Aapo2rto
This test measures how well your lungs transfer oxygen from the air into your blood to assess lung function and oxygenation status.
{"Measures the efficiency of oxygen transfer from lungs to blood","Helps diagnose and monitor conditions like ARDS, pneumonia, and respiratory failure","A normal ratio is typically above 0.75 (75%)","Lower ratios indicate poorer oxygen exchange and more severe lung disease","Results are used to guide treatment decisions in critical care settings"}
Key facts at a glance
- LOINC
- 78354-8
- Short code
- a/A PO2 Rto
- Synonyms
- a/A PO2 Rto
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 80
- Ref high
- 100
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- calculated
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
What is the Arterial-Alveolar Oxygen Ratio Test?
The arterial-alveolar oxygen partial pressure ratio (A/a ratio or P/F ratio) is a calculation that measures how effectively your lungs are transferring oxygen from the air you breathe into your bloodstream. This test compares the amount of oxygen in your arterial blood (where it has just entered from the lungs) to the amount of oxygen that should be in the alveoli (the tiny air sacs in your lungs).
How is this test performed?
This test requires a blood sample, typically drawn from an artery (usually in the wrist). The sample is then analyzed in a laboratory to measure the partial pressure of oxygen (PaO2) in your arterial blood. Your healthcare provider will also record the fraction of inspired oxygen (FiO2) you are receiving at the time of the blood draw. The A/a ratio is calculated by dividing PaO2 by the alveolar oxygen partial pressure, which is derived from FiO2 and other factors.
What do the results mean?
A normal A/a ratio is typically above 0.75 (75%). Lower ratios indicate that your lungs are not transferring oxygen efficiently. Mild impairment is indicated by ratios of 0.6-0.75, moderate impairment by 0.3-0.6, and severe impairment below 0.3. Your healthcare provider will interpret these results in context with your overall clinical condition, other test results, and symptoms.
Reference range
NCBI: PaO2 80-100 mmHgN/A
No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
◆Associated findings2
Frequently asked questions
Is drawing blood from an artery painful?
How long does it take to get results?
Are there any factors that can affect the test results?
Do I need to fast before this test?
Sources
- 1
- 2
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy