Guide to Vitamin Deficiency Blood Tests

Vitamin blood tests at a glance

Vitamin D: the most common deficiency

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)

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.

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