The Pulmonary Embolism Severity Index (PESI) is a validated prognostic score that estimates short-term mortality risk in adults with acute pulmonary embolism. It was derived and validated across large multicenter cohorts and organizes bedside information into five risk classes that correlate with 30-day outcomes. Variables include age, cancer, heart failure, chronic lung disease, pulse, systolic blood pressure, respiratory rate, temperature, mental status, arterial oxygen saturation, and selected findings such as hemoptysis. Points are summed to produce a total score which maps to classes I through V, ranging from very low to very high risk. The simplified PESI (sPESI) reduces the number of variables and retains similar prognostic accuracy, which can support rapid triage in busy settings.

PESI is most useful for early site-of-care decisions and counseling. Low-risk patients, typically classes I and II or sPESI 0, may be candidates for outpatient treatment when social supports and oxygenation are adequate. Higher classes suggest the need for inpatient management, closer monitoring, and consideration of right ventricular assessment and biomarkers as recommended by guidelines. PESI is not a diagnostic tool and it does not select reperfusion therapy on its own. It complements clinical judgment, imaging, echocardiography, and laboratory markers of right