What conditions should prompt a check of a vitamin D level?

When to Check Vitamin D Levels

Check vitamin D levels in patients with established bone disease, chronic kidney disease, malabsorption syndromes, or those at high risk for deficiency—not routinely in healthy adults. 1

High-Risk Populations Requiring Testing

Classical Indications (Strong Evidence)

• Adults ≥65 years with fall risk or fracture history should be tested, as this population has dual benefits from vitamin D on both bone density and muscle strength 2, 1
• Patients with osteoporosis or chronic kidney disease stages 3-5 require testing, as vitamin D insufficiency commonly aggravates secondary hyperparathyroidism and increases fracture risk 1
• Post-bariatric surgery patients, especially those who underwent Roux-en-Y gastric bypass, need testing due to dramatically reduced vitamin D absorption 1

Malabsorption Syndromes

• Inflammatory bowel disease (Crohn's disease, ulcerative colitis) causes vitamin D malabsorption through intestinal inflammation and reduced absorptive surface area 3
• Pancreatic insufficiency impairs fat digestion necessary for vitamin D absorption 3
• Short bowel syndrome and untreated celiac disease reduce available intestinal surface for absorption 3

Additional Risk Factors

• Dark-skinned individuals have 2-9 times higher prevalence of low vitamin D levels due to increased melanin blocking UVB synthesis 2, 3
• Institutionalized or veiled individuals with minimal sun exposure warrant testing 2
• Obesity is associated with vitamin D sequestration in adipose tissue, though bioavailability may remain adequate 3
• Pregnancy and lactation increase vitamin D demands 3

When NOT to Test

Do not test healthy adults without risk factors, as the evidence for routine population screening is insufficient and supplementation benefits are primarily seen only in those with documented deficiency 4. The 2010 Institute of Medicine report concluded that most North Americans receive sufficient vitamin D from diet and sun exposure 4.

Important Testing Considerations

Correct Test to Order

• Measure serum 25-hydroxyvitamin D [25(OH)D] as the single essential test—this accurately reflects vitamin D status 1
• Use an assay measuring both 25(OH)D2 and 25(OH)D3 for accurate assessment 1, 5
• Never measure 1,25-dihydroxyvitamin D to assess vitamin D status, as it is not a marker of vitamin D stores and is often misleadingly normal in deficiency states 1

Diagnostic Thresholds

• Deficiency: <20 ng/mL (50 nmol/L) requires treatment 1
• Insufficiency: 20-30 ng/mL (50-75 nmol/L) may warrant supplementation 1
• Target optimal range: 30-40 ng/mL (75-100 nmol/L) for health benefits 1
• Upper safety limit: 100 ng/mL 1

Additional Testing in Specific Contexts

• Measure intact PTH if chronic kidney disease stages 3-4 is present, as vitamin D insufficiency commonly aggravates secondary hyperparathyroidism 1
• Obtain serum calcium to rule out hypocalcemia and establish baseline before initiating supplementation, particularly in severe deficiency 1
• Measure serum phosphorus as baseline, especially in CKD patients where monitoring is required every 3 months during treatment 1

Common Pitfalls to Avoid

• Do not test in response to nonspecific symptoms like fatigue or general aches without other risk factors, as these symptoms have multiple etiologies 6
• Be aware of assay variability—classification of samples as "deficient" or "nondeficient" can vary by 4-32% depending on which assay is used 5
• Inflammation can reduce plasma vitamin D levels when C-reactive protein >40 mg/L, complicating interpretation 3
• Total serum 25(OH)D may underestimate vitamin D status in African Americans due to differences in vitamin D binding protein, though bioavailable 25(OH)D may be similar across races 5