It is essential to follow the practice of determining a clinical pretest probability of pulmonary embolism before proceeding with any diagnostic testing. This article simplifies the Wells score, PERC, and the revised Geneva score that help to rule out pulmonary embolism.

Modified Wells score

Key points

1. Well’s criteria risk stratifies patients for pulmonary embolism (PE) and provides an estimated pre-test probability. 
2. Wells’ is not meant to diagnose PE but to guide workup by predicting pre-test probability of PE and appropriate testing to rule out the diagnosis. There must first be a clinical suspicion for PE in the patient (this should not be applied to all patients with chest pain or shortness of breath).
3. The score is simple to use and provides clear cutoffs for the predicted probability of pulmonary embolism.
4. The score aids in potentially reducing the number of CTAs performed on low-risk pulmonary embolism patients.

In the three-tier model:

• Low-risk patients- Pulmonary embolism rule-out criteria (PERC) can be considered as well as D-dimer
• Moderate risk- Consider D-dimer or CT pulmonary angiography
• High risk- D-dimer is not recommended

In the two-tier model:

• Unlikely- Consider D-dimer
• Likely- Consider CT- pulmonary angiography

Pulmonary embolism rule-out criteria (PERC)

Key points

1. If all are negative, no further testing is required. If any of the mentioned criteria is positive, do a D-dimer. If elevated/positive, a CT-pulmonary angiography must be performed.
2. It rules out PE if no criteria are present and the pre-test probability is ≤15%.

The PERC rule is a “rule-out” tool - all variables must receive a “no” to be negative. The PERC rule can be applied to patients where the diagnosis of PE is being considered, but the patient is deemed low-risk. A patient deemed low-risk by the physician’s approach who is also <50 years of age, with a pulse <100 bpm, SaO2 >95%, no haemoptysis, no oestrogen use, no history of surgery/trauma within 4 weeks, no prior PE/DVT and no resent signs of DVT can be safely ruled out and does not require further workup.

This test rules out patients who are considered low-risk for PE based on clinical criteria alone. PERC negative patients do not require utilisation of the D-dimer, which has a high sensitivity but low specificity. Low-risk patients who are PERC negative avoid the risks associated with unnecessary testing and treatment for PE. The test is unidirectional. While PERC negative typically allows the clinician to avoid further testing, failing the rule doesn't force the clinician to order tests. It is utilised as a rule-out criterion, and not meant for risk-stratification.

Simplified revised Geneva score

Key points

1. The revised Geneva score eliminates the need for information gleaned from a chest radiograph or an arterial blood gas sample, ranging now from 0 to 22 points and results in 3 clinical probability categories: low, intermediate, and high risk.
2. A "simplified" revised Geneva score has also been used in which each of the below criteria is assigned 1 point if positive.

Probability of pulmonary embolism:

• Low probability: 0-3 points
• Intermediate probability: 4-10 points
• High probability: >10 points

It can risk-stratify patients into low, intermediate, or high risk based on history and physical exam alone. Many physicians use it as an alternative to Well’s pulmonary embolism criteria to determine those patients that are low enough risk to rule out pulmonary embolism with a D-dimer serum test and avoid the use of CT angiography and ultrasound. The revised Geneva is not meant to diagnose but to guide workup and testing by predicting the pre-test probability of PE. The Geneva score aids in the reduction of unnecessary imaging studies by identifying low-risk patients who can be ruled out for PE with a D-dimer serum test.

Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.