Using the normal A-a gradient estimate formula age/4 + 4, what is the predicted gradient for age 40?

• A. 12 mmHg
• B. 16 mmHg
• C. 18 mmHg
• D. 14 mmHg

Answer: (D)

The normal A-a gradient increases with age, and a quick estimate is age divided by 4 plus 4. This accounts for the slight decline in gas exchange efficiency as we get older.

For age 40: 40/4 = 10, then add 4 → 14 mmHg. So the predicted gradient is 14 mmHg.

The other values would correspond to different ages using the same rule (e.g., 12 mmHg fits about age 32, 16 mmHg fits about age 48, 18 mmHg fits about age 56), so 14 mmHg is the correct estimate for someone who is 40.