Can PSI be used for hospital-acquired or ventilator-associated pneumonia?

No. PSI was derived for community-acquired pneumonia and should not be used as the main triage score for hospital-acquired or ventilator-associated pneumonia. HAP and VAP involve different pathogens (gram-negatives, MRSA, Pseudomonas) and outcomes, requiring different severity tools (APACHE II, CPIS).

Does a low PSI score always mean discharge?

Not always. Social support, oral intake, oxygen requirement, and clinical trajectory matter. Low score supports outpatient care only when the full clinical context is appropriate. A low PSI Class in an immunocompromised patient or one with rapid deterioration should prompt higher-acuity management.

Should I use PSI or CURB-65 in an outpatient primary care setting?

In primary care without immediate laboratory access, CRB-65 (CURB-65 without BUN) or CURB-65 when BUN is available are more practical. PSI requires pH (ABG), BUN, sodium, glucose, hematocrit, and PaO2 — most not available in primary care settings. PSI is optimally used in emergency departments and hospital settings where these labs are routinely ordered for CAP evaluation.

What antibiotic regimens are appropriate for each PSI class?

PSI Class I–II (outpatient): amoxicillin 500 mg three times daily ± azithromycin (IDSA/ATS first-line); or respiratory fluoroquinolone (levofloxacin 750 mg daily or moxifloxacin 400 mg daily) if comorbidities present. PSI Class III (consider hospitalization): IV beta-lactam + macrolide or respiratory fluoroquinolone. PSI Class IV–V (hospitalize): IV beta-lactam + macrolide; severe CAP: add Legionella coverage; MRSA risk factors: add vancomycin or linezolid.

Is PSI more accurate than CURB-65 for identifying low-risk pneumonia?

Meta-analyses show PSI has slightly better sensitivity for identifying low-risk patients (higher negative predictive value for low-risk classification) compared to CURB-65. A Chalmers et al. meta-analysis (Thorax 2010) found PSI and CURB-65 had similar AUC for mortality prediction overall, but PSI better identified patients suitable for outpatient care. Both are guideline-endorsed; choice depends on available data and clinical context.

What is the best pneumonia severity score overall?

Both PSI and CURB-65 are guideline-endorsed for CAP severity assessment. PSI has superior discrimination for identifying low-risk patients but requires laboratory data. CURB-65 is faster and equally valid for identifying high-risk patients requiring hospitalization. IDSA/ATS 2019 guidelines recommend both tools but suggest PSI may be preferred for low-risk identification. Most emergency departments use CURB-65 as the primary tool due to its simplicity, with PSI used when more comprehensive risk stratification is needed.

Online Medical Tools — PSI / PORT Score

Printed on 9/2/2026

For informational purposes only. This is not medical advice.

Emergency

PSI / PORT Score

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).

Age

years

Sex Assigned at Birth

Nursing Home Resident

Neoplastic Disease

Liver Disease

Congestive Heart Failure

Cerebrovascular Disease

Renal Disease

Altered Mental Status

Respiratory Rate ≥ 30/min

Systolic BP < 90 mmHg

Temperature < 35°C or ≥ 40°C

Pulse ≥ 125 bpm

Arterial pH < 7.35

BUN ≥ 30 mg/dL

Sodium < 130 mEq/L

Glucose ≥ 250 mg/dL

Hematocrit < 30%

PaO2 < 60 or SpO2 < 90%

Pleural Effusion on Imaging

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How It Works

Apply the Class I screening step to identify lowest-risk patients

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.

Score all demographic, comorbidity, exam, and lab variables

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).

Assign risk class and interpret 30-day mortality

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.

Who Uses the PSI / PORT Score

Hospital Emergency Department Pneumonia Severity With Labs Available

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.

Inpatient vs Outpatient Disposition Decision

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.

Predicting Pneumonia 30-day Mortality

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.

Risk Stratification for Clinical Trials

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.

Hospital Quality Benchmarking

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.

Pro Tips

PSI is more complex than CURB-65 but has higher discrimination

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.

Class I screening identifies lowest-risk patients without extensive lab testing

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.

pH below 7.35 (30 points) and BUN ≥30 mg/dL (20 points) are major lab drivers

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.

PSI heavily weights age — may under-risk young patients with severe illness

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 severe CAP criteria for ICU should be applied independently of PSI

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 adds 10 points but warrants diagnostic consideration

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.

Blood cultures and urinary antigens in hospitalized CAP patients

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 performance may be limited in immunocompromised patients

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.

Common Questions About Your Results

PSI Class III (71–90 points, ~0.9–2.8% mortality) represents the most clinically challenging disposition zone. Factors supporting hospitalization: SpO2 <94%, bilateral or multilobar infiltrates, inability to take oral antibiotics, significant comorbidities (COPD exacerbation, decompensated heart failure), poor social support, or clinical appearance worse than score suggests. Factors supporting outpatient: SpO2 ≥94%, able to take oral medications, reliable follow-up, good social support, and score driven primarily by age rather than acute physiology.

PSI was not validated in profoundly immunocompromised patients — these populations were largely excluded from the derivation and validation cohorts. PSI may underestimate severity in immunocompromised hosts who lack typical inflammatory responses to infection. Clinical judgment and specialist input are essential; a Class II PSI in an immunocompromised patient does not confer the same safety margin as in an immunocompetent patient.

Arterial pH <7.35 contributes 30 points in PSI, but obtaining an ABG may not always be immediately feasible. If ABG is unavailable, assign 0 points for this variable and proceed with the remaining items. A venous pH (typically 0.03–0.05 lower than arterial) can be used as a surrogate — venous pH <7.30 may approximate arterial pH <7.35. When ABG is unavailable and clinical severity appears high, consider treating as higher risk pending labs.

Evidence-Based Methodology

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.

Clinical Content Trust

Last reviewed:: April 21, 2026
Guideline version:: General evidence framework v2026.04
Source set version:: Primary-source set v1

How to Interpret Your Result

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.

When to Use This Tool

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.

Limitations

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.

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.

Sources

Changelog

April 21, 2026 · trust-baseline
Clinical trust metadata enabled for this tool page with structured review/version fields.

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Frequently Asked Questions

The PSI (Pneumonia Severity Index), also called the PORT score, is a validated risk stratification tool for community-acquired pneumonia (CAP) that combines 20 variables — demographics, comorbidities, physical examination findings, and laboratory/radiographic data — to assign patients to one of 5 risk classes (I through V) with corresponding 30-day mortality estimates. It is used after CAP diagnosis to guide outpatient vs inpatient treatment decisions.

PSI Class I = age <50 with no comorbidities, normal vitals, SpO2 ≥90% (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 are outpatient-appropriate; Class III is the decision zone; Classes IV–V require inpatient care.

PSI uses 20 weighted variables (including demographics, comorbidities, laboratory values, and radiology) while CURB-65 uses only 5 bedside variables. PSI has slightly higher discrimination (C-statistic ~0.81 vs ~0.78) and is better at identifying low-risk patients for safe outpatient treatment. CURB-65 is faster and doesn't require laboratory data. IDSA/ATS 2019 guidelines endorse both; PSI is preferred for comprehensive risk assessment when labs are available.

Yes. PSI Class I–II patients have 30-day mortality below 0.6% and are the primary candidates for outpatient CAP treatment. Eligibility also requires: SpO2 ≥94% on room air, ability to take oral antibiotics, no bilateral or multilobar infiltrates, adequate social support, access to follow-up within 48 hours, and no clinical concerns beyond the score. Oral amoxicillin ± macrolide or respiratory fluoroquinolone is standard for outpatient CAP.

Step 1: Apply Class I screening — if age <50 with no comorbidities and normal vitals/SpO2 → Class I, done. Step 2: Start with age in years as base score; subtract 10 for female sex. Add: nursing home +10, cancer +30, liver disease +20, CHF +10, cerebrovascular disease +10, renal disease +10, altered mental status +20, RR ≥30 +20, SBP <90 +20, abnormal temp +15, pulse ≥125 +10, pH <7.35 +30, BUN ≥30 +20, sodium <130 +20, glucose ≥250 +10, hematocrit <30% +10, hypoxemia +10, pleural effusion +10. Step 3: Assign class by total points.

Online Medical Tools — PSI / PORT Score

PSI / PORT Score

How It Works

Apply the Class I screening step to identify lowest-risk patients

Score all demographic, comorbidity, exam, and lab variables

Assign risk class and interpret 30-day mortality

Who Uses the PSI / PORT Score

Hospital Emergency Department Pneumonia Severity With Labs Available

Inpatient vs Outpatient Disposition Decision

Predicting Pneumonia 30-day Mortality

Risk Stratification for Clinical Trials

Hospital Quality Benchmarking

Pro Tips

PSI is more complex than CURB-65 but has higher discrimination

Class I screening identifies lowest-risk patients without extensive lab testing

pH below 7.35 (30 points) and BUN ≥30 mg/dL (20 points) are major lab drivers

PSI heavily weights age — may under-risk young patients with severe illness

IDSA/ATS severe CAP criteria for ICU should be applied independently of PSI

Pleural effusion adds 10 points but warrants diagnostic consideration

Blood cultures and urinary antigens in hospitalized CAP patients

PSI performance may be limited in immunocompromised patients

Common Questions About Your Results

Evidence-Based Methodology

Clinical Content Trust

How to Interpret Your Result

When to Use This Tool

Limitations

Sources

Changelog

Related Tools

CURB-65 Score

qSOFA Score

SOFA Score

A-a Gradient

Further Reading

A Complete Guide to Emergency Medicine: Decision Tools, Scoring Systems, and Clinical Protocols

Frequently Asked Questions