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A-a Oxygen Gradient (Alveolar-Arterial) Calculator

PAO₂ = FiO₂·(P_atm − 47) − PaCO₂ / 0.8; A-a = PAO₂ − PaO₂.

Compute the alveolar-arterial oxygen gradient (A-a gradient). Alveolar PO₂ from the alveolar-gas equation: PAO₂ = FiO₂ · (P_atm − P_H₂O) − PaCO₂ / RQ, with P_H₂O = 47 mmHg and respiratory quotient RQ = 0.8. The A-a gradient = PAO₂ − PaO₂; expected age-predicted gradient ≈ (age + 10) / 4.

Priya Venkatesan By Priya Venkatesan
Published Last reviewed 1 min read

Inputs

mmHg
mmHg
mmHg
yr

Results

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How to use this calculator

  1. Fill in the inputs above using the units you already have.
  2. Values update automatically as you type — no submit button needed.
  3. Hover any result row for the underlying formula and intermediate values.

Formula

PAO₂ = FiO₂·(P_atm − 47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂.

In depth

Compute the alveolar-arterial oxygen gradient (A-a gradient). Alveolar PO₂ from the alveolar-gas equation: PAO₂ = FiO₂ · (P_atm − P_H₂O) − PaCO₂ / RQ, with P_H₂O = 47 mmHg and respiratory quotient RQ = 0.8. The A-a gradient = PAO₂ − PaO₂; expected age-predicted gradient ≈ (age + 10) / 4.