A-a Gradient & P/F Suite
Run the two bedside oxygenation indices side by side.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- FiO2
- 0.21
- PaO2
- 90 mmHg
- PaCO2
- 40 mmHg
- Age
- 40 years
Result: A-a gradient ~9.7 mmHg (expected ~14 by age); P/F ratio 429 (Normal).
The tool opens with these values already filled in. Replace them with your own.
What you enter
- FiO2 (0-1)
- PaO2
- PaCO2
- Age
What this is
The A-a gradient uses the alveolar gas equation - PAO2 = FiO2*(Patm - PH2O) - PaCO2/RQ - to estimate the alveolar-to-arterial oxygen difference, with age-adjusted upper limits of normal. The P/F ratio (PaO2/FiO2) is the ARDS Berlin Definition's severity axis - mild 200-300, moderate 100-200, severe under 100 - and applies whether the patient is on room air or a known FiO2. Sophie Well computes both from a single ABG plus FiO2 input.
When to use it
Use this when a patient is hypoxemic and you want to distinguish a V/Q-mismatch / shunt picture (elevated A-a gradient) from pure hypoventilation (normal A-a gradient with high CO2), or when you are documenting ARDS severity for a multidisciplinary chart. Both numbers are bedside-fast and require no specialized equipment beyond an ABG; together they steer the workup toward parenchymal versus pump versus ventilatory failure faster than either does alone.
How this is calculated
West JB. Respiratory Physiology. ARDS Berlin Definition: JAMA 2012;307(23):2526. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.