Which lab can be used to determine vitamin D adequacy, and how should it be interpreted?

Which lab can be used to determine vitamin D adequacy, and how should it be interpreted?

• A. Serum 25-hydroxyvitamin D; insufficiency 21-29 ng/mL, deficiency <20 ng/mL, adequate ≥30 ng/mL
• B. Serum calcium; normal range
• C. Serum phosphorus; normal range
• D. Serum glucose; normal range

Answer: (A)

Measuring vitamin D adequacy relies on a direct indicator in the blood: serum 25-hydroxyvitamin D. This form reflects vitamin D obtained from sun exposure, diet, and supplements and has a relatively long half-life, making it the most reliable measure of status. Interpretation commonly used is: deficiency when the level is below 20 ng/mL, insufficiency from about 21 to 29 ng/mL, and adequacy at 30 ng/mL or higher. Other labs like serum calcium or phosphorus aren’t direct measures of vitamin D stores; they can remain normal due to physiological regulation by PTH and other factors, so they don’t reliably indicate vitamin D adequacy. Serum glucose is unrelated to vitamin D status.

Measuring vitamin D adequacy relies on a direct indicator in the blood: serum 25-hydroxyvitamin D. This form reflects vitamin D obtained from sun exposure, diet, and supplements and has a relatively long half-life, making it the most reliable measure of status. Interpretation commonly used is: deficiency when the level is below 20 ng/mL, insufficiency from about 21 to 29 ng/mL, and adequacy at 30 ng/mL or higher. Other labs like serum calcium or phosphorus aren’t direct measures of vitamin D stores; they can remain normal due to physiological regulation by PTH and other factors, so they don’t reliably indicate vitamin D adequacy. Serum glucose is unrelated to vitamin D status.