Last updated October 12, 2026 · Report an error
Computes the alveolar-arterial (A-a) oxygen gradient using the standard published respiratory physiology formula: PAO2 = (FiO2 x (Patm - PH2O)) - (PaCO2 / 0.8), then A-a gradient = PAO2 - PaO2. Water vapor pressure is fixed at the standard 47 mmHg body-temperature value. Pure arithmetic on given blood-gas values. For general reference and educational purposes only — not a medical diagnosis. Consult a doctor for actual interpretation of arterial blood gas results.
How it works
Enter FiO2 (default 21% room air), atmospheric pressure (default 760 mmHg sea level), PaCO2, and PaO2 from an arterial blood gas. The tool computes the expected alveolar oxygen (PAO2) and subtracts the measured PaO2.
- Enter fiO2 (%).
- Enter atmospheric pressure (mmHg).
- Enter paCO2 (mmHg).
- Enter paO2 (mmHg).
- Enter age (years) — optional.
- Click Calculate to see your results.
Examples
Room air, PaCO2 40, PaO2 95
PAO2 = 0.21 x (760-47) - 40/0.8 = 149.7 - 50 = 99.7 mmHg. A-a gradient = 99.7 - 95 = 4.7 mmHg — within the commonly cited normal range for a young adult.
Who should use it
- Reviewing an arterial blood gas result.
- Respiratory physiology education.
Industry applications
- Pulmonology reference
- Clinical education
Advantages
- Uses the standard published alveolar gas equation.
- Optional age-adjusted normal-range guidance.
Limitations
- Reference ranges are approximate teaching rules, not lab-specific cutoffs.
- Does not replace clinical interpretation.
Common mistakes to avoid
- Forgetting that the normal A-a gradient range widens with age — a fixed cutoff like "15 mmHg" is not appropriate for all ages.
Best practices
- Enter the age field to get a commonly cited age-adjusted expected upper limit ((age/4)+4).
Tips
- The A-a gradient is most useful for distinguishing hypoxemia causes when interpreted alongside PaCO2 and the patient's FiO2, per standard respiratory physiology teaching.