The a/A ratio compares arterial oxygen directly to alveolar oxygen as a ratio rather than a difference, and stays more stable than the A-a gradient when FiO₂ changes.
Normal — arterial oxygen is at least 75% of alveolar oxygen; gas exchange is working as expected.
Mildly reduced — some impairment of oxygen transfer.
Moderately reduced — significant gas-exchange impairment.
Severely reduced — major shunt or V/Q mismatch.
Given: PAO₂ 99.7 mmHg, PaO₂ 90 mmHg.
0.90The a/A ratio is arterial oxygen expressed as a fraction of alveolar oxygen.
Because it remains valid across a wider range of FiO₂ without needing age correction — as a ratio, it doesn't swing as much when the oxygen setting changes, so it's better for tracking a patient over time.
Greater than 0.75 — meaning arterial oxygen is at least 75% of the alveolar oxygen the lungs calculated they should have.
From the alveolar gas equation, exactly as in step one of the A-a gradient: PAO₂ = [FiO₂ × (Patm − 47)] − (PaCO₂ ÷ 0.8).
No. Arterial oxygen can't exceed alveolar oxygen, so a ratio above 1.0 signals a measurement or entry error in the FiO₂ or ABG values.