Can Pulmonary Embolism Be Missed on CT Pulmonary Angiography?
Yes, pulmonary embolism can be missed on CTPA, particularly in patients with high pre-test clinical probability, where the negative predictive value drops to only 60%, compared to 96% in low-probability and 89% in intermediate-probability patients 1.
Diagnostic Performance of CTPA
CTPA has a sensitivity of 83% and specificity of 96% for diagnosing pulmonary embolism, meaning approximately 17% of true PE cases may be missed 1. The accuracy is heavily influenced by:
- Pre-test clinical probability: The negative predictive value varies dramatically—96% for low probability, 89% for intermediate probability, but only 60% for high probability patients 1
- Technical quality: A small proportion of CTPA examinations are technically unsatisfactory due to suboptimal contrast timing, motion artifacts, or inadequate acquisition protocols 2
- Subsegmental emboli: The clinical significance of isolated subsegmental PE detected on CTPA remains controversial, and these small emboli may be missed even on technically adequate studies 1, 2
When to Consider Additional Testing
Clinicians should pursue further investigation when there is discordance between clinical judgment and CTPA results 1. Specifically:
- High clinical probability with negative CTPA: Consider additional testing such as lower extremity venous ultrasound, repeat imaging, or ventilation-perfusion scanning before definitively excluding PE 1, 3
- Suspected concurrent DVT: Perform lower limb ultrasound if deep vein thrombosis is clinically suspected, as this can identify thromboembolic disease even when CTPA is negative 4
- Persistent clinical suspicion: In patients with intermediate probability and negative CTPA where concern persists, lower extremity venous ultrasound has sensitivity of 85% and specificity of 93% for detecting DVT 3
Special Considerations for CTEPH
CT pulmonary angiography alone may miss the diagnosis of chronic thromboembolic pulmonary hypertension (CTEPH), a potentially fatal late sequela of PE 1. Signs of pre-existing CTEPH should be actively sought on CTPA in patients investigated for suspected acute PE 1.
Clinical Outcome Data
Despite these limitations, prospective management studies demonstrate that withholding anticoagulation after negative CTPA is generally safe:
- Three-month VTE recurrence rate: 1.1% after negative CTPA, comparable to 0.9% after negative conventional pulmonary angiography 2
- Negative predictive value in real-world practice: 99.5% (95% CI 98.1-99.9%) when used as a stand-alone test 4
- False-negative rate: In clinical follow-up studies where anticoagulation was withheld, 0.4% (95% CI 0.2%-1.3%) of patients with negative CTPA developed VTE during follow-up 1
Common Pitfalls to Avoid
- Do not rely on negative CTPA alone in high-probability patients: The 60% negative predictive value is insufficient to safely exclude PE 1
- Do not ignore technical quality: Only good-quality negative CTPA studies can be relied upon to safely exclude pulmonary embolism 2
- Do not overlook alternative imaging: V/Q scanning may be preferred in patients with contraindications to CTPA (severe renal failure, contrast allergy, pregnancy) and offers lower radiation exposure 1
- Do not forget about overdiagnosis: CTPA detects more pulmonary emboli than V/Q scanning (19.2% vs 14.2%), but some of these—particularly isolated subsegmental emboli—may be of questionable clinical significance 5