Calculates the arterial-to-alveolar oxygen ratio from PaO₂ and PAO₂.
Alveolar–Arterial Oxygen Ratio: Clinical Guide
Browse Knowledge HubCalculates the arterial-to-alveolar oxygen ratio from PaO₂ and PAO₂.
Overview
Calculates the arterial-to-alveolar oxygen ratio from PaO₂ and PAO₂.
Clinical Significance
Expresses arterial oxygen tension relative to alveolar oxygen tension and can help assess gas exchange across different inspired oxygen levels.
When to Use
- Use when the required measurements are available, reliable and clinically relevant.
How It Is Calculated
- a/A ratio = PaO₂ / PAO₂.
Interpretation
- Expresses arterial oxygen tension relative to alveolar oxygen tension and can help assess gas exchange across different inspired oxygen levels.
Worked Example
Enter valid values in the displayed units. MedicalC then applies: a/A ratio = PaO₂ / PAO₂.
Limitations
- The result does not establish a diagnosis or mandate treatment.
- Measurement error may be more important than arithmetic precision.
- Published thresholds may differ between populations and settings.
Clinical Pearls
- Document the source values and units with the final result.
- Review trends rather than isolated values where appropriate.
Common Mistakes
- Using the wrong unit.
- Combining values from different time points.
- Applying the result outside the intended measurement or clinical context.
Frequently Asked Questions
What does this calculator provide?
Calculates the arterial-to-alveolar oxygen ratio from PaO₂ and PAO₂.
What formula is used?
a/A ratio = PaO₂ / PAO₂.
Can the result be used alone?
No. It must be interpreted with the full clinical assessment and appropriate investigations.
What is the main limitation?
The result depends on accurate source measurements, correct units and appropriate clinical applicability.
References
- Standard respiratory physiology relationship. — Primary, supporting or authoritative reference for the implemented relationship.
Reviewed by: MedicalC Clinical Editorial Team
Last reviewed: July 2026
Clinical Disclaimer
MedicalC calculators and clinical tools are intended to support healthcare professionals. They do not replace clinical judgement, individual patient assessment, local guidance or specialist advice.
Results should always be interpreted in the context of the patient’s history, examination, investigations, current clinical condition and applicable professional guidance.