Vitamin blood tests at a glance
| Vitamin | Test name | Normal range | Deficiency level |
|---|---|---|---|
| Vitamin D | 25-hydroxyvitamin D | 30–100 ng/mL | <20 ng/mL deficient; 20–29 ng/mL insufficient |
| Vitamin B12 | Serum B12 (cobalamin) | 200–900 pg/mL | <200 pg/mL (borderline 200–300) |
| Folate (B9) | Serum or RBC folate | Serum: 3.1–17.5 ng/mL; RBC: >140 ng/mL | Serum <3 ng/mL |
| Vitamin A | Serum retinol | 30–80 mcg/dL | <20 mcg/dL |
| Vitamin C | Plasma ascorbic acid | 0.6–2.0 mg/dL | <0.2 mg/dL (scurvy risk) |
| Vitamin E | Serum alpha-tocopherol | 5.5–17 mg/L | <5 mg/L |
| Zinc | Serum zinc (fasting AM) | 70–120 mcg/dL | <60 mcg/dL |
| Iron / ferritin | Serum ferritin | 20–200 ng/mL | <12 ng/mL definite; <30 ng/mL functional |
Vitamin D — the most common deficiency
Why Vitamin D deficiency is widespread
Vitamin D deficiency affects an estimated 1 billion people worldwide. The main source is sunlight on skin — not food. People at highest risk: those who spend little time outdoors, people with darker skin (melanin reduces Vitamin D synthesis), those living at high latitudes, elderly people, obese individuals (Vitamin D stored in fat), and people who cover their skin for cultural or religious reasons. Symptoms of deficiency include bone pain, muscle weakness, fatigue, and frequent infections. Severe deficiency causes rickets in children and osteomalacia in adults.
Vitamin B12 — a silent deficiency
B12 deficiency develops slowly — the liver stores 2–5 years of B12. By the time symptoms appear (fatigue, tingling, memory problems), deficiency may be severe. At greatest risk: strict vegans and vegetarians (B12 only in animal foods), people over 60 with atrophic gastritis, those on metformin (impairs B12 absorption), and people who have had gastric surgery. A normal serum B12 doesn't completely exclude functional deficiency — methylmalonic acid (MMA) and homocysteine are more sensitive markers of functional B12 deficiency.
Who should be routinely screened for vitamin deficiencies?
- Vitamin D: everyone — but especially elderly, indoor workers, people at high latitudes
- B12: vegans, vegetarians, over-60s, people on metformin or long-term PPIs
- Folate: women planning pregnancy (to prevent neural tube defects), people on methotrexate
- Iron/ferritin: women with heavy periods, pregnant women, frequent blood donors
- Vitamin A: people with fat malabsorption conditions (Crohn's, coeliac, cystic fibrosis)