A-a Gradient
Difference between alveolar and arterial oxygen.
What is A-a Gradient?
Difference between alveolar and arterial oxygen. A-a Gradient is intended for use in: Adults.
How to Use the A-a Gradient Calculator
- Enter FiO2 in %.
- Enter PaCO2 in mmHg.
- Enter PaO2 (arterial, from ABG) in mmHg.
- Enter Age (optional, for expected gradient) in years.
- Enter Atmospheric pressure (optional) in mmHg. Defaults to 760 mmHg (sea level)
- Click Calculate to see your result and interpretation.
Formula & Version
This calculator uses formula version 1.0, last reviewed September 2026.
Clinical Considerations & Limitations
- Assumes a respiratory quotient of 0.8 and requires a simultaneous arterial blood gas and FiO2.
- Widens normally with age and with supplemental oxygen — the age-predicted normal is only a rough guide.
References
- Alveolar gas equation, standard respiratory physiology; age-predicted normal per Mellemgaard K, Acta Physiol Scand 1966;67:10-20
Frequently Asked Questions
What is A-a Gradient?
Difference between alveolar and arterial oxygen.
Who is A-a Gradient intended for?
Adults. Always confirm applicability to your specific patient before use.
What are the limitations of A-a Gradient?
Assumes a respiratory quotient of 0.8 and requires a simultaneous arterial blood gas and FiO2. Widens normally with age and with supplemental oxygen — the age-predicted normal is only a rough guide.
What formula or reference does A-a Gradient use?
Alveolar gas equation, standard respiratory physiology; age-predicted normal per Mellemgaard K, Acta Physiol Scand 1966;67:10-20
Is A-a Gradient a diagnosis?
No. A-a Gradient produces a score or estimate to support clinical decision-making — it is not a diagnosis and should be interpreted alongside full clinical judgement.
