A–a Gradient Calculator

About this calculator

The alveolar–arterial (A–a) gradient measures the difference between the oxygen concentration in the alveoli (PAO₂, calculated from the alveolar gas equation using FiO₂, atmospheric pressure, and PaCO₂) and the oxygen actually measured in arterial blood (PaO₂ from an arterial blood gas): A–a gradient = PAO₂ − PaO₂.

A normal gradient (roughly 5–15 mmHg, increasing with age) means any low blood oxygen is due to a problem getting air into the lungs, like hypoventilation; an elevated gradient points to a problem with gas exchange itself — ventilation-perfusion mismatch, shunt, or diffusion impairment — as seen in pulmonary embolism, pneumonia, pulmonary edema, or ARDS.

Emergency and critical care physicians assessing a patient's respiratory failure, pulmonologists working up hypoxemia, and medical and nursing students learning arterial blood gas interpretation all use this calculation to narrow down the cause of low oxygen.