What labs should be ordered for a patient with suspected pulmonary embolism, particularly in the context of acute coronary syndrome, with risk factors such as immobility, cancer, or a history of deep vein thrombosis?

Laboratory Testing for Suspected Pulmonary Embolism

The laboratory workup for suspected PE centers on D-dimer testing, which should only be ordered after clinical risk stratification using validated prediction rules, with the specific approach determined by pretest probability. 1

Initial Clinical Assessment and Risk Stratification

Before ordering any laboratory tests, you must stratify the patient using validated clinical prediction rules (Wells score, Geneva score, or PERC criteria). 1, 2

Key risk factors to assess include: 1

• Recent immobilization or major surgery
• Recent lower limb trauma and/or surgery
• Clinical deep vein thrombosis
• Previous proven DVT or PE
• Pregnancy or post-partum period
• Major medical illness (including cancer)

Laboratory Testing Algorithm Based on Pretest Probability

Low Pretest Probability Patients

If all 8 PERC criteria are met (age <50, HR <100, O2 sat ≥95%, no unilateral leg swelling, no hemoptysis, no recent trauma/surgery, no prior VTE, no hormone use), order NO laboratory tests. 1, 2

If PERC criteria are not all met, order high-sensitivity D-dimer. 1

Intermediate Pretest Probability Patients

Order high-sensitivity D-dimer as the initial diagnostic test. 1

High Pretest Probability Patients

Do NOT order D-dimer—proceed directly to CT pulmonary angiography. 1

D-Dimer Interpretation

Use age-adjusted thresholds for patients over 50 years old: 1, 2

• Age ≤50 years: Standard cutoff of <500 ng/mL
• Age >50 years: Age-adjusted cutoff = age × 10 ng/mL

This age-adjusted approach maintains sensitivity >97% while dramatically improving specificity (from 14.7% to 35.2% in patients >80 years). 1, 2

If D-dimer is below the appropriate threshold, no imaging is needed and PE is ruled out. 1

If D-dimer is elevated, proceed immediately to CT pulmonary angiography. 2

Essential Baseline Laboratory Tests

Beyond D-dimer, obtain these tests in all patients with suspected PE: 1

• Arterial blood gas (to assess hypoxemia and evaluate clinical probability)
• Electrocardiogram (to assess for right ventricular strain and exclude ACS)
• Chest radiograph (to exclude alternative diagnoses)

The combination of tachypnea (>20/min), pleuritic pain, and arterial hypoxemia is highly suggestive; absence of all three effectively excludes PE. 1

Critical Pitfalls to Avoid

Never order D-dimer in high pretest probability patients—it wastes time and resources, as these patients need imaging regardless of the result. 1

Never use standard 500 ng/mL cutoff in patients over 50 years—this leads to unnecessary imaging due to poor specificity in older patients. 1, 2

Only use high-sensitivity D-dimer assays (ELISA or turbidimetric methods)—point-of-care assays have lower sensitivity (88% vs 95%) and should be avoided when laboratory-based tests are available. 2, 3

D-dimer specificity is lower in hospitalized patients due to comorbidities, but testing remains appropriate as sensitivity stays high. 2

Special Considerations for ACS Context

When PE is suspected in the context of possible acute coronary syndrome, recognize that these conditions can overlap significantly in presentation (chest pain, dyspnea, ECG changes, elevated troponin). 4

Key differentiating features favoring PE over ACS: 4

• Risk factors for VTE (immobility, cancer, prior DVT)
• Clinical DVT signs
• Unexplained dyspnea or hypoxemia
• Right ventricular pressure overload on ECG (rather than ischemic changes)

Troponin and BNP may be elevated in PE but are not diagnostic—they identify patients with poor prognosis rather than confirming diagnosis. 3