When doctors suspect a pulmonary embolism (PE) — a blood clot in the lungs — they don’t just guess. They use a combination of scoring systems and tests to rule it out safely and quickly. The most common approach starts with the Wells score or the Revised Geneva score, which estimate your pre-test probability. If your score is low, a D-dimer blood test can often rule out a PE without needing a CT scan. If your score is high or the D-dimer is positive, a CT pulmonary angiogram (CTPA) is the standard imaging test to confirm or rule out a clot. The goal is to avoid unnecessary radiation and contrast dye for people who are unlikely to have a PE.
What Is the Wells Score and How Is It Used?
The Wells score is the most widely used clinical prediction tool for pulmonary embolism. It was developed by researchers in Canada and has been validated in many studies worldwide. The score assigns points based on clinical signs, risk factors, and alternative diagnoses.
Points are given for things like:
- Clinical signs of deep vein thrombosis (DVT) — 3 points
- PE is the most likely diagnosis — 3 points
- Heart rate over 100 beats per minute — 1.5 points
- Immobilization or surgery within the past 4 weeks — 1.5 points
- Previous DVT or PE — 1.5 points
- Hemoptysis (coughing up blood) — 1 point
- Cancer (active treatment, palliative, or diagnosed within 6 months) — 1 point
A total score of 4 points or less is considered “PE unlikely.” A score above 4 is “PE likely.” Studies published in the Annals of Internal Medicine show that when the Wells score says PE is unlikely and the D-dimer is negative, the chance of having a PE is less than 2%. That is considered safe enough to stop testing.
Does the Revised Geneva Score Work the Same Way?
The Revised Geneva score is an alternative to the Wells score. It was developed in Europe and does not include the subjective judgment of “PE is the most likely diagnosis,” which some doctors find unreliable. Instead, it uses only objective clinical criteria.
The Revised Geneva score includes:
- Age over 65 — 1 point
- Previous DVT or PE — 3 points
- Surgery or fracture within 1 month — 2 points
- Active cancer — 2 points
- Unilateral leg pain — 3 points
- Hemoptysis — 2 points
- Heart rate 75-94 — 3 points; heart rate 95 or above — 5 points
- Clinical signs of DVT — 4 points
A score of 0 to 5 is low probability. A score of 6 to 11 is moderate. A score of 12 or above is high. Research from the European Respiratory Journal shows that the Revised Geneva score performs similarly to the Wells score. Both are acceptable for ruling out PE when combined with a D-dimer test. The choice often comes down to doctor preference or hospital protocol.
What Is the D-Dimer Test and When Is It Reliable?
The D-dimer test measures a protein fragment that is released when a blood clot breaks down. In a healthy person, D-dimer levels are low or undetectable. When a clot forms and dissolves, levels rise. The test is sensitive but not specific — many things can raise D-dimer levels, including pregnancy, recent surgery, infection, and cancer.
The key point: D-dimer is only useful for ruling out PE, not for diagnosing it. A negative D-dimer (below the lab’s cutoff, usually 500 ng/mL) in a patient with a low or moderate pre-test probability effectively rules out PE. A positive D-dimer means further imaging is needed.
For people over 50, the cutoff is adjusted upward. The formula is age × 10. So for a 70-year-old, the cutoff is 700 ng/mL instead of 500. This adjustment, recommended by the American College of Emergency Physicians, reduces false positives without missing many clots. A study in JAMA found that using age-adjusted cutoffs reduced unnecessary CT scans by about 20% in older patients.
What Imaging Tests Confirm or Rule Out a PE?
When the Wells or Revised Geneva score indicates PE is likely, or when the D-dimer is positive, the next step is imaging. The gold standard is the CT pulmonary angiogram (CTPA). This is a specialized CT scan that uses intravenous contrast dye to visualize the arteries in the lungs. It can detect clots as small as the subsegmental level.
A negative CTPA essentially rules out a clinically significant PE. The negative predictive value is above 99%. However, CTPA exposes you to radiation (roughly 5-15 mSv, equivalent to 1-5 years of background radiation) and carries a small risk of allergic reaction or kidney injury from the contrast dye.
The alternative is a V/Q scan (ventilation-perfusion scan), which uses a radioactive tracer to compare airflow and blood flow in the lungs. It is preferred for people with kidney problems or contrast allergies. A normal V/Q scan also rules out PE. But if the scan is “intermediate probability,” further testing is needed. The New England Journal of Medicine published a landmark trial showing that CTPA and V/Q scans have similar accuracy for diagnosing PE.
How To Rule Out Pulmonary Embolism Tests Scores — A Comparison Table
| Test / Score | What It Measures | How It Rules Out PE | Limitations |
|---|---|---|---|
| Wells Score | Clinical signs, risk factors, alternative diagnosis likelihood | Score ≤4 + negative D-dimer = PE ruled out | Subjective “PE likely” component varies by clinician |
| Revised Geneva Score | Objective clinical criteria only | Score 0-5 + negative D-dimer = PE ruled out | Less validated in some populations |
| D-Dimer Test | Blood clot breakdown product | Negative result (age-adjusted) in low/moderate probability = PE ruled out | High false positive rate; not useful in high-probability patients |
| CT Pulmonary Angiogram (CTPA) | Direct visualization of pulmonary arteries | Negative scan = PE ruled out (99%+ certainty) | Radiation exposure, contrast dye risk |
| V/Q Scan | Matching of ventilation and perfusion in lungs | Normal scan = PE ruled out | Less definitive if intermediate result; takes longer |
When Should You Not Rely on These Scores Alone?
Clinical scoring systems are powerful tools, but they are not perfect for every situation. If you have a high pre-test probability — for example, clear symptoms of DVT plus sudden shortness of breath — many guidelines recommend skipping the D-dimer and going straight to CTPA. The reasoning is that a negative D-dimer in a high-probability patient still carries a 5-10% chance of PE, which is too high to ignore.
Pregnancy is another special case. D-dimer levels naturally rise during pregnancy, making the test less reliable. The Wells score has not been well validated in pregnant women. In these cases, guidelines from the American College of Obstetricians and Gynecologists recommend using a modified approach, often starting with a V/Q scan or lower-dose CTPA.
People with kidney disease also need caution. Contrast dye used in CTPA can cause acute kidney injury. If the estimated glomerular filtration rate (eGFR) is below 30 mL/min, doctors may opt for a V/Q scan or use a different contrast protocol. There is no clinical evidence that skipping imaging entirely is safe in this group if the D-dimer is positive.
Common Misconceptions About Ruling Out Pulmonary Embolism
One widespread claim is that a normal chest X-ray rules out PE. It does not. A chest X-ray can look completely normal even with a large clot in the lung arteries. It is only useful for ruling out other causes of symptoms, like pneumonia or a collapsed lung.
Another myth is that a negative D-dimer means you are clot-free everywhere. It does not. The test is specific to clot breakdown products in the blood. A small clot in a leg vein may not raise D-dimer enough to be detected, especially if it is chronic. The test is only validated for ruling out acute venous thromboembolism in the right clinical context.
Some people believe that if you can breathe fine, you cannot have a PE. This is false. Small peripheral clots may cause no symptoms at all. The first sign can be sudden death from a large clot that breaks loose. That is why scoring systems exist — to catch the ones that are silent but dangerous.
Frequently Asked Questions
Can a D-dimer test alone rule out a pulmonary embolism?
No. A D-dimer test must be interpreted with a clinical probability score like Wells or Revised Geneva. Alone, it has too many false positives to be useful.
How long does it take to get results from a CT pulmonary angiogram?
The scan itself takes about 10 minutes. Results are usually available within a few hours, often sooner in an emergency department.
Is there a home test to rule out a pulmonary embolism?
No. There is no reliable home test. Ruling out PE requires blood work and imaging that only a hospital or clinic can provide.
What happens if the Wells score says PE unlikely but symptoms persist?
If symptoms continue or worsen, a repeat evaluation is needed. Sometimes a clot is missed on initial testing, especially if symptoms change.
