Laboratory Tests for Assessing Vitamin Status
When assessing vitamin status in patients, order serum vitamin B12 (either total B12 or active B12/holotranscobalamin) and serum 25-hydroxyvitamin D [25(OH)D] only when clinical suspicion exists based on symptoms, signs, or risk factors—not as routine screening in healthy adults.
Vitamin B12 Testing
First-Line Testing Approach
For suspected B12 deficiency, order either total B12 (serum cobalamin) or active B12 (serum holotranscobalamin) as your initial test 1. While active B12 measures the biologically active form available to cells and may be more accurate, the evidence supporting routine use over total B12 is insufficient to justify the significantly higher cost (£18 vs £2 per test) and longer turnaround time 1.
Interpretation Thresholds
- Total B12 < 180 ng/L (133 pmol/L): Confirmed deficiency
- Total B12 180-350 ng/L (133-258 pmol/L): Indeterminate—possible deficiency
- Active B12 < 25 pmol/L: Confirmed deficiency
- Active B12 25-70 pmol/L: Indeterminate—possible deficiency 1
Confirmatory Testing
When initial B12 results fall in the indeterminate range, consider measuring methylmalonic acid (MMA) as a confirmatory test 1. MMA reflects the functional status of vitamin B12 and provides more reliable diagnosis, though it is expensive (£11-80) and requires specialist equipment 1. Alternatively, homocysteine >20 µmol/L may suggest B12 deficiency in folate-replete patients 2.
Who to Test
Test patients with:
- Neurological symptoms (peripheral neuropathy, balance issues, cognitive impairment, vision problems)
- Hematological abnormalities (macrocytic anemia, unexplained fatigue)
- Risk factors: vegan/vegetarian diet, malabsorption conditions (celiac disease, atrophic gastritis), medications (metformin, PPIs, H2 blockers), autoimmune conditions 1
Vitamin D Testing
Screening Recommendations
Do not routinely screen healthy adults for 25(OH)D levels 3. The 2024 Endocrine Society guideline explicitly recommends against routine screening because outcome-specific benefits of particular 25(OH)D levels have not been established in clinical trials 3.
When to Test
Reserve 25(OH)D testing for patients with:
- Established indications (hypocalcemia, osteoporosis, malabsorption)
- Specific clinical conditions requiring monitoring
- Not for general population screening 3
Empiric Supplementation Strategy
Instead of testing, the guideline recommends empiric vitamin D supplementation (intake exceeding Dietary Reference Intakes without testing) for:
- Children and adolescents (ages 1-18 years)
- Adults ≥75 years
- Pregnant women
- Individuals with prediabetes 3, 4
Other Vitamins
Vitamin A
Serum or plasma retinol is the most commonly used marker, though it has low sensitivity (0-54%) when compared to reference standards 5. Retinol-binding protein (RBP) is an alternative with moderate specificity (76-98%) but similarly low sensitivity 5. Testing is primarily indicated in populations at high risk for deficiency rather than routine screening.
Vitamin K
PIVKA-II (Protein Induced by Vitamin K Absence) is the preferred functional marker (reference range 17.4-50.9 mAU/mL) 6. Phylloquinone (K1) in serum can be measured (deficiency <0.15 µg/L), but PIVKA-II provides better insight into cellular utilization 6. Primary indications include suspected vitamin K deficiency bleeding in newborns and monitoring anticoagulation.
Folate
Red blood cell folate is preferred over serum folate as it reflects tissue stores 7. However, folate deficiency is uncommon in many populations due to food fortification. In the reviewed data, only 2.4% of folate tests showed deficiency 8, suggesting significant overordering.
Critical Pitfalls to Avoid
- Avoid indiscriminate testing: Studies show 75% of vitamin B12 tests return normal results, indicating widespread inappropriate ordering 8
- Don't use PT/INR as a vitamin K status indicator: Coagulation assays lack sensitivity and specificity for this purpose 6
- Don't order folate routinely with B12: Only 2.4% yield positive results 8
- Recognize test limitations: Single markers cannot achieve perfect sensitivity or specificity 2, 7
- Consider functional markers: When abundance markers (serum levels) are indeterminate, functional markers (MMA, homocysteine, PIVKA-II) provide better diagnostic clarity 1, 6, 2