Diagnosing a pulmonary embolism (PE) follows a step-by-step process: doctors first assess your symptoms and risk factors using a standardized scoring system, then use blood tests and imaging to confirm or rule out a clot. No single test catches every case, so the diagnosis relies on combining clinical judgment with the right sequence of tests based on how likely a PE seems at the outset.
Symptoms That Raise Suspicion
The most common symptoms of PE are sudden shortness of breath, sharp chest pain that worsens with breathing, and cough. These symptoms overlap with many other conditions, which is part of what makes PE tricky to pin down. Some patterns vary by age and sex: younger men are more likely to cough up blood, while older patients more frequently experience fainting. As the severity of a PE increases, shortness of breath, fainting, and a rapid heart rate become more prominent regardless of who you are.
Chest pain is especially common in smaller, lower-risk PEs, appearing in roughly half of those cases. In higher-risk PEs, the dominant picture shifts toward cardiovascular stress: racing heart rate, drops in blood pressure, and loss of consciousness. None of these symptoms alone confirms a PE, but they tell the doctor whether to move forward with testing.
Clinical Scoring Systems
Before ordering any tests, doctors use structured scoring tools to estimate how likely it is that you actually have a PE. The two most widely used are the Wells Score and the Revised Geneva Score.
The Wells Score
The Wells Score assigns points based on seven factors:
- Signs of a blood clot in the leg (swelling, pain): 3 points
- PE is more likely than other diagnoses: 3 points
- Heart rate over 100 beats per minute: 1.5 points
- Immobilization or surgery in the past 4 weeks: 1.5 points
- Previous blood clot or PE: 1.5 points
- Coughing up blood: 1 point
- Active cancer or cancer treatment in the last 6 months: 1 point
A higher total means PE is more probable, and that probability determines which test comes next.
The Revised Geneva Score
The Revised Geneva Score uses a similar approach but relies entirely on objective criteria, removing the subjective “PE is the most likely diagnosis” judgment call. It factors in age (65 or older), previous clots, recent surgery or fracture, active cancer, leg pain, coughing up blood, heart rate, and leg swelling or tenderness. Scores of 0 to 3 put you in the low-risk category with roughly an 8% chance of PE. Scores of 4 to 10 indicate moderate risk, and 11 or above means high risk.
The PERC Rule: When No Testing Is Needed
For patients where the doctor’s initial suspicion is already very low (below about 15%), a tool called the Pulmonary Embolism Rule-out Criteria, or PERC rule, can eliminate the need for any blood work or imaging at all. It consists of eight yes-or-no questions:
- Are you older than 49?
- Is your heart rate above 99?
- Is your blood oxygen level below 95%?
- Are you coughing up blood?
- Are you taking estrogen (such as birth control or hormone therapy)?
- Do you have a history of blood clots?
- Have you had surgery or trauma requiring hospitalization in the past four weeks?
- Do you have swelling in one leg?
If the answer to every single question is “no,” PE is considered effectively ruled out without further testing. Even one “yes” means the workup continues.
The D-Dimer Blood Test
For patients with low-to-moderate probability scores, the next step is usually a D-dimer blood test. D-dimer is a protein fragment produced when a blood clot breaks down. A level below the standard cutoff (500 micrograms per liter) makes PE very unlikely, and no imaging is needed.
The catch is that D-dimer levels naturally rise with age, which means the standard threshold produces a lot of false positives in older adults. To address this, many hospitals now use an age-adjusted formula: for patients over 50, the cutoff becomes their age multiplied by 10. So a 70-year-old would have a cutoff of 700 instead of 500. This adjustment reduces unnecessary CT scans without missing real clots.
D-dimer is useful for ruling PE out, not for confirming it. Elevated levels can result from infection, inflammation, recent surgery, pregnancy, or cancer. A high D-dimer simply means imaging is the next step.
CT Pulmonary Angiography: The Primary Imaging Test
CT pulmonary angiography (CTPA) is the go-to imaging test when PE needs to be confirmed or excluded. It involves injecting contrast dye into a vein and taking rapid CT images of the blood vessels in the lungs. The scan directly shows whether a clot is blocking blood flow.
A meta-analysis of 12 studies covering 1,250 patients found CTPA has an overall sensitivity of about 74% and specificity of roughly 90%, with individual studies ranging from 57% to 100% sensitivity and 68% to 100% specificity. In practice, modern scanners with better resolution tend to perform at the higher end of those ranges. The test takes only seconds to perform and is available around the clock in most emergency departments, which is why it has become the standard.
For patients who score high on clinical probability, doctors typically skip the D-dimer and go straight to CTPA, since even a normal D-dimer wouldn’t be enough to rule out PE in a high-risk patient.
When CT Isn’t an Option
CTPA requires injecting iodinated contrast dye, which isn’t safe for everyone. Two groups in particular need an alternative: patients with kidney disease (because the contrast can further damage the kidneys) and patients with a history of severe allergic reactions to contrast dye.
For these patients, a ventilation-perfusion scan (V/Q scan) is the preferred alternative. This nuclear medicine test uses small amounts of radioactive material to compare airflow and blood flow in the lungs. Areas where air is reaching the lungs normally but blood flow is blocked suggest a clot. The V/Q scan avoids contrast dye entirely, making it a safer choice for patients who can’t tolerate it. People who have had a prior anaphylactic reaction to any substance, or who have atopic conditions like asthma, are at elevated risk for severe contrast reactions and are particularly good candidates for V/Q scanning instead.
Echocardiography for Severe Cases
When a PE is large enough to strain the heart, an echocardiogram (heart ultrasound) can reveal the damage in real time. A large clot increases resistance in the lung’s blood vessels, forcing the right side of the heart to work much harder. On ultrasound, this shows up as an enlarged right ventricle compared to the left, abnormal movement of the wall between the two heart chambers, and leaking of the heart’s tricuspid valve.
One particularly telling finding is called McConnell’s sign: the free wall of the right ventricle contracts poorly, but the tip of the heart moves normally. When present alongside other signs of right heart strain, it strongly supports a PE diagnosis. Echocardiography doesn’t replace CT scanning for confirming PE, but it plays a critical role when a patient is too unstable to leave the bedside for a CT scanner, or when doctors need to quickly assess how much the clot is compromising heart function to guide urgent treatment decisions.
Diagnosis During Pregnancy
Pregnancy complicates PE diagnosis in several ways. D-dimer levels rise naturally throughout pregnancy, making the standard blood test less reliable. Radiation exposure from CT scans is a concern, though the actual dose to the fetus is very low. And the stakes are high: PE is a leading cause of maternal death.
The recommended approach uses a pregnancy-adapted version of the YEARS algorithm, which combines three clinical criteria (signs of a leg clot, coughing up blood, and whether PE is the most likely diagnosis) with D-dimer cutoffs tailored to the number of criteria present. Compression ultrasound of the legs is often performed early in the workup. If it confirms a blood clot in the leg veins, that’s enough to start treatment without exposing the patient to a CT scan, since the treatment for leg clots and PE is the same. CTPA is reserved for cases where the diagnosis remains uncertain after these initial steps, and the choice of imaging depends on what’s available, how urgent the situation is, and whether a chest X-ray suggests an alternative explanation for the symptoms.