Clinical probability of pulmonary embolism

What it is and when to use it

The Wells score for pulmonary embolism estimates the pretest clinical probability of pulmonary embolism in patients with suspected PE, to guide use of D-dimer testing and imaging (CT pulmonary angiography). It assigns points for clinical signs of deep vein thrombosis, an alternative diagnosis less likely than PE, heart rate above 100, recent immobilization or surgery, prior PE or DVT, hemoptysis, and active cancer. The 2019 ESC guidelines on pulmonary embolism recognize it as one of the validated clinical prediction rules for stratifying pretest probability.

How to interpret it

There are two ways to apply it. In the three-tier scheme: 0-1 points indicates low probability, 2-6 points intermediate probability, and 7 or more points high probability. In the dichotomized scheme, now the most widely used: a score of 4 or less (PE unlikely) allows PE to be ruled out with a negative D-dimer, avoiding imaging; a score above 4 (PE likely) warrants CT pulmonary angiography. In the simplified model each item scores 1 point and the dichotomized threshold is 0-1 (unlikely) versus 2 or more (likely).

Limitations and when not to use it

It was validated in outpatients and emergency department patients with suspected PE, not in patients already hospitalized for other reasons nor as screening in asymptomatic individuals. The item "alternative diagnosis less likely than PE" is subjective and introduces interobserver variability. The score estimates probability but does not by itself confirm or exclude the diagnosis, nor does it assess the severity or mortality risk of confirmed PE: for that, indices such as the PESI and markers of right ventricular dysfunction are needed. It should not be used in isolation without integrating D-dimer and imaging according to the algorithm.

Frequently asked questions

What is the difference between the Wells score and the PERC rule?

The PERC rule is applied only when clinical probability is already low, to rule out PE without even ordering a D-dimer; the Wells score instead establishes that pretest probability and decides the next diagnostic step.

What Wells cut-off rules out pulmonary embolism with D-dimer?

Using the dichotomized scheme, a score of 4 or less (PE unlikely) combined with a negative D-dimer safely excludes PE without the need for CT pulmonary angiography.

Can the Wells score be used during pregnancy?

It is not well validated in pregnant patients and D-dimer-based algorithms perform less well in pregnancy; in this setting adapted pathways such as the pregnancy-adapted YEARS algorithm are used instead.