOT the generic 500 ng/mL cutoff) 1
• If D-dimer is below age-adjusted threshold: PE excluded, no imaging needed 1
• If D-dimer is elevated: proceed to CTPA

High Clinical Probability:

• Do NOT order D-dimer 2, 1
• Proceed directly to CTPA
• A normal D-dimer does not safely exclude PE in high-risk patients 2

Step 3: Additional Laboratory Tests

Cardiac Biomarkers (for risk stratification, NOT diagnosis):

• Troponin and BNP/NT-proBNP identify patients at higher risk for adverse outcomes and may guide treatment intensity 3
• These are ordered AFTER PE diagnosis is confirmed, not for initial diagnostic work-up

What NOT to Order

Arterial Blood Gas:

• Do NOT use ABG to exclude PE 4
• Even with PaO2 ≥80 mmHg, PaCO2 ≥35 mmHg, and A-a gradient ≤20 mmHg, 38% of patients without prior cardiopulmonary disease still had PE 4
• ABG has insufficient discriminant value for PE diagnosis 3, 4

Routine Laboratory Tests:

• CBC, BMP, coagulation studies are NOT diagnostic for PE
• Order these only if anticoagulation is being considered (baseline values before treatment)

Critical Pitfalls to Avoid

1. Never order D-dimer in high-risk patients—it wastes time and a negative result doesn't change management 2, 1

2. Don't use the fixed 500 ng/mL D-dimer cutoff in patients >50 years—this leads to unnecessary imaging. Use age-adjusted thresholds 1

3. Don't skip clinical probability assessment—ordering D-dimer without risk stratification leads to overuse and false positives 1

4. Don't rely on ABG to exclude PE—this is a common error that misses significant numbers of PEs 4

5. Know your hospital's D-dimer assay—not all D-dimer tests have equivalent sensitivity. High-sensitivity assays (ELISA) are required for safe PE exclusion 1, 3

Hemodynamically Unstable Patients

In suspected high-risk PE with hemodynamic instability:

• Initiate IV unfractionated heparin immediately (weight-adjusted bolus) without waiting for lab results 2
• Proceed directly to bedside echocardiography or emergency CTPA 2
• No role for D-dimer testing in this scenario

Special Consideration: Lower Extremity Ultrasound

While not a "laboratory test," compression ultrasound showing proximal DVT is sufficient to warrant anticoagulation without further PE testing in 30-50% of PE patients 2. This can be particularly useful when CT is contraindicated.