What is the recommended workup for vitamin D (25-hydroxyvitamin D) hypovitaminosis?

Workup for Vitamin D Hypovitaminosis

Initial Laboratory Assessment

Measure serum 25-hydroxyvitamin D [25(OH)D] as the single essential test to diagnose vitamin D hypovitaminosis. This is the only laboratory marker that accurately reflects vitamin D status, while 1,25-dihydroxyvitamin D provides no useful information about vitamin D stores and is often misleadingly normal or elevated due to secondary hyperparathyroidism 1, 2.

Primary Diagnostic Test

• Obtain serum 25(OH)D level using an assay that measures both 25(OH)D2 and 25(OH)D3 for accurate assessment 1, 3
• The preferred method is liquid chromatography-tandem mass spectrometry (LC-MS/MS), which provides superior accuracy compared to protein binding assays and can distinguish between D2 and D3 forms 4

Diagnostic Thresholds

• Deficiency: 25(OH)D <20 ng/mL (50 nmol/L) 1, 5, 6
• Insufficiency: 25(OH)D 20-30 ng/mL (50-75 nmol/L) 1, 5, 6
• Severe deficiency: 25(OH)D <10-12 ng/mL 1, 3
• Target optimal range: 30-40 ng/mL (75-100 nmol/L) for health benefits 1, 3, 7
• Upper safety limit: 100 ng/mL 1, 3, 6

Additional Laboratory Tests Based on Clinical Context

When PTH is Elevated or CKD is Present

• Measure intact PTH if chronic kidney disease stages 3-4 is present (GFR 20-60 mL/min/1.73m²), as vitamin D insufficiency commonly aggravates secondary hyperparathyroidism in this population 1
• In CKD patients, if PTH is above target range for their stage, measure 25(OH)D at first encounter and repeat annually if normal 1

Baseline Safety Monitoring

• Obtain serum calcium (corrected total calcium) to rule out hypocalcemia and establish baseline before initiating supplementation, particularly in severe deficiency 1, 3
• Measure serum phosphorus as baseline, especially in CKD patients where monitoring is required every 3 months during treatment 1

When Inflammation is Present

• Check C-reactive protein (CRP) if inflammation is suspected, as CRP >40 mg/L can significantly reduce plasma vitamin D levels and complicate interpretation 3

Risk Assessment for Targeted Testing

Measure 25(OH)D in these high-risk populations rather than screening universally:

Musculoskeletal Risk Factors

• Adults >65 years with fall risk or fracture history 1, 3
• Patients with osteoporosis or fragility fractures 6
• Individuals with proximal muscle weakness, symmetric low back pain, or bone pain on sternal/tibial pressure 5

Malabsorption Conditions

• Post-bariatric surgery patients, especially Roux-en-Y gastric bypass 3, 8
• Inflammatory bowel disease (Crohn's disease, ulcerative colitis) 3
• Celiac disease, pancreatic insufficiency, short bowel syndrome 3
• Patients requiring enteral or parenteral nutrition 3

Chronic Disease States

• Chronic kidney disease stages 3-5 1, 3
• Chronic liver disease, particularly non-alcoholic fatty liver disease 8
• Autoimmune diseases 1
• Cancer patients 1

Lifestyle and Demographic Factors

• Dark-skinned or veiled individuals with limited sun exposure 1, 3
• Institutionalized or homebound individuals 1, 9
• Pregnant or lactating women 3, 6
• Obese patients (vitamin D sequestration in adipose tissue) 3, 5

Medication-Related Risk

• Patients on medications affecting vitamin D metabolism 6

Common Pitfalls to Avoid

• Do not measure 1,25-dihydroxyvitamin D to assess vitamin D status—it is not a marker of vitamin D stores and is often misleadingly normal in deficiency states 1, 2
• Do not use active vitamin D analogs (calcitriol, alfacalcidol, doxercalciferol, paricalcitol) to treat nutritional vitamin D deficiency, as they do not correct 25(OH)D levels 3, 8
• Ensure the assay measures both D2 and D3 forms, as many patients receive ergocalciferol (D2) supplementation 1, 4
• Account for seasonal variation—vitamin D levels are typically lowest after winter 3
• Consider timing of measurement relative to supplementation—wait at least 3 months after starting treatment before rechecking levels to allow plateau 1, 3, 8

Clinical Presentation Clues

When vitamin D deficiency is symptomatic, patients typically present with:

• Symmetric low back pain 5
• Proximal muscle weakness and muscle aches 5
• Throbbing bone pain elicited by pressure over sternum or tibia 5
• Increased fall risk in elderly 1, 5

However, most cases are asymptomatic and detected through screening of at-risk populations 7, 6.