From the Guidelines
The recommended vitamin D blood test to order is the 25-hydroxyvitamin D (25(OH)D) test, which is the most accurate way to assess vitamin D status in the body. This test should be ordered as "25-hydroxyvitamin D, serum" or "Total 25-OH vitamin D." The normal range for most laboratories is 30-100 ng/mL (or 75-250 nmol/L), with levels below 20 ng/mL indicating deficiency, as defined by the Endocrine Society 1. Testing is particularly important for individuals at risk for deficiency, including those with limited sun exposure, darker skin, obesity, malabsorption disorders, or those taking certain medications.
Key Considerations
- The test should be performed after fasting for at least 8 hours for the most accurate results.
- If deficiency is detected, follow-up testing is typically recommended 3-6 months after beginning supplementation to ensure levels have normalized, as suggested by the Autoimmunity Reviews study 1.
- The 25(OH)D test is preferred over the 1,25-dihydroxyvitamin D test because it better reflects overall vitamin D stores and has a longer half-life in the bloodstream, providing a more reliable indicator of vitamin D status over time.
Population-Specific Recommendations
- The American Academy of Family Physicians and the Endocrine Society do not recommend population-wide screening for vitamin D deficiency, but rather targeted screening for individuals at risk 1.
- The Institute of Medicine recommends a daily intake of 600 IU/d for adults aged 19-70 years and 800 IU/d for adults older than 70 years, assuming minimal sun exposure 1.
Clinical Decision-Making
- Clinicians should use their judgment to determine the frequency of monitoring 25(OH)D levels, taking into account the dose of supplementation and changes in regimen or dose, as recommended by the Autoimmunity Reviews study 1.
- Monitoring of calcium levels is only required in patients with diseases such as primary hyperparathyroidism 1.
From the Research
Recommended Order for Vitamin D Blood Test
The recommended order for a Vitamin D (25-hydroxyvitamin D) blood test is not explicitly stated in the provided studies. However, the following guidelines can be inferred:
- The 25-hydroxyvitamin D (25OHD) serum concentration should not be measured in everybody, but rather in specific clinical situations where a target range of 25OHD concentrations is associated with better outcomes 2.
- Measurement of 25OHD is recommended in situations such as:
- Bone fragility (defined by low bone mineral density and/or low energy fracture)
- Malabsorptions
- Chronic kidney disease
- Phosphocalcic pathology
- Clinical signs of profound vitamin D deficiency or excess
- Biological exploration of calcium/phosphorus metabolism that includes the measurement of PTH 2
- Testing of serum 25-hydroxy-vitamin D (25OH-D) levels should be performed before prescribing vitamin D supplementation, but in some cases, moderate vitamin D doses can be started without prior testing and 25-OH-D levels can be checked later 3