What are the Canadian guidelines for obtaining a computed tomography pulmonary angiogram (CTPA) in a patient with suspected pulmonary embolism?

Canadian Guidelines for CT Pulmonary Angiography in Suspected Pulmonary Embolism

While no specific Canadian-only guidelines are provided in the evidence, the diagnostic approach follows internationally validated algorithms that are widely adopted in Canadian centers, prioritizing clinical probability assessment and D-dimer testing before proceeding to CTPA. 1

Initial Risk Stratification is Mandatory

Before ordering any imaging, you must stratify patients using validated clinical prediction rules:

• Apply the Wells score or revised Geneva score to categorize patients into low, intermediate, or high clinical probability 1
• For patients under 50 years with low probability, apply PERC criteria first - if all 8 criteria are met (age <50, HR <100, O2 sat ≥95%, no hemoptysis, no unilateral leg swelling, no recent trauma/surgery, no prior VTE, no hormone use), no further testing is needed 2
• Never skip this step - proceeding directly to CTPA without risk stratification leads to massive overuse of imaging 1

The Algorithmic Approach to CTPA

For Hemodynamically Unstable Patients (High-Risk PE)

Proceed directly to CTPA if immediately available and the patient is stable enough for transport. 1

• If CTPA is not immediately available or the patient is too unstable, perform bedside echocardiography looking for RV dysfunction 1
• If RV dysfunction is present, this confirms high-risk PE and justifies emergency reperfusion therapy without waiting for CTPA 1
• Do not delay life-saving treatment to obtain imaging in critically ill patients 1

For Hemodynamically Stable Patients (Non-High-Risk PE)

The pathway depends entirely on clinical probability:

Low or Intermediate Clinical Probability (Wells ≤6 or "PE Unlikely"):

• Order high-sensitivity D-dimer as the first test - CTPA is not indicated at this stage 1
• If D-dimer is negative using appropriate cutoffs (see below), PE is excluded and no CTPA is needed 1
• If D-dimer is positive, proceed immediately to CTPA 1

High Clinical Probability (Wells >6 or "PE Likely"):

• Proceed directly to CTPA without D-dimer testing 1
• D-dimer has insufficient negative predictive value in this population and wastes time 1

Critical D-Dimer Interpretation Rules

The D-dimer cutoff must be adjusted for age to avoid unnecessary imaging:

• For patients ≤50 years: use standard cutoff of <500 ng/mL 2
• For patients >50 years: use age-adjusted cutoff of age × 10 ng/mL 1, 2
• Age-adjusted D-dimer maintains >97% sensitivity while dramatically improving specificity, increasing from 10% to 35% in patients over 80 years 2
• This approach safely excludes PE in 30% of older patients who would otherwise undergo unnecessary CTPA 2

CTPA Technical Specifications

Multidetector CTPA is the imaging method of choice and provides:

• Sensitivity of 83% and visualization down to subsegmental vessels 1
• PE is considered diagnosed when thrombus is visible at the segmental level or above 1
• Effective radiation dose of 3-10 mSv with significant breast tissue exposure in young women 1
• Low rate of inconclusive results (3-5%) compared to V/Q scanning (50%) 1

When CTPA is Negative in High-Risk Patients

This is a critical pitfall scenario:

• In patients with high clinical probability and negative CTPA, further investigation should be strongly considered 1, 3
• The 3-month VTE risk after negative CTPA in high-probability patients may be as high as 5.2-10.4% 3
• Consider additional testing with compression ultrasonography of lower extremities or V/Q scanning 1
• However, in patients with low or intermediate probability, negative CTPA safely excludes PE with only 0.6% 3-month VTE risk 4

Alternative Imaging Options

V/Q scanning should be reserved for specific contraindications to CTPA:

• Severe iodine contrast allergy 1
• Hyperthyroidism 1
• Significant renal impairment 1
• Pregnancy when attempting to minimize radiation to breast tissue 1

V/Q scanning has lower radiation exposure (2 mSv) but 50% inconclusive rate with planar imaging, reduced to <3% with SPECT technique 1

Common Pitfalls to Avoid

• Never order CTPA as the first test in low/intermediate probability patients - you will miss the opportunity to safely exclude PE with D-dimer alone in 30-50% of cases 1
• Never use standard 500 ng/mL D-dimer cutoff in patients over 50 - this leads to massive overuse of CTPA due to poor specificity 2
• Never order D-dimer in high clinical probability patients - it wastes time and has insufficient negative predictive value 1
• Never rely on point-of-care D-dimer assays when laboratory tests are available - they have only 88% sensitivity compared to 95% for laboratory assays 1
• Be cautious interpreting negative CTPA in high-probability patients without alternative diagnosis - strongly consider additional testing 3