The a/A ratio is defined as which of the following?

The a/A ratio is defined as which of the following?

• A. PAO2 / PaO2
• B. (PAO2 − PaO2) / PAO2
• C. PaO2 / PAO2
• D. (PaO2 / PAO2) × 2

Answer: (C)

The a/A ratio measures how effectively oxygen moves from the alveoli into the arterial blood. It is defined as arterial oxygen tension divided by alveolar oxygen tension, PaO2/PAO2. This ratio reflects transfer efficiency: when the lungs are doing well, PaO2 is close to PAO2, so the ratio approaches 1. If gas exchange is impaired, PaO2 falls relative to PAO2 and the ratio drops. So PaO2/PAO2 is the correct definition because it directly compares the oxygen actually present in arterial blood to what would be available in the alveoli. The other formulations would either invert the ratio, use a difference rather than a ratio, or artificially scale the value, none of which match how the a/A ratio is defined.

The a/A ratio measures how effectively oxygen moves from the alveoli into the arterial blood. It is defined as arterial oxygen tension divided by alveolar oxygen tension, PaO2/PAO2. This ratio reflects transfer efficiency: when the lungs are doing well, PaO2 is close to PAO2, so the ratio approaches 1. If gas exchange is impaired, PaO2 falls relative to PAO2 and the ratio drops.

So PaO2/PAO2 is the correct definition because it directly compares the oxygen actually present in arterial blood to what would be available in the alveoli. The other formulations would either invert the ratio, use a difference rather than a ratio, or artificially scale the value, none of which match how the a/A ratio is defined.