Most Commonly Ordered Vitamin D Test by PCPs
The most commonly ordered vitamin D test by primary care physicians is the 25-hydroxyvitamin D [25(OH)D] serum level, which is the definitive biomarker for assessing vitamin D status. 1, 2
Why 25(OH)D is the Standard Test
Serum 25(OH)D is the barometer for vitamin D status and is universally recognized as the appropriate test to order. 2 This metabolite reflects both dietary intake and cutaneous synthesis of vitamin D, making it the most reliable indicator of total body vitamin D stores. 2
Common Pitfall to Avoid
- Never order 1,25-dihydroxyvitamin D [1,25(OH)₂D] as a screening test for vitamin D status. 2 This is the active form of vitamin D, but it provides no information about vitamin D status and is often normal or even elevated due to secondary hyperparathyroidism in vitamin D-deficient patients. 2
Target Ranges PCPs Should Know
When interpreting 25(OH)D results, the following thresholds guide clinical decision-making:
- Deficiency: <20 ng/mL (<50 nmol/L) - requires aggressive repletion 1, 2, 3
- Insufficiency: 20-29 ng/mL (50-75 nmol/L) - warrants supplementation 2
- Optimal: 30-44 ng/mL (75-110 nmol/L) - target range for health benefits 1, 4
- Upper safety limit: 100 ng/mL (250 nmol/L) - avoid exceeding this level 1
Important Measurement Consideration
Account for laboratory variability when interpreting results. 1, 4 Choosing a threshold of 30 ng/mL helps ensure the patient has a true concentration >20 ng/mL, given the measurement uncertainty inherent in 25(OH)D assays. 1, 4
Who Should Be Tested
PCPs should order 25(OH)D testing in specific populations rather than screening universally:
- Patients with musculoskeletal health problems (osteoporosis, osteomalacia, fractures) 1
- Those with cardiovascular disease or autoimmune conditions 1
- Patients with malabsorption syndromes (celiac disease, inflammatory bowel disease) 1
- Individuals with chronic kidney disease stages 3-5D 1
- Patients with secondary causes of osteoporosis (primary hyperparathyroidism) 1
Populations That Can Be Supplemented Without Testing
Some high-risk groups can receive empiric supplementation without baseline testing, as they are very likely to be deficient 1:
- Dark-skinned or veiled individuals with limited sun exposure 1
- Institutionalized patients 1
- Adults ≥65 years old without specific disease conditions 1
For these groups, start with 800 IU/day without measuring baseline levels. 1
Practical Testing Approach
When you do order the test, wait at least 3 months after initiating supplementation before rechecking levels to allow adequate time for equilibration and to assess response to therapy. 5 Seasonal variation should also be considered when interpreting results, as levels naturally fluctuate throughout the year. 1, 4