Pulmonary embolism (PE) is a potentially life-threatening condition that occurs when a blood clot (usually from the leg or pelvis) travels to the lungs and blocks an artery. Confirming the diagnosis quickly is essential to initiate appropriate treatment. There are several tests that can be used to diagnose PE.
Clinical Assessment
The first step is a clinical assessment. The doctor will take a full medical history looking for risk factors for PE such as recent surgery, cancer, previous clots, and others. Symptoms of PE may include:
- Sudden shortness of breath
- Chest pain, often sharp and worse with breathing
- Cough, sometimes with blood
- Rapid heart rate
- Lightheadedness or passing out
On physical exam, the doctor may detect:
- Rapid breathing
- Low oxygen levels
- Crackles when listening to the lungs
- Leg swelling
While symptoms and exam can raise suspicion for PE, imaging and lab tests are needed to confirm the diagnosis.
D-Dimer Blood Test
This is typically the first diagnostic test ordered. D-dimer is a protein fragment released when a blood clot dissolves. A positive D-dimer indicates there is active clotting and breakdown occurring in the body. However, many other conditions can cause a positive result, so D-dimer alone cannot confirm PE. Its main use is that a negative D-dimer in a patient with low-moderate risk virtually rules out PE.
Chest X-Ray
A chest x-ray is done to look for findings that may suggest PE such as:
- Wedge-shaped opacities indicating collapsed lung tissue
- Elevated diaphragm on affected side
- Fluid in fissure between lobes of lung
However, a chest x-ray can appear normal in many PEs, so a normal film does not rule it out. The x-ray is more useful to evaluate for other potential causes of symptoms, such as pneumonia or pneumothorax.
CT Pulmonary Angiogram
This is the gold standard test to confirm PE. It uses an injected contrast dye and CT scan to visualize the pulmonary arteries. It can detect blood clots as small as 2-3mm. Findings include:
- Filling defect in pulmonary artery from clot
- Complete occlusion of pulmonary artery branch
- Enlarged pulmonary artery on affected side
CT pulmonary angiography is highly sensitive and specific for PE. However, occasional false negatives and indeterminate scans can occur. The scan also exposes the patient to radiation and contrast dye.
Ventilation-Perfusion (V/Q) Scan
This nuclear imaging study looks at airflow and blood flow in the lungs. In PE, blood flow will be decreased in segments with blocked arteries, but airflow remains normal. Findings include:
- One or more mismatched segmental perfusion defects
- Normal or near-normal ventilation in those segments
V/Q scanning is useful in patients who have contraindications to CT angiogram. However, it is less definitive than CT and results can be equivocal. Perfusion-only scanning can be done if there are pre-existing lung conditions that would affect ventilation.
Pulmonary Angiography
Previously the gold standard, pulmonary angiography has been largely replaced by CT angiography. It involves threading a catheter into the pulmonary artery and injecting contrast dye. It provides clear visualization of clot location and extent. Disadvantages include its invasive nature, use of contrast dye, radiation exposure, and limited availability.
Pulmonary angiography is now mainly used when other tests are inconclusive or suggest a high probability of PE despite negative scans.
ECG, Echocardiogram, and Blood Tests
Other tests may be done to evaluate for PE complications or alternate diagnoses:
- ECG: to detect arrhythmias, right heart strain
- Echocardiogram: to visualize right ventricular dysfunction
- Blood tests: to assess cardiac biomarkers like troponin and BNP
These results can provide supporting evidence for PE but are not definitive for diagnosis.
Interpreting Results
No single test can diagnose PE with 100% certainty. Doctors must interpret the results in context of the clinical presentation. A high-probability V/Q scan or CT angiogram in a patient with symptoms of PE is sufficient to confirm the diagnosis. However, low or intermediate probability scans in ambiguous cases may warrant further testing to rule out PE conclusively before deciding on treatment.
Diagnostic Approach
Current clinical guidelines recommend a sequential combination of D-dimer testing, CT angiography, V/Q scanning, and pulmonary angiography to diagnose suspected PE. An example approach is:
| Setting | Recommended Approach |
|---|---|
| Outpatient, low/intermediate risk | D-dimer -> CT angiogram if positive |
| Inpatient, higher risk | CT angiogram or V/Q scan as initial test |
| Inconclusive CT or V/Q results | Pulmonary angiography |
| Renal failure or contrast allergy | V/Q scan |
| Pregnant patients | V/Q scan first due to radiation concerns |
This stepwise approach allows screening with less invasive options while proceeding to definitive tests efficiently in higher risk scenarios.
Conclusion
In summary, diagnosing PE requires a combination of clinical assessment and objective testing. While no single test provides perfect accuracy, current algorithms allow the majority of PEs to be rapidly identified or excluded using tests like D-dimer, CT angiography, V/Q scanning and selective pulmonary angiography when needed. This facilitates prompt initiation of anticoagulation treatment to prevent potentially life-threatening complications in confirmed PE patients.