PERC Calculator - Pulmonary Embolism Rule-Out Criteria

This PERC calculator (Pulmonary Embolism Rule-out Criteria) gives you a quick look at the PERC criteria, which can help you determine if the patient is likely to have a pulmonary embolism. The PERC rule was first introduced in the 2005 paper Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism, and validated two years later in Prospective multicenter evaluation of the pulmonary embolism rule-out criteria.

Right now, it makes a good supplement to the patient's Well's score (Wells score calculator). If the result of PERC is negative, then D-Dimer testing can be avoided.

Using the PERC calculator is all about checking how the patient compares to the PERC criteria. Just follow the steps below:

1. You may want to know the limitations of the PERC rule first, as, in some cases, you cannot rely on the scale. Check the last section for more details.
2. If you can apply the pulmonary embolism rule-out criteria to your patient, you can now proceed to our PERC Calculator.
3. Tick all of the conditions that refer to your patient. They are all automatically set unchecked, so you're only interested in positive checks.
4. We'll estimate the result using all of the criteria, and you will see the answer given below, together with a short comment.

In the PERC calculator, the conditions are already quite descriptive and informative. In this section, you'll find a more detailed description:

• Age ≥50 years old — is a patient older than 50?

• Heart rate over 100 beats per minute — defined professionally as tachycardia. Be careful, as the use of β-blockers can mask it.

• O₂ saturation below 95% on room air — cannot be used in patients who use additional oxygen for breathing.

• Unilateral leg swelling — it might be a sign of deep venous thrombosis, especially if the leg is painful and the complaints are fairly new.

• Hemoptysis — is a patient coughing up blood?

• Prior history of deep venous thrombosis or pulmonary embolism — if that condition has already happened, it is a sign that the patient has a tendency to suffer from this kind of illness.

• Recent trauma or surgery — a trauma or surgery that happened less than 4 weeks ago.

• Exogenous estrogen use — women on oral contraceptive pills, those on hormone replacement therapy, or those using estrogens for other reasons.

Although in the PERC calculator you see eight different factors, there are only two possible outcomes. PERC negative means that none of the risk criteria were met, and the possibility of pulmonary embolism is very low. On the other hand, just one factor is enough to support further testing, starting by examining the patient's D-Dimer level.

As mentioned in the calculator itself, the scale is helpful if you want to assess if the risk of PE (pulmonary embolism) is below <15%. We can treat it as a reference to the Well's score's diagnosis of 'PE as likely as, OR more likely than, an alternative diagnosis'. According to the Australian Family Physician, if the patient's Well's score is ≤4 (or ≤1 in a simplified version), we can apply the PERC rule. If none of the PERC criteria are met (the answer to every point is 'No' and the points are equal to zero), we can safely rule out pulmonary embolism.

Although the criteria used in this PERC calculator are very useful in determining which patients need further D-Dimer testing, there are some cases when you cannot apply the PERC rule. Patients with the below conditions were not included in the original paper that introduced this method, so the rule may be unreliable in their case:

• Presence of cancer;
• Presence of thrombophilia or a strong family history of thrombophilia;
• Using β-blockers, which may mask tachycardia;
• Transient tachycardia;
• Leg amputation;
• Massive obesity (a result ⩾30 in the BMI calculator), which may mask the leg swelling; and
• Long-term hypoxemia <95%.