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Guide to Vitamin Deficiency Blood Tests

Vitamin deficiencies are common and often missed. Blood tests can identify them before serious symptoms develop. This guide covers every major vitamin test.

Written by Suman Konda, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: · How we check our content

Quick answer

Vitamin D and B12 are the deficiencies most worth testing for: D deficiency is extremely common through low sun exposure, while B12 deficiency builds silently for years because the liver stores it, hitting vegans, older adults and metformin users hardest. The takeaway: a low-normal B12 can still mean functional deficiency, and treatment should follow the test pattern, not the calendar.

Vitamin blood tests at a glance

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VitaminTest nameNormal rangeDeficiency level
Vitamin D25-hydroxyvitamin D30–100 ng/mL<20 ng/mL deficient; 20–29 ng/mL insufficient
Vitamin B12Serum B12 (cobalamin)200–900 pg/mL<200 pg/mL (borderline 200–300)
Folate (B9)Serum or RBC folateSerum: 3.1–17.5 ng/mL; RBC: >140 ng/mLSerum <3 ng/mL
Vitamin ASerum retinol30–80 mcg/dL<20 mcg/dL
Vitamin CPlasma ascorbic acid0.6–2.0 mg/dL<0.2 mg/dL (scurvy risk)
Vitamin ESerum alpha-tocopherol5.5–17 mg/L<5 mg/L
ZincSerum zinc (fasting AM)70–120 mcg/dL<60 mcg/dL
Iron / ferritinSerum ferritinMen: 24–336 ng/mL; Women: 11–307 ng/mL<12 ng/mL definite; <30 ng/mL functional
Key points
  • Vitamin D deficiency is extremely common; sunlight is the main source, not food.
  • B12 deficiency builds over years from liver stores; vegans, older adults and metformin users are at risk.
  • Low-normal B12 (200 to 300 pg/mL) can still mean functional deficiency with symptoms.
  • Folate matters most around conception and early pregnancy.

Vitamin D: the most common deficiency

Why Vitamin D deficiency is widespread

Vitamin D deficiency affects an estimated 1 billion people worldwide.1 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: those at risk of deficiency, particularly elderly people, indoor workers, and people at high latitudes; guidelines do not recommend routine screening in people not at risk3
  • 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)

Why serum B12 can miss functional deficiency

A serum B12 in the low-normal range (200–300 pg/mL) is one of the most clinically frustrating results in laboratory medicine. Symptoms of deficiency, fatigue, tingling in the extremities, memory problems, megaloblastic anaemia, can be present with a "normal" serum B12, because the test measures total cobalamin in the blood, not the fraction that is actually delivered into cells and available for enzymatic reactions.

When serum B12 is borderline, two additional tests provide functional information. Methylmalonic acid (MMA) accumulates when B12 is functionally deficient, because the enzyme that converts methylmalonyl-CoA to succinyl-CoA requires B12 as a cofactor; a raised MMA therefore indicates tissue-level deficiency regardless of the serum B12 value. Homocysteine rises when both B12 and folate are low, making it less specific but sensitive to either deficiency.2 In practice, elevated MMA with a normal-range serum B12 in a symptomatic patient is sufficient evidence to trial B12 supplementation.

This distinction matters most in three groups: older adults with atrophic gastritis (who absorb B12 poorly even from supplements and may need intramuscular injections), people on long-term metformin (which impairs active B12 absorption in the terminal ileum), and strict vegans who started supplementing late.

Folate and pregnancy: why the timing matters

Folate deficiency during early pregnancy causes neural tube defects (NTDs), spina bifida and anencephaly, because the neural tube closes in the first 28 days of gestation, before most women know they are pregnant. Supplementation must therefore begin before conception, not after a positive pregnancy test. Most national guidelines recommend 400 micrograms of folic acid daily for all women planning a pregnancy and continuing through the first trimester; the dose rises to 5 mg daily for women who have had a previous pregnancy affected by an NTD or who take anticonvulsants that interfere with folate metabolism.

RBC folate reflects the average folate status over the past 2–3 months (the life of a red cell) and is less affected by a recent meal than serum folate, which falls within hours of poor dietary intake. For assessing longer-term folate status, particularly in pregnancy planning, RBC folate is the more reliable of the two tests.

Your next steps for vitamin deficiency testing

  1. Start with who, not what

    Routine screening isn’t recommended for everyone. Test vitamin D if you’re at risk (elderly, indoor, high latitude, darker skin), B12 if you’re vegan, over 60, or on metformin or long-term PPIs, folate when pregnancy is planned.

  2. Don’t dismiss low-normal B12 with symptoms

    Serum B12 of 200–300 pg/mL with fatigue, tingling or memory problems deserves a proper workup — methylmalonic acid and homocysteine reveal functional deficiency that serum B12 misses.

  3. Time folate before conception

    The neural tube closes in the first 28 days of gestation, before most women know they’re pregnant. Folate supplementation must begin before conception; RBC folate (not serum) reflects status over the past 2–3 months.

  4. Read ferritin with an inflammation marker

    Ferritin is also an acute-phase reactant: infection or inflammation raises it independently of iron stores. A CRP alongside tells you whether the ferritin reflects iron status or inflammation.

  5. Retest to confirm correction

    After treatment, retesting confirms the deficiency is actually corrected rather than assumed — particularly for B12, where absorption problems can make oral correction fail silently.

Vitamin blood tests at a glance

Blood tests can identify deficiencies before serious symptoms develop. This table compiles the standard tests; the article explains who should be screened and the B12 and folate nuances.

VitaminTestNormal rangeDeficiency level
Vitamin D25-hydroxyvitamin D30–100 ng/mL<20 ng/mL deficient; 20–29 insufficient
Vitamin B12Serum B12200–900 pg/mL<200 pg/mL (borderline 200–300)
Folate (B9)Serum or RBC folateSerum 3.1–17.5; RBC >140 ng/mLSerum <3 ng/mL
Vitamin ASerum retinol30–80 mcg/dL<20 mcg/dL
Vitamin CPlasma ascorbic acid0.6–2.0 mg/dL<0.2 mg/dL (scurvy risk)
Vitamin ESerum alpha-tocopherol5.5–17 mg/L<5 mg/L
ZincSerum zinc (fasting AM)70–120 mcg/dL<60 mcg/dL
Iron / ferritinSerum ferritinMen 24–336; women 11–307 ng/mL<12 definite; <30 functional

Practical notes

Vitamin D and B12 tests don't need fasting at most laboratories, but confirm when you book since they're often bundled with tests that do. More important than fasting is the supplement question: tell the lab everything you take, including multivitamins and occasional high-dose products. Supplements raise blood levels — that is expected — and hiding them only confuses the interpretation.

Ask which form was measured and in what units. Vitamin D should be the 25-hydroxy form; B12 is usually total serum B12. Units differ between labs — vitamin D may be reported in ng/mL or nmol/L, and the numbers look very different — so always read the value together with its unit and the range on your own report.

Read borderline results with their known quirks in mind. Vitamin D varies with season and sun exposure, so a winter reading naturally runs lower than a summer one. A low-normal B12 can coexist with real functional deficiency, which is why your doctor may order follow-up markers or simply treat based on symptoms — ask which approach they're taking and why.

Don't retest too soon after starting correction. Blood levels take weeks to months to reflect supplementation — typically around 2–3 months for vitamin D — so an early recheck mostly measures impatience. Ask your doctor when a follow-up test would actually be informative, and keep taking what was advised in the meantime.

In India

A 25-hydroxy vitamin D test at a large Indian laboratory typically costs around ₹1,000–₹2,500 and a vitamin B12 test around ₹800–₹1,800 — though prices vary by city and lab — and both are commonly bundled into preventive health packages.

Most large labs are NABL-accredited and offer home sample collection in major cities. The reference range and units printed on your own report are the ones that count, since both differ between laboratories.

Frequently Asked Questions

Why is vitamin D the most common deficiency?

Vitamin D deficiency affects an estimated 1 billion people worldwide, and the main source is sunlight on skin — not food. Highest risk: people who spend little time outdoors, people with darker skin (melanin reduces synthesis), those at high latitudes, and the elderly. Deficiency raises bone fracture risk and is worth testing for in at-risk groups.

Why is vitamin B12 called a silent deficiency?

The liver stores 2–5 years of B12, so deficiency develops slowly and symptoms — fatigue, tingling, memory problems — appear only when it is already severe. Greatest risk: strict vegans and vegetarians (B12 comes only from animal foods), people over 60 with atrophic gastritis, and those on long-term metformin or PPIs.

Why can serum B12 miss functional deficiency?

A serum B12 in the low-normal range (200–300 pg/mL) can coexist with real deficiency: symptoms, megaloblastic anaemia and neurological signs can be present with a "normal" result. Methylmalonic acid (MMA) accumulates when B12 is functionally deficient, so a raised MMA reveals what serum B12 misses. Symptoms with low-normal B12 deserve this workup, not reassurance.

Why does folate timing matter in pregnancy?

Folate deficiency during early pregnancy causes neural tube defects because the neural tube closes in the first 28 days of gestation — before most women know they are pregnant. Supplementation must therefore begin before conception. RBC folate reflects average status over 2–3 months and is less affected by a recent meal than serum folate.

Do I need to stop my multivitamin before a vitamin D or B12 test?
Don't stop anything without asking your doctor. The point of the test is often to see your level while on your usual routine — what matters is that the lab knows what you take, so the result is interpreted correctly. If your doctor wants a baseline off supplements, they'll tell you how long to pause.
My vitamin D report says ng/mL but my friend's says nmol/L — which is right?
Both are legitimate units used by different laboratories — they just use different scales (1 ng/mL equals 2.5 nmol/L). Never compare the raw numbers across units. Read your value only against the reference range printed on your own report, which will match its units.
My B12 came back 'low-normal' — does that mean I need treatment?
Not automatically. Symptoms, diet history and how long the level has been borderline all factor into the decision, and some doctors order follow-up markers before deciding. A low-normal result with no symptoms may simply be watched. Discuss what your doctor is weighing rather than treating the number alone.
How long after starting supplements should my levels be rechecked?
Usually around 2–3 months for vitamin D, since blood levels respond slowly — retesting after a week or two shows almost nothing. Your doctor will set the interval; ask what it is at the appointment where supplements are advised so you don't have to guess.

References

Sources cited on this page. PubMed links open the original abstract.

  1. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266–281. PMID 17634462 · doi:10.1056/NEJMra070553
  2. Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160. PMID 23301732 · doi:10.1056/NEJMcp1113996
  3. Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. "Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline." J Clin Endocrinol Metab. 2011;96(7):1911–1930. PMID 21646368 · doi:10.1210/jc.2011-0385
Medical Disclaimer: For educational purposes only. Always consult a qualified healthcare professional for diagnosis and treatment.
Written and medically reviewed by Suman Konda, Clinical Pharmacist · Sources linked to PubMed · Not medical advice: see our disclaimer