The Horowitz Index, also known as the PaO₂/FiO₂ ratio (PF ratio), is a widely applied clinical measure to evaluate oxygen exchange efficiency in patients with acute respiratory failure, most commonly those with acute respiratory distress syndrome (ARDS). It reflects how effectively the lungs transfer oxygen from inhaled air into the bloodstream by comparing arterial oxygen pressure (PaO₂) to the fraction of inspired oxygen (FiO₂). This straightforward calculation provides clinicians with a practical, standardized method to quantify hypoxemia and monitor disease progression.

One of its key strengths lies in its role in ARDS diagnosis and classification. According to the Berlin Definition, ARDS is stratified into mild, moderate, and severe categories based on PF ratio cutoffs, with lower ratios indicating more critical impairment. This grading not only assists in risk stratification but also helps guide therapeutic interventions such as adjusting ventilator settings, deciding on prone positioning, or considering advanced rescue therapies like extracorporeal membrane oxygenation (ECMO).

The PF ratio is advantageous because it adjusts PaO₂ to the FiO₂ level, making it more comparable across patients receiving different oxygen concentrations. However, it should be interpreted in context, as factors like positive end-expiratory pressure (PEEP), altitude, and hemoglobin levels may affect readings. It also does not account for other important gas exchange metrics such as carbon dioxide clearance.