Using an age-based rule, the normal A-a gradient for a 25-year-old is approximately:

• A. 8 mmHg
• B. 12 mmHg
• C. 14 mmHg
• D. 16 mmHg

Answer: (C)

The A-a gradient reflects how well oxygen moves from the alveoli into the arterial blood and it sits at a small, normally age-dependent value in healthy people. A simple way to estimate what’s normal for an age is the rule: A-a gradient in mmHg ≈ (age in years ÷ 4) + 4. For a 25-year-old, that comes to about 10–11 mmHg, and many clinicians accept roughly 8–15 mmHg as a normal range for young adults. Values around 14 mmHg fall toward the upper end of that normal span, which is still considered normal for a young adult. In other words, 14 mmHg is a reasonable approximation for a normal A-a gradient in someone who is 25.