From the Guidelines
CT pulmonary angiography (CTPA) is the preferred diagnostic test for pulmonary embolism (PE) when ventilation-perfusion (VQ) scanning is unavailable, as it directly visualizes the pulmonary arteries and can detect filling defects caused by clots with high sensitivity and specificity. According to the American College of Physicians guidelines 1, CTPA is recommended for patients with a high pretest probability of PE. The guidelines suggest that clinicians should use validated clinical prediction rules to estimate pretest probability and reserve CTPA for patients with a high pretest probability of PE.
Key Points to Consider
- CTPA has several advantages over VQ scanning, including faster acquisition time, direct visualization of clots, and the ability to identify alternative diagnoses that may explain the patient's symptoms.
- However, CTPA does have limitations, including radiation exposure, potential for contrast-induced nephropathy in patients with renal impairment, and contraindication in pregnancy.
- Clinical assessment tools like Wells score or PERC rule should be used alongside imaging to determine the pre-test probability of PE and guide appropriate diagnostic testing.
- In cases where CTPA is contraindicated, alternatives include magnetic resonance angiography (MRA) or conventional pulmonary angiography, though these are less commonly used.
Diagnostic Approach
- Clinicians should use validated clinical prediction rules to estimate pretest probability in patients in whom acute PE is being considered 1.
- Patients with a low pretest probability of PE and who meet all Pulmonary Embolism Rule-Out Criteria should not undergo d-dimer measurements or imaging studies 1.
- A high-sensitivity d-dimer measurement should be obtained as the initial diagnostic test in patients with an intermediate pretest probability of PE or in patients with low pretest probability of PE who do not meet all Pulmonary Embolism Rule-Out Criteria 1.
From the Research
Diagnosis of Pulmonary Embolism
In the absence of a VQ scan, the requirement for CT angio to diagnose Pulmonary Embolism (PE) can be evaluated based on the following points:
- CT pulmonary angiography (CTA) is the preferred initial test for diagnosing PE, but it may be indeterminate, a significant source of ionizing radiation, and contraindicated in renal insufficiency 2.
- SPECT ventilation/perfusion imaging (V/Q) is preferred in certain patients, especially when CTA is indeterminate, as it can provide a diagnosis in 93% of such cases 2.
- The agreement between CTA and V/Q scans is substantial when either test yields a positive finding, but not when including indeterminate results 2.
- In patients with a high clinical suspicion of PE, SPECT V/Q often provides a diagnosis when CTA is indeterminate 2.
Alternative Diagnostic Strategies
Alternative diagnostic strategies for suspected PE include:
- Planar ventilation-perfusion scan (V/Q) 3
- Single photon emission CT ventilation-perfusion scan (SPECT V/Q) 3
- CT pulmonary angiography (CTPA) 3, 4
- A diagnostic algorithm based on a prediction rule, D-dimer, and CT-scan 4
Clinical Practice Guidelines
The choice of diagnostic test for PE depends on various factors, including clinical probability, patient characteristics, and institutional preferences:
- Patients with a high probability of PE should undergo chest imaging, and D-dimer testing is not necessary 5.
- In patients with low or intermediate clinical probability, a D-dimer level of less than 500 ng/mL is associated with a posttest probability of PE less than 1.85% 5.
- CT pulmonary angiography is the imaging modality of choice in suspected acute pulmonary embolism, but its use should be balanced with the potential harm from overutilization and overdiagnosis 6.