PERC rule: the eight criteria and how they combine

The PERC rule (pulmonary embolism rule-out criteria) is eight criteria that must all be met: age under 50, pulse under 100 beats per minute, oxygen saturation above 94%, no unilateral leg swelling, no hemoptysis, no recent trauma or surgery, no prior pulmonary embolism or deep vein thrombosis, and no hormone use. When all eight are met, the result is called PERC-negative. A 40-year-old with a pulse of 80, saturation of 98% and none of the five history findings is PERC-negative; the same person at age 50 isn't.

This page doesn't give disposition advice. It shows how the eight criteria combine. What a PERC-negative result means for testing depends on the clinical context.

Not a medical device. Not a diagnosis or dosing recommendation.

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Kline JA et al. · 2004
The PERC logic
Worked examples
PERC reference table
Frequently asked questions

What is the PERC rule?

PERC was published by J. A. Kline and colleagues in the Journal of Thrombosis and Haemostasis in 2004. It's also called the PE rule-out criteria, the pulmonary embolism rule-out criteria, Kline PERC or the PERC block rule.

The 2004 abstract describes it as an "eight-factor block rule." "Block" means the criteria work together as one unit: every criterion has to be met, and there are no points to add.

Magent's record describes the rule as applying to patients who already have a low pretest probability of pulmonary embolism, as in the derivation paper. The calculator doesn't assign that probability; the caller does.

The PERC logic

PERC-negative = age < 50
            AND heart_rate_bpm < 100
            AND spo2_percent > 94
            AND NOT unilateral_leg_swelling
            AND NOT hemoptysis
            AND NOT recent_surgery_or_trauma
            AND NOT prior_vte
            AND NOT estrogen_use

Exact expression the calculator uses:

PERC-negative iff age < 50 and pulse < 100 bpm and SaO2 > 94% and no unilateral leg swelling and no hemoptysis and no recent trauma or surgery and no prior PE or DVT and no hormone use

If any one criterion isn't met, the result isn't PERC-negative.

The eight criteria

Criterion	Input	Met when	Not met example
Age	`age`, years (18–120)	under 50	50
Pulse	`heart_rate_bpm`, beats/min (30–220)	under 100	100
Oxygen saturation	`spo2_percent` (50–100)	above 94	94
Unilateral leg swelling	`unilateral_leg_swelling`	false	true
Hemoptysis (coughing up blood)	`hemoptysis`	false	true
Recent trauma or surgery	`recent_surgery_or_trauma`	false	true
Prior PE or DVT	`prior_vte`	false	true
Hormone use	`estrogen_use` (hormone or exogenous estrogen)	false	true

How to apply the PERC rule

Age. Under 50 meets the criterion. 49 meets it; 50 doesn't.
Pulse. Under 100 beats per minute meets it. 99 meets it; 100 doesn't.
Oxygen saturation. Above 94% meets it. 95 meets it; 94 doesn't.
Five history and examination findings. Each is met when it's absent: unilateral leg swelling, hemoptysis, recent trauma or surgery, prior PE or DVT, hormone use.
Combine. If all eight are met, the result is PERC-negative. If any one isn't, it isn't PERC-negative.

Notice that all three numeric edges are strict. Exactly 50, exactly 100 and exactly 94 each fail their criterion.

What the calculator returns

Magent's calculator returns a true/false perc_negative result and a line for each of the eight criteria, showing whether it was met and, for the numeric ones, the value used.

Worked examples

These use Magent's own expected values for the calculator. Unless stated, age is 40, pulse is 80, saturation is 98% and the five findings are absent.

Example 1: all eight met

Age 40 < 50; pulse 80 < 100; saturation 98 > 94.
No leg swelling, hemoptysis, recent trauma or surgery, prior PE or DVT, or hormone use.
Result: PERC-negative.

Example 2: age 49 vs 50

Age 49: met → PERC-negative.
Age 50: not met → not PERC-negative.

Example 3: pulse 99 vs 100

Pulse 99: met → PERC-negative.
Pulse 100: not met → not PERC-negative.

Example 4: saturation 95 vs 94

Saturation 95: met → PERC-negative.
Saturation 94: not met → not PERC-negative.

Example 5: hormone use

Everything else met; hormone use: yes.
Result: not PERC-negative.

PERC reference table

Each row is one of Magent's expected values for the calculator.

Age	Pulse	SaO2 %	Finding present	PERC-negative?
40	80	98	None	Yes
49	80	98	None	Yes
50	80	98	None	No
40	99	98	None	Yes
40	100	98	None	No
40	80	95	None	Yes
40	80	94	None	No
40	80	98	Unilateral leg swelling	No
40	80	98	Hemoptysis	No
40	80	98	Recent trauma or surgery	No
40	80	98	Prior PE or DVT	No
40	80	98	Hormone use	No

History of the PERC rule

The rule comes from J. A. Kline, A. M. Mitchell, C. Kabrhel, P. B. Richman and D. M. Courtney, Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism, Journal of Thrombosis and Haemostasis, 2004 (volume 2, issue 8, pages 1247–1255).

The abstract opens: "Overuse of the d-dimer to screen for possible pulmonary embolism (PE) can have negative consequences." The criteria "were derived from logistic regression analysis with stepwise backward elimination of 21 variables collected on 3148 emergency department patients evaluated for PE at 10 US hospitals."

It lists the result: "Eight variables were included in a block rule: Age < 50 years, pulse < 100 bpm, SaO(2) > 94%, no unilateral leg swelling, no hemoptysis, no recent trauma or surgery, no prior PE or DVT, no hormone use." Those are the same eight criteria and the same strict edges Magent's calculator uses.

The rule was then tested prospectively in two further groups: a low-risk group of 1,427 patients from two hospitals and a very low-risk convenience sample of 382 patients with shortness of breath in whom PE wasn't suspected.

When it's used

Magent's record describes PERC as applying to patients who already have a low pretest probability of PE, as in the derivation paper, with the caller supplying the age, pulse, saturation and five findings.

Where it's used

The criteria were derived from emergency department patients at 10 US hospitals.

Who uses it

The 2004 study population was emergency department patients evaluated for PE.

Limits of the calculation

It checks eight criteria. It doesn't examine anyone and doesn't diagnose or exclude pulmonary embolism.
It assumes a low pretest probability has already been judged, as in the derivation paper. Magent doesn't assign pretest probability.
It's the Kline 2004 derivation rule. Altitude-adjusted PERC isn't this calculator.
The five findings are the caller's call.
This page doesn't say what to do with a PERC-negative result.

Clinical decisions belong to a qualified clinician who knows the patient.

Wells PE is a points score for the probability of pulmonary embolism, with items such as signs of DVT, heart rate over 100 and hemoptysis. PERC is an all-or-nothing block rule with no points.
sPESI is a severity score for people already diagnosed with acute PE. PERC is applied before that.

Glossary

PE: pulmonary embolism, a blood clot in the lung arteries.
DVT: deep vein thrombosis, a clot in a deep vein, usually in the leg.
SaO2 / SpO2: oxygen saturation, the share of hemoglobin carrying oxygen.
Hemoptysis: coughing up blood.
Pretest probability: how likely a condition is before a test is done.
Block rule: a rule where every criterion has to be met.

Common mistakes

Counting 50, 100 or 94 as met. All three edges are strict: under 50, under 100, above 94.
Treating PERC as a score. There are no points. Any single unmet criterion means the result isn't PERC-negative.
Reading "PERC-negative" as a test result. It means all eight criteria were met.
Skipping the pretest-probability step. The record describes PERC as applying after a low pretest probability has been judged.

Source

Kline JA, Mitchell AM, Kabrhel C, Richman PB, Courtney DM. Clinical criteria to prevent unnecessary diagnostic testing in emergency department patients with suspected pulmonary embolism. J Thromb Haemost. 2004;2(8):1247-1255. DOI: 10.1111/j.1538-7836.2004.00790.x

Frequently asked questions

What are the PERC rule criteria?

Age under 50, pulse under 100, oxygen saturation above 94%, and no unilateral leg swelling, hemoptysis, recent trauma or surgery, prior PE or DVT, or hormone use.

What does PERC-negative mean?

All eight criteria are met. This page doesn't give disposition advice about what follows.

Is a saturation of 94% enough?

No. The criterion is above 94%, so 95 meets it and 94 doesn't.

Is age 50 under the PERC age limit?

No. The criterion is under 50.

What does PERC stand for?

Pulmonary embolism rule-out criteria.

Who developed the PERC rule?

J. A. Kline and colleagues, in a 2004 paper in the Journal of Thrombosis and Haemostasis.