Abnormal blood oxygen level Elevated alveolar-arterial (A-a) oxygen gradient An increase in the difference between the oxygen concentration in the alveoli and arterial system above the upper limit of normal. The A-a gradient, or the alveolar-arterial gradient, measures the difference between the oxygen concentration in the alveoli and arterial system. The A-a gradient has important clinical utility as it can help narrow the differential diagnosis for hypoxemia. The A-a gradient calculation is as follows: A-a Gradient = PAO2 - PaO2. In this equation, PAO2 represents alveolar oxygen pressure and PaO2 representing arterial oxygen pressure. The arterial oxygen pressure (PaO2) can be directly assessed with an arterial blood gas test (ABG) or estimated with a venous blood gas test (VBG). The alveolar oxygen pressure (PAO2) is not easily measured directly; instead, it is estimated using the alveolar gas equation. The A-a gradient may be increased in clinical conditions such as pneumonia, with poor diffusion of oxygen across the alveolar-capillary unit and thus lower oxygen levels in the arterial blood (because of the physical barrier within the alveoli, limiting the diffusion of oxygen into the capillaries).