PERC Rule for Pulmonary Embolism

Q: What does "low pretest probability" mean in practice?

The clinician's overall impression that PE is unlikely, often supported by a low Wells or Geneva score, before PERC is applied.

Q: Does PERC replace the D-dimer test?

Only in PERC-negative, low-risk patients. In everyone else, D-dimer or imaging is still needed.

Q: Why is age 50 a cut-off?

Older age is associated with higher PE risk and with more false-positive D-dimer results; 50 was the threshold chosen in the derivation study.

Understanding the PERC rule for pulmonary embolism — concept and principle

The Pulmonary Embolism Rule-out Criteria (PERC) are eight simple checks used in emergency care to decide whether a patient with possible pulmonary embolism (PE) can safely skip further testing, including the D-dimer blood test. The rule only applies to patients the clinician already judges to be at low pretest probability, generally below 15%.

Both gates must be passed: low clinical suspicion first, then a clean sheet of eight.

How it is calculated

Count how many of these criteria are present:

Age 50 or over
Heart rate 100 beats/min or more
Oxygen saturation (SpO₂) below 95% on room air
Unilateral leg swelling
Haemoptysis (coughing blood)
Surgery or trauma within the past 4 weeks
Prior PE or DVT
Hormone use, such as oral contraceptives or hormone replacement therapy

If the count is 0, the patient is "PERC negative". If one or more criteria are present, the patient is "PERC positive" and the rule cannot exclude PE.

The principle

Every test has a cost: D-dimer is often falsely positive, which leads to CT scans with radiation, contrast and incidental findings. Below a certain probability of disease, testing causes more harm than it prevents. Kline and colleagues (2004) chose criteria so that, in a low-probability group, the absence of all eight pushes the probability of PE below that testing threshold. The rule is deliberately all-or-nothing: each criterion is a single sign that alone might signal PE, so any one of them means the rule does not apply.

Worked example

Defaults: all eight criteria No.
Criteria present = 0 + 0 + 0 + 0 + 0 + 0 + 0 + 0 = 0.
Result: PERC negative — only applies when clinical pretest probability is already low (< 15%).
For comparison, the same patient aged 52 has one criterion and is PERC positive, so the usual pathway (often a D-dimer test) applies.

How to read the result

Where does your result fall?

A negative PERC in a low-risk patient supports stopping the PE work-up there. A positive PERC does not mean PE is likely; it only means the rule cannot be used, and the clinician continues with a pretest score such as Wells and a D-dimer test. The rule is meant for emergency department settings similar to those it was studied in.

Common mistakes

Applying PERC to patients who are not already low risk by clinical judgement.
Treating "PERC positive" as a diagnosis of PE.
Using a saturation measured on supplemental oxygen.

Keep in mind: For information and education only — not a diagnosis or treatment advice. Anyone with sudden breathlessness, chest pain or fainting needs urgent medical assessment. Clinical decisions must be made by a qualified professional.

Frequently asked questions

What does "low pretest probability" mean in practice?