What to Order on Lab Slip to Test Vitamin D Levels
Order a serum 25-hydroxyvitamin D [25(OH)D] test that measures both 25(OH)D2 and 25(OH)D3 forms. This is the gold standard for assessing vitamin D status in clinical practice 1.
Specific Test Name and Requirements
- Write "25-hydroxyvitamin D, total" or "25(OH)D" on the lab requisition 2
- The assay must measure both 25(OH)D2 (ergocalciferol metabolite) and 25(OH)D3 (cholecalciferol metabolite) to capture total vitamin D status 1
- If the assay separates D2 and D3, the lab should report the sum as the primary result 1
- Request results in ng/mL rather than nmol/L for consistency with clinical guidelines 1
Sample Collection Specifications
- Use serum as the sample of choice 1
- Avoid gel separator tubes if possible, as they can interfere with certain measurement methods, particularly LC-MS/MS 1
- 25(OH)D is very stable in serum, so timing of collection is not critical 1
Important Testing Considerations
- Do NOT order 1,25-dihydroxyvitamin D [1,25(OH)2D] for routine vitamin D status assessment—this is the active hormone form but does not reflect body stores 2
- Ensure your laboratory participates in external quality control programs like DEQAS (Vitamin D External Quality Assurance Scheme) or the CDC's Vitamin D Standardization Certification Program 1, 3
- Be aware that significant inter-laboratory variability exists (4-32% variation in classification of deficiency depending on assay used) 1, 3
When to Order Testing
- Population-wide screening is NOT recommended by the USPSTF or Endocrine Society 4, 3
- 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 4, 3, 5
- For dark-skinned, veiled, elderly (≥65 years), or institutionalized individuals, you may supplement with 800 IU daily without baseline testing 1, 6
Timing of Follow-Up Testing
- Recheck levels at least 3 months after initiating supplementation to allow serum concentrations to plateau and accurately reflect treatment response 1, 6, 3
- For intermittent dosing regimens (weekly or monthly), measure just prior to the next scheduled dose 1, 6
- Once stable on maintenance therapy, annual monitoring is sufficient 3