The Pneumonia Severity Index, also called the PORT score, stratifies risk in adults with community acquired pneumonia to support site-of-care decisions and counseling. It combines 20 routinely available variables that cover age, residence, comorbid illness, vital signs, mental status, and laboratory or radiographic findings. Each variable contributes a set number of points that sum to a total score. The total maps to five risk classes that correlate with short-term mortality. In the original multicenter derivation and validation by Fine and colleagues, the rule identified large groups of low-risk patients who were safe candidates for outpatient management with close follow up. Subsequent prospective comparisons have shown that PSI performs well for mortality prediction and often outperforms simpler rules when the goal is to identify patients at very low risk.

PSI is most helpful at the extremes. Classes I and II usually signal low mortality risk and potential for outpatient treatment when social factors and oxygenation are adequate. Classes IV and V indicate higher risk and support inpatient care with early reassessment for complications. Class III often requires brief observation, targeted testing, and individualized decisions. PSI does not replace clinical judgment. It should be paired with bedside evaluation, pulse oximetry, assessment of ability to safely take oral therapy, and awareness of special populations such as pregnant or severely immunocompromised patients where performance may differ.