Complete Guide to Vitamin Deficiency Blood Tests
A single blood test visit can screen for the most common nutritional deficiencies — vitamin D, B12, iron, folate, and more. This guide explains what to test, when to test, and what results actually mean in practice.
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Get My Score →The Most Common Nutritional Deficiencies in the US
Nutritional deficiency is far more common than most people realise — even in high-income countries where food is abundant. Poor food quality, restriction, malabsorption, medications, and lifestyle factors all contribute. The most prevalent deficiencies, based on NHANES data, are vitamin D (42% of US adults), iron (13% of women 20–49), vitamin B12 (6% under 60; 20% over 60), folate, magnesium, and zinc.
| Nutrient | Best Blood Test | Deficiency Threshold | Optimal Level |
|---|---|---|---|
| Vitamin D | 25-OH vitamin D | Below 20 ng/mL | 40–60 ng/mL |
| Vitamin B12 | Serum B12 + MMA if borderline | Below 200 pg/mL | 400–900 pg/mL |
| Iron | Ferritin (first); full iron panel if borderline | Ferritin below 12 ng/mL | Ferritin 50–150 ng/mL |
| Folate | Serum folate (or RBC folate for longer-term status) | Below 3 ng/mL | Above 10 ng/mL |
| Magnesium | Serum Mg (limited); RBC Mg (better) | Serum below 1.7 mg/dL | Serum 2.0–2.2 mg/dL |
| Zinc | Serum zinc (limited accuracy) | Below 60 µg/dL | 80–110 µg/dL |
| Iodine | Urine iodine (spot or 24-hr); TSH indirectly | Urine below 100 µg/L | 100–199 µg/L |
The Six Vitamins Most Commonly Deficient in Adults
Testing Vitamin D: 25-OH Is the Right Test
There are two vitamin D tests: 25-hydroxyvitamin D (25-OH D, also called calcidiol) and 1,25-dihydroxyvitamin D (calcitriol, the active form). 25-OH vitamin D is the correct screening test — it reflects total vitamin D status from sun exposure, diet, and supplements. Calcitriol (1,25-OH D) is tightly regulated and can be normal or even elevated when 25-OH D is severely depleted — ordering calcitriol instead of 25-OH vitamin D is a common testing error.
Why Ferritin Is the First Iron Test to Order
Ferritin is the iron storage protein — and the most sensitive early indicator of depleted iron stores. It falls before serum iron, before haemoglobin, and before the CBC shows any abnormality. Women are particularly vulnerable: ferritin below 30 ng/mL indicates iron depletion even with a normal CBC and haemoglobin. Many women experience fatigue, hair loss, and reduced exercise capacity with ferritin in the 15–30 ng/mL range despite never developing anaemia. Standard lab reference ranges for ferritin are set too low — the optimal level for symptom prevention is generally above 50 ng/mL.
Why Standard Labs Miss Many Deficiencies
Standard panels test for clinical deficiency (scurvy, rickets) — not subclinical insufficiency that erodes health quietly over years. Vitamin D above 20 ng/mL avoids rickets but does not optimise immunity, mood, or muscle function (optimal is 50–80 ng/mL). Ferritin above 12 ng/mL avoids severe anaemia but does not prevent fatigue symptoms (optimal is above 50 ng/mL). Always check against optimal, not just lab reference ranges.
When a Single Test Is Not Enough
Some deficiencies require confirmatory tests. B12 deficiency is best confirmed by methylmalonic acid (MMA) and homocysteine levels when serum B12 is in the 200–350 pg/mL grey zone — these functional markers rise with true deficiency even when serum B12 looks borderline. Magnesium deficiency is notoriously hard to confirm with serum magnesium alone — RBC magnesium better reflects intracellular stores. For iodine, a spot urine iodine-to-creatinine ratio is more practical than 24-hour urine for most clinical purposes.
Medical Disclaimer
This article is for educational purposes only and does not constitute medical advice. Reference ranges, supplement dosages, and nutritional information mentioned are general educational guidance from published research—not personalised recommendations. Do not use this content to self-diagnose or self-treat any condition. Always consult a qualified healthcare provider before making any changes to your health regimen, medications, or supplements.
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