Which vitamin D test should be checked on labs for deficiency?

Total Serum 25-Hydroxyvitamin D is the Appropriate Test for Vitamin D Deficiency Screening

When checking for vitamin D deficiency, total serum 25-hydroxyvitamin D [25-(OH)D] is the appropriate laboratory test to order, not 1,25-dihydroxyvitamin D. 1

Rationale for Using 25-(OH)D Testing

25-hydroxyvitamin D is the major circulating form of vitamin D in the body and is considered the best marker of vitamin D status for several reasons:

• It reflects both dietary intake and cutaneous production of vitamin D
• It has a longer half-life (2-3 weeks) compared to 1,25-dihydroxyvitamin D (4-6 hours)
• It correlates better with clinical outcomes related to vitamin D status 1, 2

Testing Methods and Considerations

Several methods are available for measuring total serum 25-(OH)D levels:

• Immunoassays (most commonly used in clinical laboratories)
• High-performance liquid chromatography (HPLC)
• Combined HPLC and mass spectrometry
• Tandem mass spectrometry (considered the reference method) 2

Important Testing Limitations

Be aware of significant variability in 25-(OH)D testing:

• Variability between different assay methods ranges from 10-20%
• Classification of samples as "deficient" or "nondeficient" may vary by 4-32% depending on which assay is used 1
• Lack of standardization between laboratories using the same methods 2
• 25-(OH)D may act as a negative acute-phase reactant, with levels decreasing during inflammation 1

Interpretation of Results

While there is no universal consensus on the definition of vitamin D deficiency, commonly used cut-points include:

• Deficiency: <20 ng/mL (<50 nmol/L) 1, 3
• Insufficiency: 20-30 ng/mL (50-75 nmol/L) 3
• Optimal: 30-80 ng/mL (75-200 nmol/L) 4

When to Test for Vitamin D Deficiency

The USPSTF does not recommend routine screening for vitamin D deficiency in asymptomatic adults 1. However, testing may be appropriate in individuals at increased risk:

• Persons with low vitamin D intake
• Those with decreased vitamin D absorption
• People with little or no sun exposure (winter season, high latitude, physical sun avoidance)
• Individuals with obesity (may have greater sequestration of vitamin D in adipose tissue)
• Those with darker skin pigmentation
• Patients with symptoms suggestive of deficiency (symmetric low back pain, proximal muscle weakness, muscle aches, bone pain) 1, 3

Common Pitfalls to Avoid

1. Ordering the wrong test: 1,25-dihydroxyvitamin D [1,25-(OH)₂D] should NOT be used to assess vitamin D status. This test is only appropriate for specific conditions like chronic kidney disease and hereditary phosphate-losing disorders 5.

2. Misinterpreting results across different populations: Laboratory reference ranges may not be appropriate for all ethnic groups. For example, African Americans typically have lower total serum 25-(OH)D levels but similar bioavailable 25-(OH)D and lower fracture risk compared to white individuals 1.

3. Relying on a single measurement: Consider the variability in testing methods when interpreting results near cut-off values. A single value of 30 ng/mL may have substantial variability, making levels of 35-40 ng/mL a more reasonable therapeutic goal 6.

4. Ignoring seasonal variation: 25-(OH)D levels can fluctuate seasonally due to changes in sun exposure.

Treatment Considerations

If deficiency is confirmed:

• Treatment may include oral ergocalciferol (vitamin D₂) at 50,000 IU per week for eight weeks 3
• After normalization, maintenance with cholecalciferol (vitamin D₃) at 800-1,000 IU daily is recommended 3
• Single large doses of 300,000-500,000 IU should be avoided 4

Remember that the goal of treatment is to normalize vitamin D levels to relieve symptoms and decrease the risk of fractures, falls, and other adverse health outcomes.