Best Method to Check Vitamin D Levels
Order a serum 25-hydroxyvitamin D [25(OH)D] test that measures both the D2 and D3 forms combined, using a laboratory certified by the CDC's Vitamin D Standardization Program, and request results in ng/mL. 1
The Standard Test
- Serum 25-hydroxyvitamin D is the single best and most reliable biochemical marker for assessing total body vitamin D status 2, 1, 3
- The test must measure both 25(OH)D2 (ergocalciferol) and 25(OH)D3 (cholecalciferol) forms, as both contribute to your patient's total vitamin D stores 2, 1
- If the laboratory reports D2 and D3 separately, ensure they provide the sum as the primary result 1
Sample Collection Specifics
- Use serum as your sample of choice 2, 1
- Avoid gel separator tubes when possible, particularly if the lab uses LC-MS/MS methodology, as gel tubes can interfere with measurement accuracy 1
- The timing of blood draw is not critical since 25(OH)D is very stable in serum 1
Laboratory Quality Matters Significantly
- Verify that your laboratory participates in the CDC's Vitamin D Standardization Certification Program or DEQAS (Vitamin D External Quality Assurance Scheme) 1
- This verification step is critical because inter-laboratory variability can cause 4-32% variation in whether a patient is classified as deficient or not, depending on which assay is used 2, 1
- Despite ongoing standardization efforts, significant differences persist among various methods and laboratories 4
Common Pitfall: Assay Method Variability
- High-performance liquid chromatography with tandem mass spectrometry (LC-MS/MS) is generally more accurate than immunoassays, but both are widely used 2, 4
- Immunoassays can be affected by heterophilic antibodies, causing falsely elevated or decreased measurements 2
- Some immunoassays, particularly older ones, significantly underestimate 25(OH)D2 levels (measuring only about 55% of actual values) 5
Result Interpretation Guidelines
- Request results in ng/mL rather than nmol/L for consistency with most clinical guidelines 1
- Be aware that total serum 25(OH)D may underestimate vitamin D status in African American patients due to differences in vitamin D binding protein, though bioavailable 25(OH)D may be similar across races 2
When to Retest After Supplementation
- Wait at least 3 months after initiating vitamin D supplementation before retesting to allow serum concentrations to plateau and accurately reflect treatment response 2, 1
- For intermittent dosing regimens (weekly or monthly), measure levels just prior to the next scheduled dose 1
- Once the patient is stable on maintenance therapy, annual monitoring is sufficient 1
Who Should Be Tested
- Do not perform population-wide screening 1
- Test selectively in high-risk populations: patients with osteoporosis, malabsorption syndromes, chronic kidney disease, chronic liver disease, darker skin pigmentation, limited sun exposure, obesity, elderly/institutionalized individuals, or those on medications affecting vitamin D metabolism 1
- For dark-skinned, veiled, elderly (≥65 years), or institutionalized individuals, you may supplement empirically with 800 IU daily without baseline testing 1