• Suspected PE where the treating clinician has already established LOW pre-test probability (< 15%) by clinical gestalt or Wells score ≤ 2
• To safely avoid D-dimer testing and further workup in genuinely low-risk patients — PERC is applied before any lab test is ordered
• Emergency department and general medicine settings where over-investigation is a recognised concern
• When CTPA use needs to be rationally curtailed — approximately 1–2% of all ED visits in the US involve CTPA for PE, with fewer than 5% of those scans confirming the diagnosis
• As part of a consistent, pre-committed diagnostic strategy — do not apply PERC selectively or intermittently

• Pregnancy — PERC has not been validated in pregnant patients; use modified YEARS or clinical specialist input
• Haemodynamic instability — this is high-risk PE by definition; proceed directly to echocardiography or CTPA
• Patients already on therapeutic anticoagulation — clinical scenario has already been adjudicated
• Patients with symptom onset > 3 days before presentation — D-dimer sensitivity decreases; the full PERC model was derived from acute presentations
• Moderate or high pre-test probability patients — applying PERC outside the low-PTP window produces an unacceptable false-negative rate

PERC is built on the concept of the "test-treatment threshold" — a disease prevalence below which the harms of diagnostic testing outweigh the benefits. Kline et al. derived this threshold at approximately 1.8% for PE. Below this prevalence, the probability of a false-positive D-dimer leading to unnecessary CTPA, contrast nephropathy, radiation exposure, or iatrogenic anticoagulation injury exceeds the probability that testing will identify a clinically meaningful PE. PERC identifies patients whose post-test probability — after applying 8 objective criteria — falls below this threshold.

• Age < 50 years
• Heart rate < 100 bpm (resting)
• SpO2 ≥ 95% on room air
• No unilateral leg swelling (visual assessment of calf asymmetry)
• No haemoptysis
• No exogenous oestrogen use (OCP, HRT, testosterone — any hormonal therapy)
• No prior venous thromboembolism (DVT or PE, objectively documented)
• No recent surgery or trauma requiring hospitalisation or endotracheal intubation within 4 weeks

• ALL 8 criteria must be ABSENT (each = 0) to apply PERC rule-out — PERC score of 0
• A single positive criterion (any PERC item present) mandates D-dimer testing — do not interpret partial negativity as reassuring
• PERC score of 0 in a low-PTP patient: post-test PE probability ≈ 1.0% — below treatment threshold, no further testing warranted
• PERC score > 0: proceed to D-dimer with age-adjusted threshold (age × 10 µg/L if > 50 years)

The PROPER trial (Freund et al., JAMA 2018) was the first cluster-randomised controlled trial of PERC vs conventional care in 14 French EDs (n = 1914 low-risk patients). Primary outcome: 1 symptomatic PE (0.1%) at 3 months in the PERC group vs 0 in the control group — meeting the non-inferiority margin. Secondary benefits of the PERC strategy: 10% reduction in CTPA use (13% vs 23%), 36–40 minute reduction in ED length of stay, and 3.3% reduction in hospital admission rates. This trial provides the only RCT-level evidence that PERC-based care does not increase VTE events.

• Heart rate: must be resting rate — do not score based on a single anxious or exertion-related reading; reassess after 5–10 minutes if initially > 100
• SpO2: must be room air — if the patient is on supplemental O2 at any flow rate, this criterion cannot be assessed and PERC is invalid
• Exogenous oestrogen: includes ALL hormonal therapies — OCP, HRT, testosterone, progesterone-only pills; not limited to oestrogen-containing preparations
• Leg swelling: requires visual comparison of calves — if bilateral swelling is present, the criterion is negative (PERC only flags unilateral swelling as a DVT proxy); do not falsely reassure
• Prior VTE: must have been objectively documented — patient-reported "blood clot" without imaging confirmation is insufficient and should be treated as positive
• Surgery criterion: specifically requires hospitalisation OR endotracheal intubation — minor outpatient procedures in the past 4 weeks do not trigger this criterion

The dramatic rise in CTPA use has exposed a new clinical challenge: incidental detection of subsegmental PE (SSPE), which affects small distal pulmonary arteries and may not require anticoagulation. Studies estimate that 5–10% of CTPA-detected PEs are isolated SSPE with uncertain clinical significance. Kline noted in his JAMA 2018 editorial that CTPA is performed in approximately 1–2% of 120 million annual US ED visits, yet < 5% of scans are positive. PERC is one of the few evidence-based tools specifically designed to interrupt this cycle upstream — before any test is ordered.

• Establish PTP first: clinical gestalt < 15%, or Wells PE score ≤ 2. If not low → do not apply PERC, proceed directly to D-dimer ± CTPA.
• Low PTP + ALL 8 PERC criteria absent (PERC score = 0) → PE excluded. No D-dimer. No imaging. Document pre-test probability and PERC assessment explicitly in the clinical record.
• Low PTP + ANY PERC criterion present (PERC score > 0) → Obtain high-sensitivity D-dimer. Use age-adjusted threshold (age × 10 µg/L) in patients > 50 years.
• D-dimer negative (below threshold) → PE excluded. Discharge with appropriate safety-netting.
• D-dimer positive (above threshold) → Proceed to CTPA. Interim LMWH only if CTPA delay > 4 hours and clinical suspicion is high.

When applying PERC, explicitly document: (1) pre-test probability estimate and the basis for designating it as low (gestalt or Wells score), (2) each PERC criterion with its assessed value, and (3) the decision reached. In medicolegal settings, undocumented application of PERC is indistinguishable from no assessment. This is particularly important given that PERC leaves a small residual miss rate (~1%) — documentation demonstrates that the clinical threshold was properly evaluated.

An emergency physician and clinical scientist at Indiana University School of Medicine (subsequently at Wayne State University). Kline developed PERC in response to what he characterised as an epidemic of over-testing in suspected PE — driven by fear of litigation, the proliferation of high-sensitivity D-dimer assays, and the clinical instinct to always do more. His core argument, published in 2004, was mathematically rigorous: when pre-test probability is below ~1.8%, a positive D-dimer is statistically more likely to be a false positive than a true signal of PE, making D-dimer testing actively harmful in that population.

Through the 1990s and 2000s, CTPA became rapidly accessible across US emergency departments and the rate of PE testing expanded dramatically — driven partly by genuine clinical need, partly by medicolegal pressure, and partly by the ease of ordering a sensitive test. The paradox: as more CTPAs were ordered, more incidental and subsegmental PEs were found, many of which were treated with anticoagulation despite uncertain clinical significance, exposing patients to bleeding risk without proven benefit. PERC was the first clinical tool to address this problem not by improving PE detection, but by rationally defining when PE detection is unnecessary.

• 2004 (J Thromb Haemost): PERC derived in 3148 ED patients. 8 criteria identified via logistic regression. Test-treatment threshold defined at 1.8%.
• 2008 (J Thromb Haemost): Prospective multicenter validation in 8138 patients across 13 sites. False-negative rate 1.0% in low-PTP + PERC-negative group. Safety confirmed in North American ED populations.
• 2011–2012: European studies report higher false-negative rates (6–8%) in populations with PE prevalence > 20%. Controversy about European applicability. Root cause identified as failure to apply gestalt-based low-PTP precondition.
• 2012 (Ann Emerg Med): Singh et al. meta-analysis across 13 cohorts, n = 13,885 — pooled sensitivity 97%. Performance stable across high- and low-prevalence populations when applied correctly.
• 2017 (Lancet Haematol): PERCEPIC study prospectively validates PERC in European EDs using strict < 15% gestalt criterion. Zero missed PEs at 3 months. European controversy resolved.
• 2018 (JAMA): PROPER trial — first RCT. PERC non-inferior to conventional care in 1914 French ED patients. 10% CTPA reduction, 37-minute ED stay reduction. PERC endorsed as standard of care in low-risk ED patients.