Diagnostic Approach to Suspected Pulmonary Embolism
You should order plasma D-dimer (Option A) as the next test to help rule out pulmonary embolism in this patient.
Clinical Probability Assessment
The first critical step is determining this patient's pretest probability of PE using validated clinical prediction rules 1, 2. This patient presents with:
- Low-risk features: No hemoptysis, no leg edema, no prior VTE history, ambulatory (walks one mile daily), normal oxygen saturation (96%), hemodynamically stable 1
- Age and vital signs: Tachypneic (22/min) and tachycardic (95/min), but these are nonspecific findings 3
- Recent surgery consideration: CABG was 5 years ago, not recent 3
Based on these clinical features, this patient likely falls into the low to intermediate pretest probability category for PE 1, 2.
Why D-dimer is the Correct Next Step
For patients with low or intermediate pretest probability of PE, high-sensitivity D-dimer measurement should be obtained as the initial diagnostic test before proceeding to imaging 1. The American College of Physicians explicitly states that clinicians should not use imaging studies as the initial test in patients who have low or intermediate pretest probability of PE 1.
Age-Adjusted D-dimer Threshold
Since this patient appears to be older (given 5-year-old CABG history), you should use an age-adjusted D-dimer threshold (age × 10 ng/mL) rather than the standard 500 ng/mL cutoff if the patient is over 50 years old 1, 2. This age-adjustment improves specificity while maintaining safety, as D-dimer specificity decreases with age (dropping to only 10% in patients >80 years) 2.
D-dimer Interpretation Strategy
- If D-dimer is below the age-adjusted threshold: PE can be safely excluded without any imaging, and no further testing is indicated 1, 2
- If D-dimer is elevated: Then proceed to CT pulmonary angiography 1
Why Not the Other Options
Option B (V/Q Scan): Not Appropriate
Ventilation-perfusion scanning is reserved for patients who have a contraindication to CTPA or when CTPA is not available 1. This patient has no contraindications to contrast (normal renal function implied, hemodynamically stable) 1.
Option C (CT Pulmonary Angiography): Premature
CTPA should only be obtained directly in patients with HIGH pretest probability of PE 1. The American College of Physicians explicitly recommends against using imaging studies as the initial test in patients with low or intermediate pretest probability 1. Proceeding directly to CTPA without D-dimer testing in low-to-intermediate probability patients leads to unnecessary radiation exposure, contrast risks, and potential overdiagnosis of clinically insignificant PE 2, 3.
Option D (No Further Testing): Incorrect
No further testing would only be appropriate if this patient met all Pulmonary Embolism Rule-Out Criteria (PERC) 1, 2. However, this patient has tachypnea (22/min) and tachycardia (95/min), which likely violates PERC criteria 3.
Critical Pitfalls to Avoid
- Do not skip clinical probability assessment: Always stratify pretest probability before ordering tests 1, 2
- Do not order CTPA with low clinical suspicion: This leads to overdiagnosis and unnecessary anticoagulation complications 2, 3
- Do not ignore age-adjusted D-dimer thresholds: Using fixed 500 ng/mL cutoff in elderly patients results in excessive false positives and unnecessary imaging 1, 2
- Remember D-dimer has high negative predictive value but poor positive predictive value: An elevated D-dimer alone does not diagnose PE 2
Algorithmic Summary
- Assess clinical probability (low/intermediate/high) using Wells score, Geneva score, or gestalt 1, 2
- Low/intermediate probability → Order high-sensitivity D-dimer 1
- D-dimer negative (below age-adjusted threshold) → PE excluded, no imaging needed 1, 2
- D-dimer positive → Proceed to CTPA 1
- High probability → Skip D-dimer, go directly to CTPA 1