When should a vitamin D (25-hydroxyvitamin D) test be ordered?

When to Order Vitamin D Testing

Vitamin D testing should NOT be ordered routinely for the general population; instead, reserve testing for patients at high risk for deficiency based on specific clinical conditions or risk factors. 1

General Population Screening

• Population-wide screening is not recommended by the U.S. Preventive Services Task Force, as there is insufficient evidence that screening asymptomatic individuals improves health outcomes. 1
• The Endocrine Society similarly recommends against population-level screening, stating there is no evidence of benefit from universal testing. 1
• No national primary care professional organization currently recommends routine vitamin D screening for all patients. 1

High-Risk Populations Who Should Be Tested

Musculoskeletal Conditions

• Test patients with suspected vitamin D deficiency who present with symmetric low back pain, proximal muscle weakness, muscle aches, or throbbing bone pain elicited with pressure over the sternum or tibia. 2
• Order testing as part of osteoporosis management or fall prevention in older adults, as recommended by the American Congress of Obstetricians and Gynecologists, American Geriatric Society, and National Osteoporosis Foundation. 1

Chronic Diseases Requiring Testing

• Patients with cystic fibrosis should have annual vitamin D testing, with additional testing 3-6 months after any dosage change. 1
• Patients with chronic kidney disease warrant testing due to high risk of deficiency from renal losses or decreased synthesis of 1,25-dihydroxyvitamin D. 3
• Patients with chronic liver disease should be tested due to impaired vitamin D metabolism. 4
• Patients with malabsorption syndromes (Crohn's disease, celiac disease) require testing due to impaired intestinal absorption. 3

Additional Risk Factors Warranting Testing

• Darker skin pigmentation (requires more sun exposure for equivalent vitamin D production). 1
• Limited sun exposure (less than 5% of skin exposed regularly, use of UVB-blocking sunscreens, concealing clothing). 1
• Older individuals (reduced skin synthesis capacity compared to younger persons). 1
• Institutionalized patients are at high risk, though these patients should receive empiric supplementation (800 IU daily) without requiring baseline testing. 5
• Women planning pregnancy or in early pregnancy should have vitamin D assessed, particularly those with cystic fibrosis or other risk factors. 1

When NOT to Test

• Do not test before discussing recommended dietary allowance (RDA) with patients, as it is not necessary to evaluate 25(OH)D levels before recommending standard vitamin D intake (600 IU/day for adults 19-70 years, 800 IU/day for adults >70 years). 1
• Do not test healthy individuals without risk factors, as this represents low-value care without demonstrated benefit. 1

Testing Methodology Considerations

• Use assays that measure both 25(OH)D2 and 25(OH)D3 forms for accurate total vitamin D status assessment. 6
• Be aware of significant inter-laboratory variability (10-20% between methods), which can result in 4-32% variation in classifying patients as deficient versus non-deficient depending on the assay used. 6
• Verify laboratory participation in the CDC's Vitamin D Standardization Certification Program to ensure quality and comparability of results. 6

Monitoring After Supplementation

• Wait at least 3 months before retesting after initiating vitamin D supplementation to allow serum levels to reach plateau and reflect true response to therapy. 4
• For patients with cystic fibrosis or chronic kidney disease, recheck 3-6 months after any dosage adjustment. 1, 4
• Once target levels are achieved, annual monitoring is sufficient for most patients on stable maintenance therapy, preferably measured at the end of winter months when levels are typically lowest. 4

Common Pitfalls to Avoid

• Avoid testing too early after starting supplementation (before 3 months), as vitamin D has a long half-life and levels need time to stabilize. 4
• Account for seasonal variation when interpreting results, as levels measured in late winter/early spring will be lower than those in late summer/early fall. 4
• Do not over-interpret small differences between serial measurements given the 10-20% inter-assay variability. 6