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Printed on 7/19/2026
For informational purposes only. This is not medical advice.
The Pneumonia Severity Index (PSI), also known as the PORT score, is a validated risk-stratification tool for community-acquired pneumonia (CAP). It combines demographics, comorbidities, physical exam findings, and laboratory/radiographic markers to assign a risk class from I to V. Compared with CURB-65, PSI is more comprehensive and often better at identifying low-risk patients suitable for outpatient care.
Formula: PSI score = demographic points (age, sex, nursing home) + comorbidity points + exam points + lab/radiographic points; risk classes I-V by total score (with class-I low-risk pathway).
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Before calculating the full PSI score, apply the Class I screening criteria: age under 50 years, no active cancer, heart failure, cerebrovascular disease, renal disease, or liver disease, no altered mental status, and no vital sign abnormalities (pulse <125 bpm, SBP ≥90 mmHg, RR <30/min, temp 35–39.9°C), and SpO2 ≥90%. Patients meeting ALL these criteria are automatically Class I — no further scoring needed.
For patients not meeting Class I criteria, score 20 variables with their respective weights: demographic (age as direct points, female sex -10, nursing home +10), comorbidities (cancer +30, liver disease +20, CHF +10, cerebrovascular disease +10, renal disease +10), physical exam (altered mental status +20, RR ≥30 +20, SBP <90 +20, temp <35 or ≥40 +15, pulse ≥125 +10), laboratory/radiology (pH <7.35 +30, BUN ≥30 +20, sodium <130 +20, glucose ≥250 +10, hematocrit <30% +10, PaO2 <60 or SpO2 <90% +10, pleural effusion +10).
PSI Class I = screening criteria met, 30-day mortality <0.1%. Class II = up to 70 points, ~0.6% mortality. Class III = 71–90 points, ~0.9–2.8% mortality. Class IV = 91–130 points, ~8–9% mortality. Class V = above 130 points, ~27–31% mortality. Classes I–II = outpatient treatment appropriate; Class III = observation or brief admission; Classes IV–V = inpatient; consider ICU criteria independently.
Emergency physicians, hospitalists
PSI provides the most comprehensive CAP risk stratification when laboratory data is available. Unlike CURB-65, PSI incorporates comorbidities (liver disease, cancer, CHF), abnormal laboratory values (pH, BUN, sodium, glucose, hematocrit), and radiographic findings (pleural effusion), making it the most discriminating tool for identifying low-risk patients.
Emergency physicians, hospitalists, pulmonologists
PSI Class I–II (30-day mortality <0.6%) identifies patients suitable for outpatient oral antibiotic treatment — the largest proportion of ED CAP patients. PSI's high sensitivity for low-risk identification has been shown to reduce inappropriate hospitalizations in validated clinical pathways.
Clinicians, palliative care teams
PSI provides quantitative 30-day mortality estimates (Class I <0.1% through Class V ~27–31%) that can be used for prognostication discussions with patients and families, particularly for severe Class IV–V pneumonia in elderly or comorbid patients.
Clinical researchers, pneumonia trialists
PSI is the standard risk stratification variable in major CAP clinical trials. Its 5-class system enables stratified randomization and subgroup analysis that supports fine-grained epidemiological and outcomes research.
Quality improvement teams, hospital administrators
PSI class distribution in admitted CAP patients serves as a quality metric. High proportions of Class I–II hospitalizations suggest potential for safe outpatient treatment pathway expansion. PSI-based quality benchmarking has been used in pay-for-performance programs and readmission reduction initiatives.
PSI has a C-statistic of ~0.81 for 30-day mortality vs ~0.78 for CURB-65 in most meta-analyses. The additional discrimination is clinically most valuable at identifying the lowest-risk patients (Class I–II) who can safely be discharged. Both tools are endorsed by IDSA/ATS guidelines — use PSI when more granularity or lab-based stratification adds clinical value.
The PSI Class I screening step identifies young patients without comorbidities and with normal vitals. These patients are automatically Class I regardless of lab values — they do not need BMP, ABG, or chest X-ray to be classified as very low risk. This makes Class I identification as rapid as CRB-65 in the appropriate patient.
In patients where labs are obtained, pH <7.35 (respiratory or metabolic acidosis) and markedly elevated BUN are the highest-weight laboratory items. A patient with otherwise moderate PSI scores can jump to Class IV or V if acidosis is present — reflecting the poor prognostic significance of acid-base disturbance in severe CAP.
Because PSI uses age in years as direct points, older patients start with higher baseline scores. A 28-year-old with bilateral pneumonia, SpO2 82%, and sepsis may only score Class III due to young age offsetting severity. Always apply ICU criteria independently when the clinical picture is severe regardless of PSI class.
IDSA/ATS 2019 guidelines define severe CAP requiring ICU by major criteria: septic shock needing vasopressors, or respiratory failure needing mechanical ventilation. Or 3 or more of 9 minor criteria: RR ≥30, PaO2/FiO2 ratio <250, multilobar infiltrates, confusion, BUN ≥20 mg/dL, WBC <4000/mm3, platelets <100,000/mm3, temperature <36°C, or hypotension requiring aggressive fluids. These criteria apply regardless of PSI class.
Pleural effusion (10 points) can represent parapneumonic effusion, which if complicated (exudate, low pH, elevated LDH, loculated) may require drainage. A pleural effusion in CAP should trigger diagnostic thoracentesis if large or if the patient is not improving within 48–72 hours — regardless of PSI class.
IDSA/ATS 2019 guidelines recommend blood cultures for all hospitalized CAP (PSI Class III–V), urinary pneumococcal antigen for moderate-severe CAP (Class III–V), and urinary Legionella antigen for severe CAP, high-risk occupational exposures, or epidemic settings. Positive Legionella antigen guides targeted azithromycin or levofloxacin therapy.
PSI was derived from a general CAP cohort. Immunocompromised patients (HIV, solid organ transplant, hematologic malignancy on chemotherapy) were largely excluded from validation cohorts. These patients may have blunted inflammatory responses that reduce the sensitivity of PSI variables. Apply PSI in immunocompromised hosts with caution and supplement with specialist input.
PSI/PORT Score developed by Fine et al. (NEJM 1997) from 14,199 patients across the Pneumonia Patient Outcomes Research Team (PORT) cohort study. C-statistic 0.81. PSI Class I-II 30-day mortality: 0.1-0.6%. IDSA/ATS 2007 and 2019 CAP Guidelines reference both PSI and CURB-65. A meta-analysis by Chalmers et al. (Thorax 2010) confirmed PSI equivalent to CURB-65 for outcome prediction but more complex to calculate.
Your PSI result estimates short-term mortality risk in community-acquired pneumonia and groups patients into Class I through V. Class I-II generally represent low-risk presentations often suitable for outpatient treatment when clinically stable. Class III is an intermediate zone where observation or short inpatient stay may be appropriate. Class IV-V are high-risk groups where inpatient care is typically recommended, with Class V carrying very high mortality and often requiring higher-acuity monitoring.
PSI is a disposition support tool, not a replacement for bedside judgment. Use it together with oxygen requirement, trajectory over time, ability to take oral medications, social support, and clinician concern for early decompensation.
Use PSI at initial evaluation of adults with suspected or confirmed community-acquired pneumonia when deciding between outpatient treatment, observation, and admission. It is most useful when the disposition decision is uncertain and you want a validated risk estimate beyond gestalt.
PSI is particularly valuable in ED and acute-care workflows where objective, reproducible triage helps reduce both unnecessary admissions and unsafe discharges.
PSI can under-represent acute instability in younger patients because age contributes a large portion of baseline points. A younger patient with severe hypoxemia may still score lower than an older patient with mild illness. Conversely, older adults may score high due to age and comorbidities despite relatively stable physiologic findings.
The model was developed for community-acquired pneumonia and is not designed for hospital-acquired, ventilator-associated, or profoundly immunocompromised populations. It should also be interpreted cautiously when key lab variables are missing. Clinical judgment and dynamic reassessment remain essential.
For related assessments, see CURB-65 Score, qSOFA Score and SOFA Score.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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