Nutrient Deficiency Test: What It Reveals

By Dr. Matt Gianforte, D.C. | Functional Medicine Clinician

A nutrient deficiency test measures a vitamin or mineral, or a marker of how the body is using it, in blood, red blood cells, or urine. The most useful tests are targeted: ferritin for iron stores, methylmalonic acid alongside B12, 25-hydroxyvitamin D, and folate, chosen because symptoms, diet, medications, or digestive problems point to a specific gap. A normal result narrows the question but does not always rule a deficiency out.

A nutrient deficiency test usually enters the picture after a familiar dead end. Someone feels tired, foggy, achy, slow to recover, and vaguely unlike themselves, yet the standard CBC and metabolic panel come back “normal.” That result matters, but it doesn't answer every question. Basic labs are designed to catch disease patterns, not to map whether iron stores are thin, whether vitamin B12 is being used properly, or whether inflammation is distorting the picture.

• A nutrient deficiency test works best as a targeted root-cause tool, not as a reflex screen for everyone.
• Routine screening isn't needed for many healthy adults. For vitamin D, the USPSTF found the evidence insufficient to recommend for or against screening adults without symptoms, partly because assays vary and there is no agreed cutoff (USPSTF, 2021), and the Endocrine Society's 2024 guideline suggests against routine 25(OH)D testing in the general population (Demay et al., 2024).
• No single method tells the whole story. Serum, RBC, urine, hair, and functional markers answer different questions.
• The strongest nutrient assessment usually relies on biochemical markers interpreted alongside inflammation status and symptoms, rather than one isolated number, because inflammation changes many nutrient biomarkers, including ferritin (INSPIRE review, J Nutr 2015).
• “Normal” doesn't always rule out deficiency, especially for B12 and iron, where marker choice and context can change the conclusion.

Why a Nutrient Deficiency Test Belongs in Root-Cause Care

A common scenario goes like this. A patient has months of fatigue, brain fog, dry skin, hair shedding, low mood, or slower recovery after workouts and stress. Their primary labs look acceptable. They're told everything is fine, yet their body says otherwise.

That disconnect is one reason functional and integrative clinicians look deeper. A nutrient deficiency test isn't a replacement for a broader workup. It sits beside other root-cause tools such as thyroid evaluation, stool testing, hormone assessment, and inflammatory markers. The decision to test depends on whether the symptoms, diet pattern, medication history, and clinical context point toward a plausible deficiency.

Standard labs often miss subclinical patterns

The field of micronutrient surveillance has moved far beyond waiting for late-stage deficiency signs. WHO set up its Vitamin and Mineral Nutrition Information System in 1991 to track micronutrient deficiencies using blood biomarkers rather than symptoms alone. Even WHO notes that some markers only work at the population level: serum retinol at or below 0.70 µmol/L is used to estimate vitamin A deficiency in populations, but it is not useful for judging one individual's status (WHO, 2011).

That shift matters in practice. Symptoms like fatigue, poor focus, brittle nails, or neuropathy are nonspecific. They can reflect poor sleep, thyroid dysfunction, chronic inflammation, gut dysfunction, low iron availability, or impaired B12 utilization. Conventional screening panels aren't built to sort that out.

Clinical reality: A normal CBC can coexist with low iron stores, and a normal serum B12 can coexist with functional B12 problems.

Root-cause care asks a different question

Instead of asking, “Is there obvious disease?” root-cause care asks, “What process is dragging function down?” That's why a patient who feels depleted may need a more structured look at nutrient status, especially if there's a history of restrictive eating, digestive symptoms, heavy menstrual losses, long-term acid suppression, metformin use, or repeated high stress.

For readers trying to understand how this systems-based model works, the article on functional medicine principles gives the broader framework. If you're still trying to connect symptoms to a specific nutrient, the guide to nutrient deficiency symptoms is a useful companion to this article. IV nutrient protocols are another option some people explore, but they don't replace proper testing.

The Five Main Ways to Test for Nutrient Deficiency

Not all nutrient tests measure the same thing. One method may tell you what's circulating today. Another may show what has been available to cells over time. Another may reveal whether the body can use the nutrient.

Blood serum testing

Serum testing measures nutrients in the fluid portion of blood. It's the most common entry point because it is widely available and useful for markers like 25-hydroxyvitamin D, serum iron studies, and total B12.

Its strength is speed and practicality. Its weakness is that serum often reflects recent intake, transport, and shifts caused by illness more than true tissue sufficiency.

RBC and intracellular testing

Red blood cell testing aims to assess nutrients inside cells rather than outside them. This approach is commonly used when clinicians want a longer-view marker for nutrients such as magnesium or folate.

RBC testing can be helpful, but it isn't automatically superior. It still has method limits, and not every nutrient has a well-standardized intracellular test.

Urine testing

Urine-based testing often looks at metabolites rather than the raw nutrient itself. This matters for methylmalonic acid, which helps assess B12 status, and for some organic acid patterns that point toward impaired nutrient-dependent pathways.

Hydration and kidney function can affect interpretation. Urine testing works best when it answers a focused question, not when it's used as a catch-all.

Hair mineral analysis and functional markers

Hair testing tries to capture longer-term mineral exposure and patterns over months. It can be interesting for trace minerals or environmental exposure, but external contamination and inconsistent interpretation limit how heavily it should be weighted.

Functional markers ask a more useful clinical question in many cases: not just “is the nutrient present,” but “is metabolism behaving as if the nutrient is enough?”

• Ferritin helps assess iron storage.
• Methylmalonic acid helps assess B12 function.
• Homocysteine gives added context for B12, folate, and B6-related metabolism.
• 25-hydroxyvitamin D is the standard marker of vitamin D status; the active hormone, 1,25-dihydroxyvitamin D, is not used to judge status.

Some commercial services package multiple methods together. Patients comparing options often look at specialty panel companies and direct-to-consumer tools before deciding whether to use a clinician-guided pathway. For a broader lab strategy, Hack Your Labs offers useful context on choosing the right panel instead of the biggest panel.

Comparing Blood, RBC, Urine, Hair, and Functional Markers

A patient comes in with fatigue, restless legs, and hair shedding. Another has numbness, brain fog, and a normal CBC. A third wants a broad nutrient panel because social media told them to. Those are three different testing decisions.

That distinction matters because routine nutrient screening is often not recommended for healthy adults without risk factors or symptoms. In practice, integrative and functional clinicians use targeted nutrient testing for a different reason. The goal is to explain persistent symptoms, medication effects, restricted diets, malabsorption, or recovery problems when standard labs do not fully answer the question.

What each method is actually good for

No single method wins across the board. The useful question is whether you need a snapshot of what is circulating, a better estimate of intracellular status, evidence of recent excretion, or proof that a pathway is struggling.

Nutrient Testing Methods Compared	Best Detects	Key Limitation	Best Use Case
Serum blood	What is circulating at the time of the draw	Can swing with recent intake, inflammation, and shifts in binding proteins	First-pass screening for iron studies, B12, folate, zinc, copper, vitamin D
RBC or intracellular	Longer-term cell-level exposure for select nutrients	Standardization is uneven, and not every RBC marker has strong outcome data	Magnesium, some B vitamin questions, cases where serum looks normal but symptoms persist
Urine	Excretion patterns and some functional output	Hydration, kidney function, and timing can change the result	Iodine, some organic acids, selected provocation or spot testing questions
Hair	Longer-term mineral exposure pattern	External contamination and lab interpretation vary widely	Supporting context for mineral trends or toxic element exposure, not a stand-alone diagnosis
Functional markers	Whether physiology looks impaired	Often needs companion labs to avoid false conclusions	B12, folate, B6, methylation, iron use, mitochondrial and amino acid metabolism

Serum testing is usually the starting point because it is accessible, standardized, and clinically familiar. It is also the method most likely to be overread. A normal serum value can reflect short-term intake rather than meaningful tissue sufficiency.

RBC testing can add value, but it is not automatically better. It is most useful when the clinical question is intracellular availability and when the marker has at least some practical validation. RBC magnesium is a common example. Some clinicians prefer it to serum magnesium in a symptomatic patient because serum magnesium is tightly regulated and may stay in range even when intake is poor, although RBC magnesium is not standardized across labs.

Urine testing is strongest when the nutrient is best understood through excretion. Iodine is the clearest example. If a patient is relying on skin-based self-checks, this review of the iodine patch test and why urinary iodine is more useful helps clarify the trade-off.

Hair testing is where the most confusion arises. It may offer clues about longer-term exposure, especially for minerals or environmental metals, but collection quality, shampoos, dyes, and laboratory algorithms can distort the picture. Hair results can support a hypothesis. They should not outweigh better clinical data.

Where interpretation usually breaks down

The main error is treating every marker as if it measures body stores directly.

Ferritin is a good example. It is one of the better nutrient-related markers available, but it is still an acute phase reactant. The WHO ferritin guideline uses a serum ferritin below 15 µg/L to indicate iron deficiency in apparently healthy people aged 5 and older, including adults. With infection or inflammation, values below 70 µg/L may still mean deficiency in adults. On the high side, ferritin above 150 µg/L in menstruating women, or above 200 µg/L in men and other women, may signal a risk of iron overload, and WHO says ferritin should not be used alone to make that call. Context changes interpretation.

B12 creates a different problem. Total serum B12 can look acceptable while tissue-level function is poor. In that setting, the pattern deserves more weight. Symptoms, methylmalonic acid, homocysteine, CBC indices, diet, metformin use, acid-suppressing drugs, and GI history often tell a more accurate story than one serum number.

Functional markers often answer the question patients care about. Is the body using the nutrient well enough to run the pathway? That is why methylmalonic acid can outperform serum B12 alone, and why homocysteine can add context for folate, B12, and B6 questions. These markers are not perfect either. Kidney function, inflammation, genetics, and medications can affect them.

At-home testing changes access, not physiology. It can be reasonable for selected markers if the collection method is reliable and the follow-up plan is clear. The trade-off is interpretation. A broad panel without a focused question often creates more noise than clarity.

Key Nutrients and the Markers That Actually Matter

A common scenario in practice looks like this. Someone has fatigue, poor recovery, hair shedding, or numbness, and they have already been told their basic labs are normal. The next step is not to order every nutrient marker available. The right move is to choose the marker that answers the clinical question being asked: Are stores low, is intake poor, or is the body failing to use the nutrient well?

Iron, vitamin D, and B12

For iron, ferritin usually carries the most weight because it reflects iron stores earlier than hemoglobin changes show up. It still needs to be read in context with a CBC, symptoms, and inflammatory burden, because ferritin can rise during inflammation and hide a true deficiency picture. A low-normal ferritin in a patient with hair loss, restless legs, heavy periods, or endurance training deserves more attention than a single “within range” flag suggests.

Vitamin D is different. The marker to order is 25-hydroxyvitamin D, but the harder question is whether testing is useful in that person at all. Healthy adults without risk factors often do not need routine screening. In symptom workups or higher-risk cases, the result can guide dosing and follow-up. Home collection kits appeal to many patients, but convenience does not fix assay variation or interpretation errors. When a low result does call for repletion, the guide to choosing a vitamin D3 and K2 supplement explains the practical side, and a maintenance option such as D3 2000 softgels should still be dosed against follow-up testing.

B12 is where many standard workups fall short. Serum B12 can miss functional deficiency, so functional medicine practitioners look at methylmalonic acid, homocysteine, CBC indices, diet pattern, medications, and GI history together. The NIH Office of Dietary Supplements notes that methylmalonic acid above 0.271 µmol/L suggests deficiency, and homocysteine above 15 µmol/L can support concern, though homocysteine is less specific (NIH Office of Dietary Supplements B12 fact sheet). MMA can also run high with reduced kidney function and in older adults.

One nuance matters here. NICE's 2024 guideline recommends total B12 or active B12 as the first test in most people. The exceptions are pregnancy, where it uses active B12, and suspected recreational nitrous oxide use, where homocysteine or MMA comes first. MMA and homocysteine are read against the lab's reference ranges, and a high homocysteine should be weighed against other causes such as low folate (NICE NG239).

A 2016 study of 316 cancer patients showed how much the cutoff matters. Using common thresholds (serum B12 below 300 pg/mL, MMA above 260 nmol/L, homocysteine above 12 µmol/L), the share of patients classed as deficient ranged from 8.9% to 17.4%, and an MMA cutoff of 413.5 nmol/L gave 86% sensitivity and 99% specificity (Vashi et al., 2016).

Zinc, copper, folate, and magnesium

Folate, magnesium, zinc, and copper are easy to test badly.

RBC folate often gives a better longer-view picture than serum folate, especially if recent diet or supplementation could distort a serum result, although British haematology guidelines treat serum folate as an equally good first-line test. It also should not be read alone. A folate result without B12 context can create false confidence, especially if neurologic symptoms are present.

Magnesium has a similar trade-off. Serum magnesium is widely available and useful for severe deficiency, but it may stay normal even when intake is poor or symptoms suggest a problem. Some clinicians use RBC magnesium for that reason, while recognizing that it is not a perfect gold standard and can vary by lab method.

Zinc and copper usually make more sense as a pattern than as isolated numbers. High-dose zinc can drive copper lower over time. Inflammation, low protein intake, GI dysfunction, and hormone status can also change the picture. If this is part of the clinical question, it helps to understand the copper zinc ratio and how it changes interpretation.

Nutrient	Marker that usually matters most	Why it matters	Practical interpretation point
Iron	Ferritin, with CBC and inflammation context	Best routine marker for iron stores	Low or low-normal ferritin can matter before anemia appears
Vitamin D	25-hydroxyvitamin D	Best standard marker for vitamin D status	Useful when the result will change dosing or follow-up
Vitamin B12	MMA, interpreted with B12 and CBC context	Picks up functional deficiency better than serum B12 alone	MMA above 0.271 µmol/L suggests deficiency
Homocysteine	Homocysteine	Adds context for B12, folate, and B6 questions	Above 15 µmol/L can support concern, but it is not specific
Folate	RBC folate	Better reflects longer-term status than serum folate	Best read alongside B12 status
Magnesium	Serum magnesium or RBC magnesium, depending the question	Serum is accessible. RBC may add context in selected cases	Method and lab variation matter
Zinc and copper	Serum or plasma zinc, copper, and ceruloplasmin together	The relationship is often more useful than one isolated value	Interpret with diet, supplements, and inflammation in mind

The test that matters most is the one that can change a decision. Sometimes that means ordering one focused marker. Sometimes it means not testing yet, correcting the obvious risk first, and retesting only if the clinical picture stays unresolved.

Who Should Test and How to Prepare for Accurate Results

A common scenario in practice is the patient whose CBC and metabolic panel are "normal," but fatigue, hair shedding, muscle cramps, poor recovery, or tingling have not improved. That is the point to decide whether testing will change care, not to order every nutrient marker on reflex.

For healthy adults without symptoms or clear risk factors, routine nutrient screening is often not recommended. As noted earlier, major guidelines do not support universal vitamin D screening in otherwise healthy adults. Integrative and functional medicine clinicians still use targeted nutrient panels often, but usually for a different reason. The goal is to investigate a persistent pattern, sort out which markers are worth checking, and avoid testing that adds cost without changing the plan.

Who is most likely to benefit

Testing is more useful in people with a real pretest reason to look. Functional medicine practitioners usually start with a careful history (diet, digestion, medications, menstrual losses, and training load) and then order the one or two markers most likely to change the plan, rather than a broad panel.

• Persistent symptoms: ongoing fatigue, brain fog, low mood, neuropathy, restless legs, muscle cramps, brittle nails, hair loss, slow exercise recovery
• Dietary restriction: vegan or very low animal-food intake, low-protein diets, highly limited food variety, long-term calorie restriction
• Absorption concerns: chronic diarrhea, bloating with suspected celiac disease, inflammatory bowel disease, prior bariatric or intestinal surgery, long-term acid suppression
• Medication exposure: metformin, proton pump inhibitors, some anticonvulsants, oral contraceptives, and other long-term drugs that can alter absorption, storage, or metabolism
• Higher-demand states: pregnancy, heavy menstrual blood loss, endurance training, and recovery after illness

Symptoms alone do not tell you which nutrient is low. They do tell you when testing may be worth the effort.

How to prepare so the result is actually useful

The biggest testing mistake is ordering the right marker at the wrong time, under the wrong conditions, or without enough clinical context to interpret it.

• Match the test to the decision. If the question is iron status, ferritin with CBC and inflammation context is usually more useful than a broad "micronutrient panel." If the question is B12 function, MMA may matter more than serum B12 alone.
• List every supplement. High-dose biotin can distort some lab tests, including thyroid and vitamin D assays (NIH ODS). Iron, zinc, magnesium, and B12 supplements can also shift results enough to confuse interpretation if the lab draw is timed poorly.
• Keep repeat testing consistent. Use the same lab when possible, draw under similar conditions, and avoid comparing a nonfasting afternoon sample to a fasting morning sample if you want a clean trend.
• Wait out acute illness when possible. Infection, hard training, inflammatory flares, and poor sleep can temporarily change markers such as ferritin and make the result harder to use.
• Bring the full clinical picture. Menstrual history, GI symptoms, medications, alcohol intake, thyroid symptoms, and diet history often explain more than the isolated number.

At-home testing has a place. It can improve access and make an initial screen easier to get done. The trade-off is lower control over collection quality, less ability to add related markers once a result looks borderline, and less context for interpretation. At-home tests work best as a first pass in selected cases, followed by confirmation in a standard lab when symptoms are significant or the result sits in a gray zone.

Nutrient testing also should not substitute for a broader workup. Fatigue, hair loss, constipation, palpitations, and mood changes can overlap with thyroid disease, iron deficiency, sleep problems, and blood sugar issues. If the history points that way, pair nutrient evaluation with a complete thyroid panel instead of assuming one deficiency explains everything.

Why Normal Results Can Still Mean Deficiency

A common scenario in practice looks like this. A patient has fatigue, brain fog, tingling, hair shedding, or muscle cramps, gets a standard lab panel, and is told everything is normal. The symptoms are still real. The problem is often the testing strategy, not the fact that symptoms were imagined.

A normal result usually means the value fell inside that lab's reference range on that day, using that assay. It does not confirm that tissue levels are adequate, that the marker chosen was the right one, or that the nutrient is not contributing to the case.

B12 is the clearest example

There is no gold-standard test for B12 deficiency. British haematology guidelines say the clinical picture matters most, that serum B12 and active B12 both have a grey zone, and that firm cutoffs aren't possible because lab methods differ; if someone has strong clinical features but a “normal” result, treatment shouldn't be delayed (Devalia et al., 2014). NICE adds that B12 deficiency should not be ruled out just because anemia or enlarged red cells are absent.

In practice, this is why serum B12 alone is a weak stopping point, and why the article on vitamin B12 and fatigue focuses on symptoms as much as numbers. A patient with neuropathy, poor concentration, glossitis, or unexplained fatigue may still need methylmalonic acid, homocysteine, or holotranscobalamin, depending on the clinical picture and test access.

Reference ranges have limits

Many nutrient markers were built to detect clear deficiency, not to answer a more nuanced question: could this nutrient still be part of why this person feels poorly?

Labs also set ranges from population data. Population normal and functional sufficiency are not always the same thing. Research reviews note that cutoffs for many nutrient biomarkers are hard to interpret, are often derived only in adults, and depend on lab methods that are not yet fully harmonised (Lamers, 2019). That matters clinically. Ferritin can look acceptable in the setting of inflammation. Serum magnesium can sit in range while intracellular status is still low. Zinc and copper can shift with infection, stress, medications, and hormone use.

One more trade-off matters here. Serum tests are fast and widely available, but they are often the least informative for long-term intracellular status. RBC markers can better reflect longer exposure for some nutrients, but they are not available for everything and still need clinical context. Functional markers can clarify whether a nutrient-dependent pathway is struggling, yet they are less specific and can be influenced by more than one deficiency.

A normal result can mean “not clearly abnormal on this marker today.” It does not always mean “this nutrient is not part of the problem.”

This is why routine nutrient screening is not recommended for every healthy adult. It often adds cost without changing care. But in root-cause workups for persistent symptoms, restrictive diets, gut issues, medication use, heavy training, or poor recovery, the decision is often whether to test, when to test, and which marker is worth ordering first. That decision usually matters more than whether a basic serum value landed inside the lab range.

From Results to a Root-Cause Protocol

A patient comes in with fatigue, muscle tension, poor recovery, and a stack of “normal” labs. The next step is not ordering every nutrient panel available. The next step is deciding which abnormal pattern is actionable, which result may be misleading, and what would change care.

Start with the findings that can explain the case

The useful question is simple. Which result best matches the symptom pattern and has a plausible mechanism behind it?

A low ferritin in a person with hair shedding, breathlessness on exertion, and heavy menstrual loss deserves attention before a borderline serum magnesium result. Low B12 with neuropathy, brain fog, vegan intake, or metformin use deserves more weight than a mildly off value with no clinical fit. In practice, findings can be ranked by three things: symptom match, test quality, and how likely the result is to change the plan.

Then identify why the deficiency pattern developed in the first place. Intake is only one category. Absorption problems, inflammatory burden, medication effects, alcohol use, restrictive dieting, low digestive capacity, and increased demand from training, illness, or chronic stress are common reasons a patient does not hold onto nutrients well.

Build the plan in the right order

Correction works better when the foundation is addressed before the supplement stack gets bigger.

1. Fix the intake problem. Increase protein if intake is low. Add iron-rich, magnesium-rich, or folate-rich foods when the history supports it. Regular meals matter because erratic eating alone can keep symptoms going.
2. Remove barriers to absorption. Review acid-suppressing drugs, metformin, laxative use, GI symptoms, prior bowel surgery, celiac risk, and ongoing diarrhea or constipation. A patient cannot supplement around unresolved malabsorption forever.
3. Match the supplement to the pattern. Dose, form, and route matter. Oral B12, such as methylcobalamin tablets, is often enough. Sometimes it is not. Magnesium glycinate, citrate, and malate do not behave the same in practice, especially if loose stools are already part of the case; a gentler form such as magnesium glycinate powder is often easier on the gut.
4. Track response. Symptom change, tolerance, and repeat markers decide whether the plan is working.

That order matters because more product is not always better care.

Practitioner-guided programs can be helpful when they are tied to a clear clinical question and a follow-up plan. Supplements should be chosen to match the documented gap, not the longest label.

Retesting should answer a specific question

Retesting is useful when it helps answer one of three questions: Did the marker move, did the patient feel better, and does the response fit the working diagnosis?

Some corrections are straightforward. Others stall. Ferritin may barely rise because blood loss continues. B12 may improve on paper while neurological symptoms lag behind. Magnesium may help sleep and muscle tension even when the lab marker changes very little. Those are different problems, and they call for different next steps.

If a patient feels better and the marker improves, stay the course long enough to rebuild stores. If the marker improves without symptom change, reconsider the diagnosis. If neither improves, check adherence, form, dose, timing, gut function, inflammatory load, and whether the original test was the right one to follow.

The protocol should stay narrow enough to test a hypothesis. That is how results become treatment instead of more noise.

Frequently Asked Questions About Nutrient Deficiency Testing

Should everyone get a nutrient deficiency test

No. Routine screening is often not recommended for healthy adults without risk factors. A nutrient deficiency test is most useful when symptoms, diet, medication use, or absorption concerns create a clear clinical question.

What's the best nutrient deficiency test for fatigue

There isn't one universal best test. Fatigue may justify targeted assessment of iron status, B12 with functional follow-up, vitamin D in selected cases, thyroid markers, and inflammation, depending on the pattern.

Can a normal B12 blood test still miss deficiency

Yes. Serum B12 alone can miss functional deficiency, and no single test can rule B12 problems in or out in every setting. MMA and, in some cases, active B12 or homocysteine add useful context.

Are at-home nutrient tests accurate

Some are useful for convenience and baseline screening. They're less useful when the result is borderline, symptoms are complex, or a full interpretation needs inflammatory and clinical context.

What's the best marker for iron deficiency

Ferritin is usually the key marker, but it has to be interpreted with context because inflammation can push it upward. Iron studies and CBC findings help complete the picture.

Is serum or RBC testing better

Neither is universally better. Serum is often the practical first step, while RBC testing may help in selected cases where intracellular or longer-view status matters more.

How often should nutrient levels be rechecked

That depends on the nutrient, the severity of the issue, and the treatment plan. Retesting is most useful when the same marker is repeated under similar conditions and the result will change the next decision.

References

• US Preventive Services Task Force. Screening for Vitamin D Deficiency in Adults: Recommendation Statement. 2021. USPSTF
• Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024;109(8):1907-1947. PubMed
• Raiten DJ, Sakr Ashour FA, Ross AC, et al. Inflammation and Nutritional Science for Programs/Policies and Interpretation of Research Evidence (INSPIRE). J Nutr. 2015;145(5):1039S-1108S. PubMed
• World Health Organization. Vitamin and Mineral Nutrition Information System (VMNIS). WHO
• World Health Organization. Serum retinol concentrations for determining the prevalence of vitamin A deficiency in populations. 2011. WHO
• World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. 2020. WHO
• NIH Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. ODS
• National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). 2024. NICE
• Vashi P, Edwin P, Popiel B, Lammersfeld C, Gupta D. Methylmalonic Acid and Homocysteine as Indicators of Vitamin B-12 Deficiency in Cancer. PLoS One. 2016;11(1):e0147843. PMC
• Devalia V, Hamilton MS, Molloy AM. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol. 2014;166(4):496-513. PubMed
• Stevens GA, Beal T, Mbuya MNN, et al. Micronutrient deficiencies among preschool-aged children and women of reproductive age worldwide: a pooled analysis of individual-level data from population-representative surveys. Lancet Glob Health. 2022;10(11):e1590-e1599. PubMed
• Lamers Y. Approaches to improving micronutrient status assessment at the population level. Proc Nutr Soc. 2019;78(2):170-176. PubMed
• NIH Office of Dietary Supplements. Biotin: Fact Sheet for Health Professionals. ODS

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