Use this scale to rule out pulmonary embolism if the clinical probability is low (<15%) and there were no tests run beforehand.

Remember, that there are limitations for using the PERC scale. See the appropriate section before using the calculator.
Age ≥50
No
HR ≥100 bmp
No
O₂ saturation <95%
No
Unilateral leg swelling
No
Hemoptysis
No
Prior history of PE/ DVT
No
Recent trauma or surgery
No
Hormone use
No
Result
0
Pulmonary embolism can be ruled out in this patient, as the overall risk is below 2%.

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. If the result of PERC is negative, then D-Dimer testing can be avoided.

How to use PERC calculator?

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, take a look at the panel on the left of your screen.
  3. Select "Yes" to all of the conditions that refer to your patient. They are all automatically set as 'No', 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.

PERC criteria (pulmonary embolism rule out criteria) description

In the PERC calculator, you can see a short description for some of the points hovering over the particular criteria. In this section, you'll find a more detailed description:

  • Age ≥50 years old - is a patient older than 50?
  • Heart rate over 100/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 is 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 using estrogens for other reasons.

PERC - negative and positive results

Although in the PERC calculator you see eight different factors, there are only two possible outcomes. PERC negative means that none of the risk criterion 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.

PERC rule limitations

Although the criterion 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 that 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, which may mask the leg swelling,
  • long-term hypoxemia <95%.
Aleksandra Zając, MD