How should pretest probability guide imaging decisions in suspected pulmonary embolism?

• A. Assess pretest probability (Wells or PERC), then apply imaging appropriately (CTPA if not low, D-dimer if low) based on likelihood ratios and risks.
• B. Imaging all patients regardless of probability.
• C. Only use D-dimer for all patients.
• D. Use MRI exclusively.

Answer: (C)

Guiding imaging decisions starts with estimating pretest probability. By using clinical decision rules like Wells or the PERC rule, you categorize how likely a pulmonary embolism is before doing imaging. If the probability is low, a negative D-dimer result can effectively rule out PE because D-dimer is very sensitive. In that scenario you can avoid imaging, sparing the patient from radiation and contrast exposure. If the probability is not low, a positive or indeterminate D-dimer is not as helpful, and you proceed to imaging with CT pulmonary angiography to confirm or exclude PE. If the probability is high enough, you typically move straight to imaging without relying on D-dimer. D-dimer usefulness decreases with higher clinical suspicion and in older patients, where age-adjusted thresholds can help reduce unnecessary tests. While CT imaging is the standard acute test, MRI is not the first-line choice due to practicality and accessibility. The overall goal is to balance missing a PE against the risks and costs of imaging by tailoring the workup to how likely PE is in each patient.