PERC Calculator
Use our free Perccalculator Calculator to get personalized health results. Based on validated medical formulas and clinical guidelines.
Formula
PERC Rule: All 8 criteria must be NEGATIVE to rule out PE
The PERC rule evaluates 8 binary criteria. If ALL 8 are absent in a patient with low pretest probability (< 15% or Wells score <= 4), PE probability is < 2% and no further testing is needed. If ANY criterion is present, the PERC rule is not satisfied and further workup is indicated.
Worked Examples
Example 1: PERC-Negative Low-Risk Patient
Problem: A 38-year-old woman presents with mild chest discomfort. No prior VTE, no surgery, no estrogen use, no hemoptysis, no leg swelling, HR 82, SpO2 98%. Wells score is 1 (low risk). Apply the PERC rule.
Solution: PERC Criteria Assessment:\nAge >= 50: No (age 38)\nHR >= 100: No (HR 82)\nSpO2 < 95%: No (SpO2 98%)\nPrior DVT/PE: No\nRecent surgery/trauma: No\nHemoptysis: No\nEstrogen use: No\nUnilateral leg swelling: No\n\nAll 8 criteria are NEGATIVE.
Result: PERC: Negative (0/8 criteria met) | PE effectively ruled out | No D-dimer or CTPA needed
Example 2: PERC-Positive Patient Requiring Workup
Problem: A 55-year-old man on no medications presents with acute dyspnea. HR 105, SpO2 93%, no prior VTE, no recent surgery. Wells score is 3 (low risk). Apply PERC.
Solution: PERC Criteria Assessment:\nAge >= 50: YES (age 55)\nHR >= 100: YES (HR 105)\nSpO2 < 95%: YES (SpO2 93%)\nPrior DVT/PE: No\nRecent surgery/trauma: No\nHemoptysis: No\nEstrogen use: No\nUnilateral leg swelling: No\n\n3 of 8 criteria are POSITIVE.
Result: PERC: Positive (3/8 criteria met) | Cannot rule out PE | Proceed to D-dimer testing
Frequently Asked Questions
What is the PERC rule and when should it be applied?
The PERC (Pulmonary Embolism Rule-out Criteria) rule is a clinical decision tool designed to identify emergency department patients with such low risk of pulmonary embolism that no further diagnostic testing is needed. It was developed by Kline and colleagues in 2004 and validated in subsequent multicenter studies. The rule consists of 8 clinical criteria that can be assessed at the bedside without any laboratory testing. Crucially, the PERC rule should ONLY be applied to patients already determined to have a low pretest probability of PE (typically less than 15%) using gestalt clinical assessment or a validated scoring system like the Wells criteria. If all 8 PERC criteria are negative in a low-risk patient, the probability of PE is less than 2%, which falls below the test threshold and no further workup is indicated.
What are the eight PERC criteria and why were they selected?
The eight PERC criteria are: age 50 years or older, heart rate 100 bpm or greater, oxygen saturation below 95% on room air, prior history of DVT or PE, recent surgery or trauma within the past 4 weeks, hemoptysis, exogenous estrogen use (oral contraceptives, hormone replacement therapy), and unilateral leg swelling. These criteria were selected through logistic regression analysis of a large derivation cohort of emergency department patients evaluated for possible PE. Each criterion independently contributes to PE risk, and their combination provides high sensitivity for identifying patients who may have PE. The criteria encompass physiological signs of PE (tachycardia, hypoxemia), known risk factors (prior VTE, surgery, estrogen use), demographic factors (age), and clinical findings suggestive of DVT (leg swelling) or PE (hemoptysis).
How sensitive and specific is the PERC rule for pulmonary embolism?
The PERC rule has a sensitivity of approximately 97-98% and a negative predictive value exceeding 99% when applied to low-pretest-probability patients. This means that fewer than 2% of patients who satisfy all PERC criteria (all negative) will actually have a PE. The specificity is relatively low at approximately 20-22%, meaning many patients who fail the PERC rule will not have PE but will require further testing. The high sensitivity is the critical performance metric because the primary goal of the PERC rule is to safely rule out PE without missing cases. The landmark PROPER trial (2018) demonstrated that a PERC-based strategy was noninferior to conventional D-dimer-based workup, with a 3-month PE rate of 0.1% in PERC-negative patients. This performance makes the PERC rule one of the most reliable clinical decision tools in emergency medicine.
What is the relationship between the PERC rule and D-dimer testing?
The PERC rule and D-dimer testing serve complementary roles in the PE diagnostic pathway but operate at different steps. The PERC rule is applied first, before any laboratory testing, and if all criteria are negative in a low-risk patient, D-dimer is NOT needed. This is important because D-dimer testing, while highly sensitive, has poor specificity and leads to many false-positive results that trigger unnecessary CT pulmonary angiography (CTPA). Studies show that up to 30-40% of D-dimer tests ordered in the ED are abnormal, but only 5-10% of subsequent CTPAs are positive for PE. By applying the PERC rule first, clinicians can avoid this cascade of testing in truly low-risk patients. If the PERC rule is not satisfied, then D-dimer testing becomes the next appropriate step, followed by CTPA if the D-dimer is elevated.
What happens if a patient fails the PERC rule?
Failing the PERC rule (having one or more positive criteria) does not mean the patient has PE; it simply means that PE cannot be safely ruled out by clinical criteria alone and further diagnostic evaluation is warranted. The next step depends on the overall clinical pretest probability. For patients with low to moderate pretest probability who fail PERC, a D-dimer test should be ordered. If the D-dimer is negative (below the age-adjusted or standard cutoff), PE is effectively ruled out. If the D-dimer is positive, CT pulmonary angiography is the next step. For patients with high pretest probability, the PERC rule should not have been applied in the first place, and CTPA should be obtained directly. It is essential to understand that the PERC rule is a rule-out tool only and cannot be used to rule in PE or estimate the probability of PE in positive cases.
Can the PERC rule be used in hospitalized or critically ill patients?
The PERC rule was specifically developed and validated for use in the emergency department setting in ambulatory patients presenting with symptoms potentially suggestive of PE. It has not been validated for use in hospitalized patients, critically ill ICU patients, or postoperative patients, and should not be applied in these settings. Hospitalized patients often have different baseline characteristics including higher rates of immobility, recent procedures, and comorbidities that fundamentally alter the pretest probability of PE. Additionally, the PERC rule criteria such as oxygen saturation and heart rate may be abnormal for reasons unrelated to PE in critically ill patients. For inpatients suspected of having PE, direct diagnostic imaging with CTPA or ventilation-perfusion scanning is generally recommended, guided by clinical assessment and possibly D-dimer testing depending on the clinical context.