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The most accurate way to measure how much vitamin D is in your body is the 25-hydroxy vitamin D blood test. A level of 20 nanograms/milliliter to 50 ng/mL is considered adequate for healthy people. A level less than 12 ng/mL indicates vitamin D deficiency.

nanograms/milliliter (ng/ml) and nanomoles/Liter (nmol/L)
1 ng/ml = 2.5 nmol/L