What tests should I order to check vitamin D and vitamin B levels?

Ordering Vitamin D and Vitamin B Tests

To check vitamin D levels, order serum 25-hydroxyvitamin D (25(OH)D), and for vitamin B levels, order serum vitamin B12 (cobalamin) and serum folate. 1, 2, 3

Vitamin D Testing

Test to Order:

• Serum 25-hydroxyvitamin D (25(OH)D) - This is the primary circulating form of vitamin D and the best indicator of overall vitamin D status 1, 2
• Ensure the assay measures both 25(OH)D2 and 25(OH)D3 for accurate assessment 1, 4

Interpretation of Results:

• Vitamin D insufficiency: 21-29 ng/mL (52.5-72.5 nmol/L) 2
• Target range for optimal health: ≥30-40 ng/mL (75-100 nmol/L) 1, 2
• Upper safety limit: 100 ng/mL (250 nmol/L) 1, 2

When to Test:

• Testing is not recommended for the general asymptomatic population 1, 2
• Testing is appropriate for high-risk individuals:
  • Patients with osteoporosis, osteomalacia, or history of fractures 2
  • Patients with chronic kidney disease, malabsorption syndromes, liver disease 2
  • Patients with symptoms of vitamin D deficiency (bone pain, muscle weakness) 2
  • Elderly, especially institutionalized individuals 1, 2
  • Individuals with darker skin pigmentation or limited sun exposure 2
  • Obese individuals (BMI >30) 2

Vitamin B Testing

Tests to Order:

• Serum Vitamin B12 (cobalamin) - Primary test for B12 status 1, 3
• Serum Folate - Important to check alongside B12 1, 3
• Additional tests if B12 deficiency is suspected:
  • Complete blood count (CBC) - To detect macrocytic anemia 3
  • Methylmalonic acid (MMA) - More sensitive marker for B12 deficiency 1

Interpretation:

• B12 deficiency: <221 pmol/L 5
• Folate deficiency: <13.6 nmol/L 5

When to Test:

• Regular monitoring for patients after bariatric surgery 1
• Patients with symptoms of B12 deficiency (fatigue, weakness, neurological symptoms) 3
• Patients with macrocytic anemia 3
• Patients on medications that may affect B12 absorption (metformin, proton pump inhibitors) 3

Important Considerations

Timing of Testing:

• When monitoring response to vitamin D supplementation, wait at least 3 months after starting treatment before retesting 1, 2
• Consider seasonal variations in vitamin D levels when interpreting results 2

Common Pitfalls to Avoid:

1. Overscreening - Testing individuals without risk factors contributes to unnecessary healthcare costs 2
2. Underscreening - Failing to test high-risk individuals can lead to missed opportunities for treatment 2
3. Improper timing - Testing too soon after starting supplementation can lead to inaccurate results 2
4. Laboratory variability - Different assay methods may yield different results (10-20% variability) 1, 4
5. Not checking both B12 and folate together - High folate can mask B12 deficiency 1, 3

Special Considerations:

• For certain high-risk groups (dark-skinned individuals, elderly, institutionalized), vitamin D supplementation (800 IU/day) may be started without baseline testing 1, 2
• For patients with pernicious anemia, parenteral B12 is the recommended treatment (not oral B12) 3, 6
• There may be an association between vitamin B12 and vitamin D deficiency, so consider testing both when one is deficient 7

By following these guidelines, you can appropriately order and interpret vitamin D and vitamin B tests to optimize patient care and avoid unnecessary testing.