What is the Pneumonia Severity Index?
The Pneumonia Severity Index (PSI), also known as the PORT score (Pneumonia Outcomes Research Team), is a validated clinical prediction tool used to estimate the severity and 30-day mortality risk of community-acquired pneumonia (CAP). Developed by Fine et al. in 1997, it stratifies patients into five risk classes (I through V) based on demographics, comorbidities, physical examination findings, and laboratory/imaging results.
The PSI was specifically designed to identify low-risk patients (Classes I–III) who may be safely treated as outpatients, thereby reducing unnecessary hospitalizations and healthcare costs while maintaining patient safety.
PSI Scoring System
(Female: subtract 10 from age points)
| Variable | Points |
|---|---|
| Demographics | |
| Age (male) | Age in years |
| Age (female) | Age − 10 |
| Nursing home resident | +10 |
| Comorbidities | |
| Neoplastic disease | +30 |
| Liver disease | +20 |
| Congestive heart failure | +10 |
| Cerebrovascular disease | +10 |
| Renal disease | +10 |
| Physical Exam | |
| Altered mental status | +20 |
| Respiratory rate ≥ 30/min | +20 |
| Systolic BP < 90 mmHg | +20 |
| Temperature < 35°C or ≥ 40°C | +15 |
| Pulse ≥ 125/min | +10 |
| Laboratory & Imaging | |
| Arterial pH < 7.35 | +30 |
| BUN ≥ 30 mg/dL | +20 |
| Sodium < 130 mEq/L | +20 |
| Glucose ≥ 250 mg/dL | +10 |
| Hematocrit < 30% | +10 |
| PaO2 < 60 mmHg | +10 |
| Pleural effusion | +10 |
Risk Classes & Mortality
| Risk Class | Score Range | 30-Day Mortality | Recommended Disposition |
|---|---|---|---|
| Class I | ≤ 50 (age < 50, no comorbidities) | 0.1% | Outpatient |
| Class II | ≤ 70 | 0.6% | Outpatient |
| Class III | 71 – 90 | 0.9 – 2.8% | Brief observation / Outpatient |
| Class IV | 91 – 130 | 8.2 – 9.3% | Inpatient |
| Class V | > 130 | 27 – 31% | Inpatient (consider ICU) |
PSI Decision Algorithm Diagram
Community-Acquired Pneumonia
Community-acquired pneumonia (CAP) is a common and potentially serious infection acquired outside of hospitals or healthcare facilities. It remains a leading cause of hospitalization and death worldwide, particularly among the elderly and immunocompromised.
- Most common pathogen: Streptococcus pneumoniae (pneumococcus), followed by Haemophilus influenzae, Mycoplasma pneumoniae, and respiratory viruses
- Atypical pathogens: Legionella, Chlamydophila pneumoniae, and Mycoplasma — often present with milder symptoms
- Risk factors: Age > 65, smoking, COPD, diabetes, heart failure, immunosuppression, alcoholism
- Diagnosis: Clinical presentation (cough, fever, dyspnea) + radiographic confirmation (chest X-ray or CT)
Outpatient vs. Inpatient Treatment
The PSI helps guide the critical decision of where to treat the patient. Appropriate site-of-care decisions improve outcomes and reduce healthcare costs:
| Risk Class | Setting | Typical Treatment |
|---|---|---|
| I – II | Outpatient | Oral amoxicillin or macrolide (azithromycin, doxycycline); follow-up in 48–72 hours |
| III | Brief ED observation or short hospital stay | Oral or IV antibiotics based on clinical judgment; discharge if improving in 24 hours |
| IV | Inpatient (medical ward) | IV beta-lactam + macrolide or IV respiratory fluoroquinolone |
| V | Inpatient (consider ICU) | IV broad-spectrum antibiotics; assess for sepsis, respiratory failure, vasopressor need |
Worked Example
A 65-year-old male with congestive heart failure (CHF), respiratory rate of 32/min, and BUN of 35 mg/dL, no other findings:
A score of 115 places this patient in Risk Class IV (91–130 points) with a 30-day mortality of approximately 9.3%. This patient should be admitted to the hospital for inpatient treatment with IV antibiotics.
Frequently Asked Questions
What is the difference between PSI and CURB-65?
Both are validated pneumonia severity scores. CURB-65 is simpler (5 variables: Confusion, Urea, Respiratory rate, Blood pressure, Age ≥ 65) and easier to calculate at the bedside. PSI is more comprehensive (20 variables) and better validated for identifying low-risk patients suitable for outpatient treatment. The ATS/IDSA guidelines recommend either tool.
Does PSI apply to hospital-acquired pneumonia?
No. The PSI was validated specifically for community-acquired pneumonia (CAP). Hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP) involve different pathogens and risk profiles, requiring separate scoring systems and treatment guidelines.
Can young patients with no comorbidities still be Class I?
Yes. Class I is specifically defined as patients under 50 years old with no comorbidities (neoplastic disease, liver disease, CHF, cerebrovascular disease, renal disease), no abnormal vital signs, and no altered mental status. These patients can be identified before calculating the full score.
Should the PSI score override clinical judgment?
No. The PSI is a decision-support tool, not a replacement for clinical judgment. Factors not captured by the PSI — such as ability to take oral medications, reliable follow-up, home support, substance abuse, and homelessness — may influence the disposition decision.