Ruling out a pulmonary embolism (PE) follows a step-by-step process that starts with a clinical risk assessment, moves to a blood test in most cases, and only reaches imaging if those earlier steps can’t clear you. The goal is to safely exclude a blood clot in the lungs without exposing low-risk patients to unnecessary CT scans and radiation. Here’s how that process works at each stage.
The First Step: Clinical Risk Scoring
Before any blood draw or scan, your doctor evaluates your overall likelihood of having a PE using a standardized scoring system. The most widely used is the Wells score, which assigns points based on seven factors: signs of a blood clot in the leg (3 points), PE being the most likely explanation for your symptoms (3 points), heart rate above 100 (1.5 points), a history of blood clots (1.5 points), recent surgery or being bedridden for three or more days in the past four weeks (1 point), coughing up blood (1 point), and active cancer (1 point). A total score of 4 or below classifies you as “PE unlikely,” while anything above 4 means “PE likely.”
Another option is the Revised Geneva Score, which uses similar but slightly different variables. It factors in age over 65, heart rate in three tiers (with a rate of 95 or higher earning 5 points), leg pain, and leg swelling with tenderness. Scores of 0 to 3 are low probability, 4 to 10 intermediate, and 11 or higher high probability. Both scoring systems accomplish the same thing: sorting patients into risk categories that determine what happens next.
When No Testing Is Needed at All
For patients who already appear very low risk, there’s an even earlier off-ramp called the PERC rule (Pulmonary Embolism Rule-out Criteria). It applies to adults aged 18 to 50 and checks eight simple factors: age under 50, heart rate under 100, oxygen saturation 95% or above, no coughing up blood, no recent surgery or trauma within four weeks, no history of blood clots, no unilateral leg swelling, and no use of oral estrogen. If all eight criteria are met and your doctor’s initial impression already puts you in the low-risk category, PE can be ruled out on clinical grounds alone, with no blood test and no imaging.
PERC exists because overtesting carries its own risks. CT scans involve radiation and contrast dye, and D-dimer blood tests (described below) frequently come back falsely elevated, triggering a cascade of further workups that weren’t needed. PERC stops that cascade before it starts in the right patient.
The D-Dimer Blood Test
If you don’t meet all eight PERC criteria but your clinical risk score is low or intermediate, the next step is a D-dimer blood test. D-dimer is a protein fragment released when the body breaks down blood clots. A level below 500 ng/mL generally rules out PE without further testing. The test is extremely sensitive, meaning a negative result is very reliable. The catch is that D-dimer rises with age, inflammation, infection, pregnancy, surgery, and cancer, so a positive result doesn’t confirm PE. It just means imaging is needed.
For patients over 50, the standard 500 ng/mL cutoff produces a high rate of false positives. An age-adjusted formula addresses this: multiply your age by 10 to get your personal threshold. A 70-year-old, for example, would use 700 ng/mL as the cutoff instead of 500. A large meta-analysis confirmed this approach safely excludes PE while significantly reducing unnecessary CT scans in older adults.
The YEARS Algorithm
A newer approach called the YEARS algorithm combines clinical assessment and D-dimer interpretation into a single streamlined step. It checks just three criteria: signs of deep vein thrombosis in the leg, coughing up blood, and whether PE is the most likely diagnosis. If none of these three apply to you, a D-dimer below 1,000 ng/mL rules out PE. If one or more apply, the threshold drops to the standard 500 ng/mL. By raising the D-dimer cutoff for the lowest-risk patients, the YEARS algorithm reduces the number of CT scans ordered without missing clots.
CT Pulmonary Angiography: The Definitive Scan
When D-dimer comes back elevated, or when clinical probability is high from the start, the gold-standard imaging test is CT pulmonary angiography (CTPA). This is a contrast-enhanced CT scan focused on the blood vessels in your lungs. It takes only seconds and can directly visualize a clot. CTPA has a sensitivity around 93% and specificity above 98%, meaning it catches the vast majority of clots and rarely flags something that isn’t there.
A normal CTPA in a patient with low or intermediate clinical probability effectively rules out PE. In high-probability patients, a negative CTPA is still very reassuring but may occasionally be supplemented with additional testing if suspicion remains strong.
When a V/Q Scan Is Used Instead
Not everyone can get a CTPA. The scan requires iodine-based contrast dye injected into a vein, which can be a problem for people with kidney disease or a history of contrast allergies. In those cases, a ventilation-perfusion (V/Q) scan is the main alternative. This nuclear medicine test uses a small amount of radioactive tracer to compare airflow and blood flow in the lungs. Mismatched areas, where air reaches the lungs but blood doesn’t, suggest a clot is blocking flow.
V/Q scans work well when the result is clearly normal or clearly abnormal. The limitation is that results sometimes land in an “indeterminate” zone, which doesn’t rule anything in or out and may require further workup.
Leg Ultrasound as a Supporting Test
Because most pulmonary emboli originate from blood clots in the deep veins of the legs, compression ultrasonography of the lower extremities plays a supporting role in the diagnostic process. The test has about 95% sensitivity and 92% specificity for detecting deep vein thrombosis. If a leg ultrasound finds a clot, treatment begins regardless of whether PE is confirmed on imaging, since the treatment is the same: anticoagulation.
Leg ultrasound is particularly useful when CTPA is unavailable or contraindicated. Finding a DVT in a patient with respiratory symptoms consistent with PE is enough to start treatment, effectively making the question of whether a clot has already reached the lungs less urgent from a management standpoint.
Ruling Out PE During Pregnancy
Pregnancy complicates the process because D-dimer levels naturally rise throughout gestation, making the standard test far less specific. At the same time, both CTPA and V/Q scans involve radiation exposure to the fetus, so doctors want to minimize unnecessary imaging.
A pregnancy-adapted version of the YEARS algorithm helps solve this problem. It uses the same three clinical criteria (leg clot signs, coughing up blood, PE as the most likely diagnosis) with the same D-dimer thresholds. In the original study published in the New England Journal of Medicine, this approach safely avoided CT scans in 39% of pregnant women with suspected PE. For women who do have leg symptoms, a compression ultrasound is performed first. If it confirms a clot, treatment starts without a CT scan at all.
How the Full Process Comes Together
The diagnostic pathway is designed as a funnel. At each step, low-risk patients are filtered out safely, and only those who genuinely need imaging proceed to a scan. In practice, it looks like this:
- Very low suspicion: If you’re under 50 and meet all eight PERC criteria, PE is ruled out clinically. No further testing.
- Low to intermediate suspicion: A D-dimer blood test is drawn. If it’s below the appropriate threshold (standard or age-adjusted), PE is ruled out. If it’s elevated, you proceed to CTPA.
- High suspicion: D-dimer is skipped entirely because even a normal result wouldn’t be reassuring enough. You go straight to CTPA.
This layered approach means that many patients who arrive at the emergency department with chest pain or shortness of breath can be safely cleared with nothing more than a clinical assessment and a blood test. For those who do need imaging, CTPA provides a fast, highly accurate answer. The system isn’t perfect, but when the steps are followed in order, the chance of missing a clinically significant PE is extremely low.