A–a Gradient Calculator
Alveolar–arterial oxygen gradient with the age-expected value.
Enter your values to see the result.
PAO₂ = FiO₂ × (Patm − 47 mmHg) − PaCO₂ ÷ RQ. The expected gradient on room air is roughly age ÷ 4 + 4 mmHg, and it rises with FiO₂. For education and clinical decision support only. Results do not replace professional judgment, local protocols or the product information, so always verify doses and values before acting on them.
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How to use the A–a Gradient Calculator
Alveolar–arterial oxygen gradient with the age-expected value.
- 1 Choose mmHg or kPa and enter FiO₂, PaO₂ and PaCO₂.
- 2 Enter age, and adjust barometric pressure at altitude.
- 3 Read the A–a gradient, alveolar PAO₂ and a/A ratio.
Example and practical tips
On room air with PaO₂ 70 and PaCO₂ 36 mmHg, PAO₂ is 104.7 mmHg and the A–a gradient is 34.7 mmHg, raised for a 45-year-old.
Frequently asked questions
What is a normal A–a gradient?
Roughly age ÷ 4 + 4 mmHg on room air. It increases with age and with higher inspired oxygen.
What does a normal gradient with low oxygen mean?
The lungs transfer oxygen normally, so hypoventilation or low inspired oxygen, such as at altitude, is the likely cause.
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