What is the best diagnostic test to rule out pulmonary embolism (PE) in a patient with asthma?

Diagnostic Approach to Rule Out PE in Asthmatic Patients

In asthmatic patients with suspected pulmonary embolism, use the same diagnostic algorithm as non-asthmatic patients: stratify pretest probability, apply PERC criteria if low risk, obtain high-sensitivity D-dimer (with age-adjusted cutoffs for patients >50 years), and proceed to CT pulmonary angiography only if D-dimer is elevated or pretest probability is high. 1, 2

Asthma Does Not Impair PE Diagnostic Testing

• A history of asthma does not reduce the diagnostic performance of either ventilation-perfusion scanning or CT pulmonary angiography for detecting pulmonary embolism 3
• Nondiagnostic results occur at comparable rates in asthmatic versus non-asthmatic patients for both V/Q scanning (3.3% vs 3.8%) and CTPA (1.6% vs 1.5%) 3
• False-negative rates are not higher in asthmatic patients for either imaging modality (V/Q: 0.4% vs 0.9%; CTPA: 1.9% vs 1.5%) 3
• The overlapping symptoms between asthma exacerbation and PE do not necessitate any modification to the standard PE diagnostic algorithm 3

Step 1: Assess Pretest Probability

• Use validated clinical prediction rules such as the Wells score or revised Geneva score to stratify patients into low, intermediate, or high pretest probability categories 1, 2, 4
• The Wells score assigns points for clinical signs of DVT (3 points), PE as likely as alternative diagnosis (3 points), heart rate >100 bpm (1.5 points), immobilization/surgery (1.5 points), previous PE/DVT (1.5 points), hemoptysis (1 point), and malignancy (1 point) 4
• Low probability patients have ~3-13% PE prevalence, intermediate probability ~16-30%, and high probability ~36-65% 5, 4

Step 2: Apply PERC Criteria for Low Probability Patients

• If pretest probability is low AND all 8 PERC criteria are met, no further testing is needed 1, 2, 5
• The 8 PERC criteria are: age <50 years, heart rate <100 bpm, oxygen saturation ≥95%, no unilateral leg swelling, no hemoptysis, no recent trauma/surgery, no prior VTE, and no hormone use 5
• PERC has 97% sensitivity and safely rules out PE in 22% of low-risk patients without any testing 5, 4

Step 3: D-Dimer Testing Strategy

• For low probability patients who do not meet all PERC criteria, or for all intermediate probability patients, obtain high-sensitivity D-dimer as the initial test 1, 2
• Never obtain D-dimer in high probability patients—proceed directly to CTPA 1, 2, 5
• For patients ≤50 years old, use the standard cutoff of <500 ng/mL 2, 5
• For patients >50 years old, use age-adjusted cutoff: age × 10 ng/mL 1, 2, 5
• Age-adjusted D-dimer maintains >97% sensitivity while increasing specificity from 10% to 35% in patients over 80 years, allowing PE exclusion in 30% of older patients versus only 6.4% with standard cutoffs 5

Step 4: Imaging When D-Dimer is Elevated

• If D-dimer is below the appropriate threshold, PE is ruled out and no imaging is needed 1, 2
• If D-dimer is elevated or pretest probability is high, proceed directly to CT pulmonary angiography 1, 2
• CTPA has >95% sensitivity for segmental or larger emboli and is the primary imaging modality for hemodynamically stable patients 2, 4
• Reserve ventilation-perfusion scanning for patients with contraindications to CTPA (renal insufficiency, contrast allergy) or when CTPA is unavailable 1

Critical Pitfalls to Avoid

• Do not assume asthma limits imaging options—both CTPA and V/Q scanning perform equally well in asthmatic patients as in non-asthmatic patients 3
• Do not order D-dimer indiscriminately without pretest probability assessment, as this increases false positives and unnecessary imaging 5
• Do not use standard 500 ng/mL D-dimer cutoff in patients over 50 years, as poor specificity leads to excessive imaging 1, 5
• Do not apply PERC to patients over 50 years old, as age <50 is a required criterion 5
• In hospitalized patients, D-dimer has limited utility due to frequent elevation from comorbidities—consider proceeding directly to imaging if clinical suspicion is moderate to high 5, 4