When a patient has a new shortness of breath (especially if it begins abruptly), or a chest pain that hurts with every breath (“pleuritic” pain), we think about the possibility of Pulmonary Embolism (PE), a blood clot in the lung. But since these symptoms are not rare, usually due to other conditions, we don’t want to expose people to radiation of CT scans unless there’s some likelihood of the diagnosis. Most importantly, we know not to order tests “just in case” on people with low likelihood of having the disease we’re looking for, since we’d get lots of false-positives which can even be dangerous.
Certainly if a patient seems ill, breathing very fast, or with low oxygen in their blood, we worry about a PE. We’d also worry if they had signs of a blood clot in the leg (which is where most clots begin before they break off & float to the lung) — pain, tenderness, redness, or swelling in the inner part of the thigh. But for people without such signs, who are only short of breath, the following risks are significant for a PE:
- Previous history of a blood clot in the lung or leg
- A leg cast for 3 or more days
- General anesthesia in the past 4 weeks
- Active cancer
- Maybe HIV infection
- Maybe obese women with hypertension who smoke
- Covid-19 infection (current or recent)
Such patients need a blood test called a D-dimer, which is elevated whenever there’s a clot, or for lots of other reasons as well. If it’s normal, there’s no clot, therefore no PE. If high, we consider testing for a PE, based on their other risks as above. We actually don’t go looking for other causes of a high D-dimer — all we care about is whether it means a PE. The D-dimer should always be performed in an ER, where they get immediate results.