From the Guidelines
The most accurate laboratory test for measuring vitamin D (25-hydroxyvitamin D) levels is High-Performance Liquid Chromatography (HPLC) or Liquid Chromatography-Tandem Mass Spectrometry (LC/MS-MS).
Key Considerations
- These assays should measure both 25(OH)D2 and 25(OH)D3 and indicate the sum of the two compounds as the main result 1.
- Laboratories should participate in an external quality control scheme, such as the UK-based DEQAS, which provides materials with documented commutability with human sera 1.
- An internal quality control program should be carried out using different levels of samples and valuable criteria for rejecting or accepting analytical runs 1.
- Serum is the sample of choice, and results should be expressed in ng/mL 1.
- It is recommended to avoid using population-based reference values and instead use a recommended range for 25(OH)D to avoid confusion 1.
From the Research
Laboratory Tests for Measuring Vitamin D Levels
The most accurate laboratory test for measuring vitamin D (25-hydroxyvitamin D) levels is a topic of interest in clinical medicine.
- The most abundant vitamin D metabolite, 25-hydroxyvitamin D, is considered the best marker for estimating vitamin D status and is therefore the most commonly measured in clinical practice 2, 3, 4.
- Automated immunoassays are the most frequently used methods for measuring 25-hydroxyvitamin D, but they do not meet the requirements of today's standards for certain patient groups 2, 3.
- Liquid chromatography-tandem mass spectrometry is considered the desired method of choice for measuring 25-hydroxyvitamin D in certain individuals due to its high accuracy and precision 2, 5.
- Other methods, such as high-performance liquid chromatography (HPLC), are also available but have limitations, including lack of sensitivity for determining trace amounts of 25-hydroxyvitamin D2 and D3 5.
Comparison of Assays
- Different assays for measuring 25-hydroxyvitamin D have been compared, and significant differences remain among various methods and laboratories 3.
- Standardization of analytical methods is essential for developing and implementing evidence-based clinical guidelines and achieving comparable results via different methods 5.
- The measurement of free 25-hydroxyvitamin D is also of interest, and new immunoassays have been developed to measure it directly, but calculated values using levels of total 25-hydroxyvitamin D, albumin, and vitamin D binding protein are still commonly used 6.