A pulmonary embolism (PE) is a blockage in a lung artery, most often caused by a blood clot that travels from another part of the body. This original clot, a deep vein thrombosis (DVT), usually forms in the deep veins of the legs or pelvis. Because a PE can restrict blood flow, lower blood oxygen levels, and strain the heart, a timely and accurate diagnosis is necessary. To determine if a patient has a PE, doctors use a systematic process that combines clinical evaluation with specific diagnostic tests to confirm or rule out the condition.
Recognizing the Signs and Risk Factors
The diagnostic journey begins with a thorough clinical evaluation, focusing on the patient’s symptoms and personal risk profile. The signs of a pulmonary embolism can vary significantly from person to person, depending on the size of the clot and the individual’s overall health. The most frequently reported symptoms are a sudden onset of shortness of breath, chest pain that worsens with deep breathing, and a persistent cough, which may sometimes produce bloody sputum.
Beyond these common indicators, other signs may also point toward a PE. These can include a rapid heart rate (tachycardia), unexplained sweating, wheezing, or feelings of lightheadedness and dizziness. In some cases, symptoms related to the original DVT in the leg may be present, such as localized pain, swelling, warmth, or a reddish discoloration of the skin.
A doctor’s suspicion is further guided by an assessment of risk factors, as a PE rarely occurs without a predisposing condition. The presence of one or more of these factors helps build a clinical picture of a patient’s likelihood of having a PE. Key risk factors include:
- A personal or family history of blood clots, including DVT or a previous PE
- Long periods of immobility, such as extended bed rest after a major surgery or long-haul flights
- Active cancer, which can make the blood more prone to clotting
- Cardiovascular diseases like heart failure or stroke
- Pregnancy and the use of estrogen-based medications like hormonal birth control
- Obesity and recent trauma, particularly fractures of the pelvis or leg
Diagnostic Blood Tests
Following the initial clinical assessment, blood tests are an early step in the diagnostic process, with the D-dimer test being the most important. This test measures D-dimer, a protein fragment produced when a blood clot dissolves in the body. Its primary value is its ability to help rule out a pulmonary embolism, particularly in patients with a low or moderate probability of the condition.
A normal D-dimer result indicates that significant clot formation is not occurring. In this scenario, it is highly unlikely the patient has a PE, and doctors can often rule out the diagnosis without proceeding to more invasive imaging.
Conversely, an elevated D-dimer result does not confirm a PE. It only suggests the possibility of a clot, as other conditions like recent surgery, infection, cancer, and pregnancy can also raise D-dimer levels. A positive result serves as a trigger for confirmatory imaging rather than a definitive answer.
Confirmatory Imaging Techniques
When a pulmonary embolism is suspected after initial assessments, or if a D-dimer test is positive, doctors turn to advanced imaging techniques to visualize the pulmonary arteries and confirm the presence of a clot. These tests provide direct evidence of a blockage, allowing for a definitive diagnosis.
CT Pulmonary Angiography (CTPA)
The most widely used and preferred imaging method for diagnosing a PE is the CT pulmonary angiography (CTPA). This non-invasive procedure uses computed tomography (CT) scanning and a special contrast dye to create detailed, three-dimensional images of the lungs’ arteries. During the test, an iodine-containing contrast agent is injected into a vein, which travels through the bloodstream to the pulmonary arteries, making them appear bright on the scan. A computer then processes a series of cross-sectional images to build a comprehensive map of the pulmonary vasculature.
If a blood clot is present, it will appear as a dark filling defect within the bright, dye-filled artery, blocking the normal flow of blood. The high resolution of modern scanners allows for the detection of even small clots. While highly accurate, the CTPA involves exposure to radiation and carries a small risk of an allergic reaction or kidney issues related to the contrast dye.
Ventilation/Perfusion (V/Q) Scan
A ventilation/perfusion (V/Q) scan is a key alternative to the CTPA. It is often used for patients who cannot receive the contrast dye required for a CTPA, such as those with severe kidney disease or a strong allergy to iodine, as well as for pregnant women to minimize radiation exposure. The V/Q scan is a nuclear medicine test performed in two parts to evaluate both airflow (ventilation) and blood flow (perfusion) in the lungs.
For the ventilation part, the patient inhales a harmless radioactive gas or aerosol, and a scanner takes pictures to show how air is distributed. For the perfusion part, a small amount of a radioactive tracer is injected into a vein, and the scanner tracks its path to map blood circulation. A PE is indicated by a “mismatch” between the two scans: an area of the lung that shows normal ventilation but has no corresponding blood flow.
Supplementary Imaging
In addition to the primary diagnostic tools, other imaging tests may be used to provide supporting evidence. A venous ultrasound of the legs, specifically a compression ultrasonography, is often performed to look for the source DVT. This test uses sound waves to check for clots in the deep veins of the thigh and calf. Finding a DVT in a patient with symptoms of a PE significantly increases the likelihood of the diagnosis.
An echocardiogram, an ultrasound of the heart, can also be a valuable tool, especially in patients who are hemodynamically unstable. While it typically cannot visualize the clot in the lung itself, it can detect signs of strain and dysfunction in the right ventricle of the heart, which must work harder to pump blood past the obstruction. These findings help doctors assess the severity of the PE.
The Diagnostic Pathway
Detecting a pulmonary embolism follows a logical pathway that integrates a patient’s clinical presentation with a sequence of tests. This diagnostic algorithm is designed to confirm or exclude PE accurately while minimizing unnecessary testing. The process begins with estimating the patient’s pre-test probability—the likelihood of having a PE before definitive tests are performed.
To standardize this estimation, clinicians use validated clinical decision rules like the Wells’ score or the Pulmonary Embolism Rule-out Criteria (PERC). These tools assign points based on specific clinical criteria, such as heart rate, recent surgery, or signs of DVT. The total score categorizes the patient into a low, moderate, or high-risk group.
The diagnostic algorithm unfolds based on this risk stratification. For a patient deemed very low-risk by the PERC rule, a PE can often be safely ruled out with no further testing. If a patient has a low or moderate Wells’ score, the next step is a D-dimer blood test. A negative D-dimer in this group effectively excludes a PE, while a positive result prompts a move to imaging.
Patients categorized as high-risk, or those in lower-risk groups with a positive D-dimer, proceed directly to a confirmatory imaging test. The CT pulmonary angiography (CTPA) is the most common choice to visualize the pulmonary arteries. This structured approach ensures the diagnostic process is tailored to each patient’s clinical picture.