Vitamin D Status and Its Association With Quality of Life, Physical Activity, and Disease Activity in Rheumatoid Arthritis Patients

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To the Editor:
We have read with interest the recently published article entitled “Vitamin D Status and Its Association With Quality of Life, Physical Activity, and Disease Activity in Rheumatoid Arthritis Patients” by Raczkiewicz et al.1 We would like to comment on an incorrect use about measurement unit of serum 25(OH) vitD.
Some medical tests report serum 25(OH) vitD results in nanogram per milliliter (ng/mL). In some laboratories, vitamin D blood test results are also given in units of nanomole per liter (nmol/L). This category of measurement units is defined by the density. To convert a test result measured in nmol/L to one measured in ng/mL, divide the nmol/L number by 2.5. For example, 100 nmol/L is the same as 40 ng/mL (100 ÷ 2.5).
A nanogram is one-billionth of a gram. A milliliter measures fluid volume equal to 1/1000 L. A deciliter (dL) measures fluid volume equal to 1/10 L. Authors of this article have used ng/dL as serum 25(OH) vitD measurement unit, but 1 ng/dL equal to 0.01 ng/mL.
Serum levels of 25(OH) vitD have been used as a measure of adequacy for vitamin D. The Endocrine Society has described that serum 25(OH) vitD concentration less than 20 ng/mL is vitD deficiency.2 In this article, serum 25(OH) vitD concentration less than 20 ng/dL has been defined as vitD deficiency. We believe that using ng/dL for the measurement unit of serum 25(OH) vitD concentration is incorrect. The unit of measurement ng/mL should be used instead of ng/dL by the authors. We hope that this brief text will help contribute to the accuracy of the study.

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