How to calculate PSI (Pneumonia Severity Index) for community-acquired pneumonia?

How to Calculate PSI (Pneumonia Severity Index) for Community-Acquired Pneumonia

The PSI uses a two-step process to stratify patients into 5 mortality risk classes based on 20 variables including demographics, comorbidities, vital signs, and laboratory findings, primarily to identify low-risk patients who can be safely treated as outpatients. 1

Step 1: Determine if Patient Qualifies for Risk Class I

First, assess whether the patient meets ALL of the following criteria for automatic assignment to Class I 1:

• Age < 50 years
• Zero comorbid conditions (no neoplastic disease, liver disease, congestive heart failure, cerebrovascular disease, or renal disease)
• Normal or mildly abnormal vital signs
• Normal mental status

If the patient meets all these criteria, they are Class I with <1% mortality risk and can be treated as an outpatient. 1

Step 2: Calculate Point Score for Classes II-V

If the patient does NOT qualify for Class I, calculate points based on the following 20 variables 1:

Demographic Factors

• Age in years (for males) or Age in years minus 10 (for females)
• Nursing home resident: +10 points

Comorbid Conditions (each adds points)

• Neoplastic disease: +30 points
• Liver disease: +20 points
• Congestive heart failure: +10 points
• Cerebrovascular disease: +10 points
• Renal disease: +10 points

Physical Examination Findings

• Altered mental status: +20 points
• Respiratory rate ≥30 breaths/min: +20 points
• Systolic blood pressure <90 mm Hg: +20 points
• Temperature <35°C or ≥40°C: +15 points
• Pulse ≥125 beats/min: +10 points

Laboratory and Radiographic Findings

• Arterial pH <7.35: +30 points
• Blood urea nitrogen ≥30 mg/dL: +20 points
• Sodium <130 mmol/L: +20 points
• Glucose ≥250 mg/dL: +10 points
• Hematocrit <30%: +10 points
• PaO2 <60 mm Hg or O2 saturation <90%: +10 points
• Pleural effusion on chest radiograph: +10 points

Risk Class Stratification and Management

After calculating the total point score, assign the patient to a risk class 1:

• Class I: Meets Step 1 criteria (≤0.1% mortality) → Outpatient treatment
• Class II: ≤70 points (0.6% mortality) → Outpatient treatment
• Class III: 71-90 points (2.8% mortality) → Outpatient or short hospital observation
• Class IV: 91-130 points (8% mortality) → Hospitalization required
• Class V: >130 points (35% mortality) → Hospitalization required

Important Clinical Caveats

The PSI has significant limitations that require clinical judgment to override the score 1, 2:

• Underestimates severity in young patients without comorbidities who develop severe respiratory failure, because hypoxia alone does not generate enough points 1
• Does not include COPD or diabetes as risk factors, despite their clinical relevance 1
• Complex calculation requiring 20 variables makes it challenging in emergency settings 2
• Social factors (lack of outpatient support, medication adherence concerns) are not included but must be considered 1

Alternative: CURB-65 Score

The CURB-65 is a simpler alternative using only 5 variables, with recent evidence suggesting it may provide equal or better outcomes 3, 4:

One point for each of:

• Confusion
• Urea >7 mmol/L (20 mg/dL)
• Respiratory rate ≥30/min
• Blood pressure (SBP <90 or DBP ≤60 mm Hg)
• Age ≥65 years

Score interpretation: 0-1 points = outpatient; 2 points = consider admission; ≥3 points = hospitalization with ICU assessment 3, 2

A 2023 study found CURB-65 associated with lower 30-day mortality (8.6% vs 9.7%) compared to PSI, with similar other outcomes 4.

Clinical Implementation

Use PSI or CURB-65 as adjuncts to clinical judgment, not sole determinants 5. For patients with PSI Class IV-V or CURB-65 ≥3, promptly evaluate for ICU admission using the 2007 IDSA/ATS severe CAP criteria (septic shock requiring vasopressors or need for mechanical ventilation) 1, 3.