Nutrient Deficiency Blood Tests: Vitamin D, B12, Iron, Folate, Magnesium

A vitamin deficiency test is a blood test that measures one or more specific nutrients to find out whether your levels are too low. The five that account for most real, treatable deficiencies in US adults are vitamin D, vitamin B12, iron, folate, and magnesium. You rarely need all of them. Which ones are worth running depends on your symptoms, your diet, your medications, and your medical history, not on a package price.

Key takeaways

• Iron deficiency is the one most often missed. Ferritin below 30 ng/mL means depleted iron stores even when your hemoglobin and full blood count look completely normal.
• Vitamin D thresholds are more contested than most articles admit. The National Academy of Medicine treats below 12 ng/mL as deficiency, not below 20.
• The US Preventive Services Task Force says there is insufficient evidence to screen asymptomatic adults for vitamin D deficiency at all.
• Serum magnesium measures roughly 1% of your body's magnesium, so a normal result does not rule out depletion.
• Folate deficiency became rare in the US after mandatory grain fortification in 1998, which is why it is usually tested alongside B12 rather than on its own.

What a vitamin deficiency test actually covers

There is no single blood test that checks every nutrient. What exists is a set of individual tests, each measuring one thing, which a clinician selects based on what they are actually looking for.

That matters because of how these are marketed. Direct-to-consumer panels often bundle twenty or thirty analytes together and present the result as a complete picture of your nutritional status. In practice, a handful of those markers carry nearly all the useful information, several are unreliable in the way they are measured, and a few are essentially never acted on.

The five covered here are the ones where a low result genuinely changes what happens next. Each has a specific marker worth knowing:

A good blood test for vitamin deficiency is a short, targeted list built around your situation. Testing.com covers how individual nutrient markers are used and interpreted.

Who should be tested, and who should not

This is where most people go wrong, in both directions. Some test far more than they need to. Others have a textbook risk factor and never get checked.

Strong reasons to run a test for vitamin deficiency include heavy or prolonged menstrual bleeding, a vegan or strictly vegetarian diet maintained without reliable fortified foods, gastric bypass or other bariatric surgery, celiac disease or inflammatory bowel disease, chronic kidney disease, long-term use of metformin or proton pump inhibitors, heavy alcohol use, pregnancy or planning one, being over 65, and unexplained anemia or neurological symptoms like numbness and tingling.

Weaker reasons include general tiredness with no other findings, wanting a baseline out of curiosity, or following an influencer recommendation. Tiredness is worth investigating, but a scattergun panel is usually the least efficient way to do it.

The distinction matters financially as well as clinically. A vitamin deficiency test ordered because you have a documented reason is usually covered by insurance as part of a diagnostic workup. The same test bought as part of a wellness bundle rarely is, and the result lands without anyone to interpret it against your history.

The vitamin D situation deserves particular attention because the official position surprises people. The US Preventive Services Task Force reviewed the evidence and concluded that it is insufficient to assess the balance of benefits and harms of screening asymptomatic adults for vitamin D deficiency. It also found that treating identified deficiency did not reduce cancer, type 2 diabetes, or death in community-dwelling adults, nor fractures in people who were not at high fracture risk. The American Academy of Family Physicians, summarizing this in its review of vitamin D screening, recommends against screening the general population.

That is a long way from how vitamin D testing is usually promoted. It does not mean the test is useless. It means the case for testing people with risk factors or symptoms is much stronger than the case for testing everyone.

Vitamin D

The correct test is 25-hydroxyvitamin D. There is another form, 1,25-dihydroxyvitamin D, which is sometimes ordered by mistake and does not reflect your vitamin D stores.

Here is where the published thresholds genuinely disagree, and knowing this will save you a lot of confusion.

The National Academy of Medicine, formerly the Institute of Medicine, treats levels below 12 ng/mL as usually indicating deficiency, considers 12 to 20 ng/mL the normal range for adequate exposure, and holds that above 20 ng/mL, 97.5% of people have adequate intake. The Endocrine Society, in its 2011 guidance, classifies 21 to 29 ng/mL as insufficiency, which implies a target above 30.

Those two positions are not compatible, and the difference is not trivial. Under one, a result of 24 ng/mL is fine. Under the other, it is insufficient and warrants supplementation. The AAFP review notes plainly that these values are not universally accepted.

What to do with that as a reader: a result under 12 is a real deficiency that should be addressed. A result between 12 and 20 is low-ish and worth discussing, particularly if you have bone health concerns or limited sun exposure. A result in the twenties sits in genuinely contested territory, and whether it gets treated depends on your clinician's school of thought and your individual risk, not on a settled fact.

Risk factors that make testing more worthwhile include darker skin, limited sun exposure, living at northern latitudes, obesity, malabsorption conditions, chronic kidney or liver disease, and taking medications that affect vitamin D metabolism. The NIH Office of Dietary Supplements covers intake recommendations, and MedlinePlus describes the test itself.

Vitamin B12

B12 is the deficiency with the highest stakes, because untreated it causes nerve damage that can become permanent. It is also the one where the standard test is least satisfying.

Serum B12 is the usual first-line measurement, reported in pg/mL. Most labs set the lower limit somewhere around 200 pg/mL, with a borderline zone roughly from 200 to 300. The problem is that serum B12 measures total circulating B12, including a portion bound to a protein that does not deliver it to your cells. Some people with symptoms have a serum level that looks acceptable.

That is why two follow-up markers exist. Methylmalonic acid, or MMA, rises when cells lack B12, and the NIH Office of Dietary Supplements describes it as among the most sensitive markers of B12 status. Homocysteine also rises in B12 deficiency, though it rises in folate deficiency too, which makes it less specific. If your serum B12 is borderline and your symptoms fit, MMA is the test that resolves the question.

This is also why B12 is one of the few nutrients where it is worth pushing past a borderline result. A serum level of 240 pg/mL with numb feet and five years of metformin is not reassuring, and a test for vitamin deficiency that stops at the first number can miss a problem that becomes irreversible.

Who is actually at risk, per MedlinePlus: people with celiac or Crohn's disease, people with alcohol use disorder, people with malabsorption conditions, anyone eating little or no animal food, anyone who has had digestive surgery including gastric bypass, and people taking medications that interfere. Those medications include metformin, proton pump inhibitors and H2 blockers, colchicine, phenytoin, and some antibiotics.

The medication group is the one most often overlooked. Metformin is taken by millions of people with type 2 diabetes for decades at a time, and it reduces B12 absorption. If you have been on metformin for years and have tingling in your feet, B12 belongs on the list of things to check, not least because diabetic neuropathy and B12 neuropathy look alike.

Symptoms MedlinePlus associates with low B12 include fatigue, irregular heart rhythm, unintended weight loss, and tingling or numbness in the hands and feet, alongside more general signs like weakness, mood changes, memory problems, and hair thinning. The Merck Manual covers the clinical picture in depth.

Iron

If you read only one section, make it this one, because iron deficiency is the most commonly missed finding in this whole group.

Ferritin is the key marker. It reflects your stored iron. The Merck Manual's iron deficiency guidance gives a normal range of 30 to 300 ng/mL and states that a level below 30 ng/mL is specific for iron deficiency.

Now the part that matters. Iron deficiency develops in stages. In the first stage, your bone marrow iron stores are depleted and ferritin falls below 30, while your hemoglobin and serum iron remain completely normal. Anemia only appears in the second stage. The classic small, pale red cells only show up in the third.

This means you can be genuinely iron deficient, with fatigue, hair shedding, breathlessness on stairs, restless legs, and poor exercise tolerance, while your complete blood count is reported as entirely normal. If a doctor checks only a CBC and tells you your iron is fine, they have not actually measured your iron. This happens constantly, particularly to women with heavy periods.

The complication to know about. Ferritin is an acute phase reactant, meaning it rises during inflammation, infection, and some cancers. In someone with an inflammatory condition, a ferritin anywhere up to 100 ng/mL can still represent iron deficiency. So a ferritin of 45 in a healthy person is reassuring, while the same 45 in someone with rheumatoid arthritis or chronic infection is not. This is why transferrin saturation is often measured alongside it. Saturation below 15% suggests impaired production of red cells, and values below 10% are typical in established deficiency.

MedlinePlus covers the ferritin test and the wider iron panel separately, and Testing.com's ferritin page explains the interpretation issues well.

The American Society of Hematology covers the clinical picture, and the NIH iron fact sheet sets out intake requirements by age and sex.

One safety note that runs the other way. Do not take iron supplements without a confirmed deficiency. Iron overload is harmful, hereditary hemochromatosis is more common than people realize, and iron tablets are a leading cause of poisoning in young children.

Folate

Folate is included in most panels, and it is usually the least informative of the five in the US context.

Mandatory folic acid fortification of enriched grain products began in the United States in 1998. Since then, dietary folate deficiency has become uncommon in the general population. It still occurs with heavy alcohol use, malabsorption conditions, pregnancy where demand rises sharply, and with certain medications including methotrexate, some anticonvulsants, and sulfasalazine. The NIH folate fact sheet covers intake requirements across these groups.

Two measurements exist. Serum folate reflects recent intake and can swing with what you ate in the last few days. Red blood cell folate reflects status over the preceding few months and is the more stable measure, though it is ordered less often.

The reason folate is nearly always tested alongside B12 is not that both are commonly deficient. It is that the two deficiencies produce the same type of anemia, and more importantly, that folate can conceal a B12 problem. Supplementing folate corrects the anemia caused by B12 deficiency while doing nothing about the nerve damage, which continues to progress silently. That is the single strongest argument for not supplementing folate alone when B12 status is unknown, and it applies to high-dose B-complex products sold over the counter.

For anyone who is pregnant or planning a pregnancy, folate matters for a different reason, which is neural tube defect prevention, and that is handled with supplementation before conception rather than with testing.

Magnesium

Magnesium testing comes with the largest caveat of the five.

Serum magnesium, the standard measurement, captures only about 1% of the magnesium in your body. Roughly half sits in bone, and most of the rest is inside your cells. Your body defends the blood level tightly, pulling from bone and cells to keep it steady. The NIH states directly that serum levels do not accurately reflect total body magnesium or concentrations in specific tissues.

So a normal magnesium result tells you that your circulating level is not dangerously low. It does not confirm that your stores are full.

Typical adult ranges run around 1.7 to 2.2 mg/dL, with the NIH giving 0.75 to 0.95 mmol/L, and the Merck Manual defining hypomagnesemia as below 1.8 mg/dL. Deficiency is uncommon in the general population at roughly 2%, but far more common in specific groups: people taking loop or thiazide diuretics, people on long-term proton pump inhibitors, people with poorly controlled diabetes, people with chronic diarrhea or malabsorption, and people with heavy alcohol use.

Magnesium is also worth checking when potassium or calcium is low and will not correct with supplementation, because low magnesium blocks the correction of both.

What the big panels get wrong

Direct-to-consumer nutrient panels have become a large business, and a few things are worth knowing before you buy a comprehensive micronutrient test.

Testing more does not mean learning more. Running thirty analytes on an asymptomatic person produces a predictable result: by chance alone, one or two will fall outside their reference range. Those flagged values generate anxiety, follow-up appointments, and sometimes supplements, without any underlying problem.

Some of the markers are measured in ways that are not well validated. Intracellular or white-cell nutrient assays are marketed as superior to standard serum testing. Reference ranges for them are not standardized across laboratories, mainstream guidelines do not endorse them, and the same sample can produce different interpretations at different labs.

Results are rarely paired with a plan. A number on a dashboard with a red or green marker beside it is not clinical interpretation. Ferritin is the clearest example: without knowing whether you have an inflammatory condition, the number cannot be read properly at all.

None of this means a vitamin and mineral deficiency test is a bad idea. It means a targeted one ordered for a reason beats a comprehensive one ordered out of curiosity, almost every time. If you want to see what a focused panel typically includes, August's lab test directory sets out the common ones.

How to prepare, and what distorts your results

A few practical points cover most of what goes wrong.

Biotin is the biggest interference risk. High-dose biotin, common in hair, skin, and nail supplements, interferes with many laboratory immunoassays and can distort results including vitamin D and B12. Most labs advise stopping it for at least two days before testing. Mention it even if it is only part of a multivitamin.

Recent supplementation skews things. Taking a B12 supplement shortly before a serum B12 test can produce a normal-looking result that masks a tissue deficiency. The same applies to iron tablets and serum iron. If you are testing to find out whether you need supplements, ideally test before starting them, not after.

Recent illness raises ferritin. Because inflammation pushes ferritin up, testing during or shortly after an infection can hide iron deficiency entirely. Waiting a few weeks after an acute illness gives a cleaner picture.

Fasting is rarely required. Serum iron shows diurnal variation and is often drawn in the morning, and some panels ask you to fast if a lipid or glucose test is included. Ferritin, vitamin D, B12, and folate generally do not require fasting, but follow whatever instruction covers your full order.

Tell them everything you take. Multivitamins, protein powders, greens powders, and fortified drinks all contribute, and several contain far more than you would expect.

What to do once you have your results

Three principles cover most situations.

Treat the cause, not the number. A low ferritin in a woman with heavy periods and a low ferritin in a man of 60 are the same number with very different implications. Unexplained iron deficiency in an adult man or a postmenopausal woman warrants investigation of the gastrointestinal tract, because occult blood loss is a leading cause and it is the finding you least want to miss.

Retest before committing to a story. Single values move. Repeat testing after a reasonable interval, usually two to three months for most of these markers, tells you more than one snapshot.

Do not supplement blind. Iron without a confirmed deficiency risks overload. Folate without knowing your B12 status can mask progressing nerve damage. Magnesium in someone with reduced kidney function can accumulate dangerously. Fat-soluble vitamin D builds up in a way that water-soluble vitamins do not. This is the practical argument for running a vitamin deficiency test before buying supplements rather than after: once you are taking something, the result stops telling you what you wanted to know.

If a result needs discussing and you cannot get a timely appointment, August's telehealth services can connect you with a clinician, though the draw itself and any repeat testing have to happen at a lab.

If your results are normal and you still feel unwell, that is worth taking seriously rather than retesting repeatedly. Thyroid function, sleep apnea, depression, perimenopause, and chronic infections all produce the same fatigue picture and are more common than most of the deficiencies on this list.

To talk through symptoms before deciding what to test, you can describe them to August, a free AI health assistant, which can help you organize what you are experiencing ahead of a clinical conversation. It is a starting point, not a diagnosis, and interpreting results is a job for a clinician who can see your full history.