How this calculator works
The A-a gradient subtracts measured arterial oxygen pressure from estimated alveolar oxygen pressure. The simplified alveolar gas equation uses inspired oxygen, local barometric pressure, arterial carbon dioxide and the respiratory exchange ratio. The age estimate is a rough room-air reference, not a diagnostic boundary at every FiO2. High inspired oxygen and non-steady-state conditions reduce the usefulness of this simplified model. Negative gradients can occur with measurement or model differences and are displayed rather than forced to zero. The result is an estimate for information only and is not medical advice.
Formula
PAO2 = FiO2 x (PB - 47 mmHg) - PaCO2/R. A-a = PAO2 - PaO2. Age-reference estimate (mmHg) = (age in years + 10)/4. Inverse modes algebraically rearrange these equations.
Worked example
At FiO2 21%, PB 760 mmHg, PaCO2 40 mmHg and R 0.8, PAO2 = 0.21 x 713 - 50 = 99.73 mmHg. If PaO2 is 90 mmHg, the A-a gradient is 9.73 mmHg. At age 40, the reference estimate is 12.5 mmHg.
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Frequently asked questions
Why enter barometric pressure?
Inspired oxygen pressure falls at altitude. Use local absolute pressure rather than a weather report corrected to sea level.
Is the age estimate a diagnosis?
No. It is a rough reference for room air. Interpretation also depends on oxygen delivery, blood gas quality and clinical circumstances.
Can I solve for another gas value?
Yes. Select the unknown value. Each inverse mode validates the solved oxygen fraction and pressures before displaying a result.