Calculate a patient’s PERC rule result from the 8 pulmonary embolism rule-out criteria to see if PE can be excluded without a D-dimer or CT scan.
PERC Rule Formula
The PERC rule is a checklist of eight criteria. Each criterion scores 1 point when it is present and 0 points when it is absent, and the points are added together:
PERC = Age + HR + SpO2 + Swelling + Hemoptysis + Surgery + VTE + Estrogen
A patient is PERC negative only when the total is 0, meaning every criterion is absent. Any score of 1 or more is PERC positive.
Variables:
- Age is 1 when the patient is 50 years or older
- HR is 1 when the heart rate is 100 bpm or higher
- SpO2 is 1 when the oxygen saturation on room air is under 95 percent
- Swelling is 1 when there is unilateral leg swelling
- Hemoptysis is 1 when the patient is coughing up blood
- Surgery is 1 when there was surgery or trauma needing general anesthesia in the past 4 weeks
- VTE is 1 when there is a prior deep vein thrombosis or pulmonary embolism
- Estrogen is 1 when the patient uses estrogen, such as oral contraceptives or hormone replacement
When a PERC positive result sends you on to a D-dimer, the cutoff itself can be raised for age in patients over 50:
D-dimer cutoff = Age * 10 (age over 50, ug/L FEU)
The calculator applies each threshold for you. In exact mode you enter the age, heart rate and oxygen saturation and the tool decides whether those three criteria are met, then you answer the remaining five findings Yes or No. The pretest probability selector at the top acts as a gate, because PERC is only valid once you have already judged the patient low risk. If you mark the patient as not low risk, the tool tells you the rule does not apply. A memory aid for the eight items is HAD CLOTS: Hormones, Age, DVT or PE, Coughing blood, Leg swelling, O2 low, Tachycardia, Surgery.
PERC Criteria and Age-Adjusted D-dimer Reference
The first table lists the eight criteria, the reassuring finding that scores 0, and what makes the criterion count against the patient.
| Criterion | Reassuring (0 points) | Counts against PERC (1 point) |
|---|---|---|
| Age | Under 50 years | 50 years or older |
| Heart rate | Under 100 bpm | 100 bpm or higher |
| Oxygen saturation | 95 percent or higher on room air | Under 95 percent on room air |
| Leg swelling | None or symmetric | Unilateral leg swelling |
| Hemoptysis | Absent | Coughing up blood |
| Recent surgery or trauma | None in past 4 weeks | Within 4 weeks, under general anesthesia |
| Prior DVT or PE | No history | Any prior venous clot |
| Estrogen use | None | Oral contraceptive or hormone replacement |
The second table is the age-adjusted D-dimer cutoff you use if a PERC positive result leads to a D-dimer in a patient over 50. It replaces the fixed 500 ug/L threshold and clears more older patients without imaging while keeping the miss rate low.
| Age | Age-adjusted cutoff (ug/L FEU) | Standard cutoff |
|---|---|---|
| 50 or under | 500 | 500 |
| 55 | 550 | 500 |
| 60 | 600 | 500 |
| 65 | 650 | 500 |
| 70 | 700 | 500 |
| 75 | 750 | 500 |
| 80 | 800 | 500 |
Example Problems
Example 1: a PERC negative patient.
A 44 year old with pleuritic chest pain has a heart rate of 88, an oxygen saturation of 98 percent on room air, no leg swelling, no hemoptysis, no recent surgery, no prior clot and no estrogen use. You judge the pretest probability low. Every criterion scores 0, so the PERC total is 0. The patient is PERC negative, and you can rule out PE without a D-dimer.
Example 2: one criterion turns it positive.
A 55 year old with the same clean history now meets the age criterion, because 55 is over 50. The PERC total is 1, so the patient is PERC positive and the rule cannot exclude PE. Since the age is over 50, the age-adjusted D-dimer cutoff becomes 55 times 10, or 550 ug/L, instead of the fixed 500.
Frequently Asked Questions
What does a PERC negative result mean?
PERC negative means all eight criteria are absent. In a patient you have already judged low risk, that lowers the chance of pulmonary embolism to under about 2 percent, which is low enough that further testing tends to cause more harm than it prevents. You can stop the workup without a D-dimer or CT scan. It does not mean PE is impossible, only that testing is not worthwhile.
Can you use the PERC rule on every patient?
No. PERC only applies after you have decided the patient is low risk by clinical judgment, usually a gestalt estimate under 15 percent or a low Wells score. In moderate or high risk patients a negative PERC does not rule out PE, so you still need a D-dimer or imaging. The rule was also not validated in pregnancy, active cancer, or strong thrombophilia, and beta blockers can hide a fast heart rate.
How is the PERC rule different from the Wells score?
The Wells score estimates how likely PE is in the first place and sorts patients into risk groups. PERC is the next step for the low risk group, asking whether that low risk patient can skip testing entirely. Wells sets the pretest probability, and PERC decides whether a low probability patient needs a D-dimer at all.