Which Vitamins Can Be Checked
All major vitamins have laboratory assays available for clinical testing, including fat-soluble vitamins (A, D, E, K) and water-soluble vitamins (B-complex including B1/thiamine, B12, folate, and C), though the clinical utility and availability of specific assays varies considerably. 1
Routinely Available and Clinically Useful Vitamin Tests
Water-Soluble Vitamins
Vitamin B12 (cobalamin): Serum B12 levels are widely available and should be checked at regular intervals (3,6, and 12 months in first year, then annually) in at-risk populations. 1 Always check B12 levels before treating folate deficiency, as folate supplementation can mask B12 deficiency while allowing irreversible neurological damage to progress. 2, 3
Folate (Vitamin B9): Both serum folate and red blood cell folate can be measured, with regular monitoring recommended at 3,6, and 12 months initially, then annually. 1 Red blood cell folate better reflects tissue stores than serum folate. 4, 5
Thiamine (Vitamin B1): Can be measured, though if clinical suspicion is high (rapid weight loss, poor dietary intake, vomiting, alcohol abuse, edema, neuropathy symptoms), treatment should be initiated immediately without waiting for blood results. 1
Fat-Soluble Vitamins
Vitamin D: Serum 25-hydroxyvitamin D (25-OHD) is the standard test, with levels ≥75 nmol/L considered sufficient. 1 Ensure total 25-hydroxyvitamin D (D3 and D2) is measured if patient is on vitamin D2 supplements like ergocalciferol. 1 This is one of the most commonly checked vitamins in clinical practice. 1
Vitamin A (retinol): Serum vitamin A levels can be checked, particularly indicated for patients with steatorrhea, night blindness, protein malnutrition, or following malabsorptive procedures. 1 Monitoring every 3 months then annually is recommended after malabsorptive bariatric procedures. 1
Vitamin E (tocopherol): Serum vitamin E levels are available and should be checked at least annually following malabsorptive procedures, or if unexplained anemia or neuropathy develops. 1
Vitamin K: Vitamin K1 and PIVKA-II (protein induced by vitamin K absence) levels can be measured, particularly important following malabsorptive procedures with annual monitoring recommended. 1
Additional Functional Markers
Indirect Assessment of Vitamin Status
Homocysteine: Elevated plasma homocysteine reflects deficiency of either vitamin B12 or folate, serving as a functional marker of inadequate vitamin status. 2, 4, 5
Methylmalonic acid (MMA): Elevated MMA is specific for vitamin B12 deficiency and helps distinguish B12 from folate deficiency when homocysteine is elevated. 4, 5
Clinical Context for Testing
High-Risk Populations Requiring Regular Monitoring
Post-bariatric surgery patients: Comprehensive vitamin monitoring at 3,6, and 12 months in first year, then at least annually, including B12, folate, vitamin D, calcium, ferritin, and fat-soluble vitamins (A, E, K) especially after malabsorptive procedures. 1
Patients on home parenteral nutrition (HPN): Baseline and annual monitoring of vitamin A, D, C, B12, and folate, with 24-45% showing subnormal levels of vitamins A, D, or C. 1
Pregnant women post-bariatric surgery: Screening during each trimester should include ferritin, folate, vitamin B12, calcium, vitamin D, and vitamin A, with vitamins E and K monitored if clinically indicated. 1
Important Clinical Caveats
Testing Limitations
Laboratory availability varies: Good markers of overall status are available only for a limited number of vitamins, and few clinical laboratories are equipped to measure all trace elements and vitamins. 1
No single test is perfect: Each test has limitations in sensitivity, specificity, or predictive value, and interpretation can be challenging. 6, 4, 5
Vitamin C: While mentioned as commonly deficient (45% in HPN patients), specific testing recommendations are less standardized than for other vitamins. 1
Critical Testing Sequence
Never treat folate deficiency without first checking B12: This is the single most important pitfall to avoid, as folate will correct the anemia of B12 deficiency but allow neurological damage to progress unchecked. 2, 3
Morning collection preferred: For iron studies and when making critical supplementation decisions, morning blood draw (ideally before 10:00 AM) is recommended, avoiding iron-containing supplements/foods for 24 hours. 3