Vitamin D monitoring: which method suits your risk group?

The same laboratory result may carry very different implications in a neonate, an older adult, a pregnant patient, or a population living at high latitude, because the biology of vitamin D status, the likelihood of deficiency, the assay’s limitations, and the clinical purpose of testing do not remain constant across groups.

There is also a policy problem hiding inside what appears to be a laboratory question. A population may face limited sunlight exposure, low dietary intake, darker skin pigmentation, institutional living, malabsorption, or other systemic barriers to adequate vitamin D status, yet routine blood screening for everyone may not be the most effective response. The 2024 Endocrine Society guidance moves away from universal 25-hydroxyvitamin D testing in healthy populations and toward empiric supplementation for selected groups, including children aged 1–18, pregnant individuals, adults aged 75 and older, and people with prediabetes.

That shift does not make measurement irrelevant. It makes the choice of measurement more consequential.

Start with the question, not the instrument

Vitamin D status assessment options are often discussed as though they were interchangeable laboratory services. They are not. Before selecting an assay, we need to define what we are trying to establish:

• whether an individual has a biochemical marker consistent with low vitamin D stores;
• whether a treatment or supplementation plan is changing those stores;
• whether a newborn result may be distorted by a metabolite that the assay cannot separate;
• whether a laboratory is producing results that can be compared across time, sites, or population groups;
• or whether a community has a broader nutritional-equity problem that cannot be solved through testing alone.

For most nutritional-status questions, the relevant marker is total serum 25-hydroxyvitamin D, written as 25(OH)D. It reflects circulating vitamin D metabolites formed after vitamin D is processed in the liver and is substantially more useful for assessing vitamin D reserves than the active hormone 1,25-dihydroxyvitamin D.

The practical comparison looks like this:

The distinction between clinical care and population surveillance is central. In clinical practice, a test is ordered for a person with a particular history, symptom pattern, treatment question, or risk profile. In public health, a measurement system must also support comparison: between regions, seasons, age groups, ethnic groups, or periods before and after a fortification intervention. A method that is acceptable for one purpose may be insufficient for the other.

The most accurate vitamin D result is not automatically the most useful public health decision; the value lies in matching the measurement to the population question.

The gold standard: why LC-MS/MS earns that position

Liquid chromatography-tandem mass spectrometry, or LC-MS/MS, is widely regarded as the gold standard for measuring total 25(OH)D. Its advantage is not simply that it is more sophisticated. The method separates the compounds before quantifying them, allowing the laboratory to distinguish 25(OH)D2 from 25(OH)D3 and to resolve interfering isomers such as 3-epi-25(OH)D3.

That separation matters because vitamin D measurement is not a single uncomplicated signal. The blood sample contains related metabolites, and the ability of an assay to tell them apart affects the reported concentration. For routine adult samples, this may not always produce a clinically decisive difference. In certain demographic groups, especially infants and neonates, it can be much more important.

LC-MS/MS methods can cover a linear analytical range of approximately 5 to 550 nmol/L for 25(OH)D2 and 25(OH)D3, with a limit of quantification around 10 nmol/L, or 2 ng/mL, in the methods described in the available technical evidence. Reported intra-assay and inter-assay coefficients of variation are generally below 6% to below 10%, depending on the method and validation conditions. Those figures do not mean every laboratory using mass spectrometry produces identical results, but they illustrate why the technique is valued when analytical specificity and reproducibility are priorities.

The practical strength of LC-MS/MS is greatest when the result must withstand scrutiny across several layers:

1. The patient or study group has unusual metabolite patterns. Neonates are the clearest example because 3-epi-25(OH)D3 can represent a much larger fraction of measured vitamin D metabolites than it does in adults.

2. The distinction between D2 and D3 matters. A method that reports only a combined value may not answer questions about the source of vitamin D or the response to a particular supplementation approach.

3. The result is being used for research or surveillance. Comparisons across laboratories require methods with documented performance and, ideally, alignment with recognized reference or standardization programs.

4. The concentration is near the lower analytical range. When values approach a method’s limit of quantification, uncertainty around the result becomes more important, particularly if a decision is being made from a single measurement.

5. The sample contains compounds likely to interfere with measurement. Separating the analytes and relevant isomers can reduce the risk that the laboratory reports a value that is analytically precise but biologically misleading.

Routine immunoassays should not be dismissed as unusable. They are widely available and can be appropriate when their performance has been established for the intended population and clinical purpose. The problem begins when convenience is treated as equivalence. A result from one platform may not be directly interchangeable with a result from another, particularly when laboratories differ in calibration, antibody behavior, metabolite cross-reactivity, and the way they handle 25(OH)D2 or 3-epi-25(OH)D3.

For longitudinal monitoring, the most defensible approach is often to use the same laboratory and, where possible, the same assay platform. A change in method can create an apparent biological change even when the person’s underlying vitamin D status has not materially shifted.

The neonatal challenge: when the reported number may be too high

Infant and neonatal testing requires a separate level of caution. In neonatal samples, the C3-epimer 3-epi-25(OH)D3 may be present at considerably higher relative concentrations than in adults, reaching up to 58% of total 25(OH)D3 in neonatal samples described in the evidence. In adults, the corresponding proportion is much lower, with one cited figure of approximately 3.3 ng/mL for the adult concentration context.

The reason this matters is straightforward: some routine immunoassays, and some LC-MS/MS methods that do not resolve the epimer, may count 3-epi-25(OH)D3 as ordinary 25(OH)D3. The resulting total can be overestimated. In a neonatal setting, where clinical interpretation may be based on a narrow range of concentrations and where physiology is changing rapidly, that is not a minor technical footnote.

A neonatal result therefore needs to be read alongside three questions:

• Does the assay distinguish 3-epi-25(OH)D3 from 25(OH)D3?
• Was the method validated specifically for neonatal or infant samples?
• Is the result being interpreted as part of a broader assessment rather than as an isolated number?

This is where a laboratory’s technical description becomes clinically relevant. The phrase LC-MS/MS alone is not enough. A method may use mass spectrometry but still fail to resolve the metabolites that matter in a particular age group. The critical issue is not the prestige of the instrument; it is the analyte separation and validation behind the reported value.

For population researchers, the neonatal epimer also complicates comparisons between age groups. If one survey uses an assay that includes the epimer and another uses a method that separates it, the apparent difference in vitamin D status may partly reflect analytical design rather than a true difference in exposure, intake, or physiology. That is why demographic monitoring must document the assay method, calibration approach, sample type, and metabolite resolution rather than publishing a concentration without its laboratory context.

Risk groups and the 2024 move away from universal screening

At-risk populations vitamin D tracking sits at the intersection of individual medicine and population health. We know that vitamin D status can be shaped by latitude, season, sunlight exposure patterns, skin pigmentation, age, mobility, institutional living, dietary intake, pregnancy, childhood growth, and conditions that affect absorption or metabolism. Yet the presence of a risk factor does not automatically mean that every person requires the same testing schedule.

The 2024 Endocrine Society guideline advises against routine 25(OH)D screening in healthy individuals. For several demographic groups, it endorses empiric supplementation without requiring baseline screening, including:

• children aged 1–18;
• pregnant individuals;
• adults aged 75 and older;
• individuals with prediabetes.

This recommendation is best understood as a response to the limits of universal testing. Screening millions of healthy people can consume laboratory capacity, expose communities to inconsistent results, and encourage decisions based on thresholds that may not have the same meaning across every demographic or clinical context. When the intervention is low-risk and the group has a recognized vulnerability, a policy may reasonably favor supplementation guidance over a blood test for every individual.

That does not mean older adults, pregnant patients, children, or people with prediabetes should never be tested. A clinician may still order 25(OH)D when there is a specific diagnostic question, an unusual clinical presentation, a condition affecting absorption or metabolism, or a need to monitor a treatment plan. The point is that routine testing should not be mistaken for the default entry point to care.

For public health teams, the difference between risk-based testing and universal screening has practical consequences. A community with limited access to nutritious foods may benefit more from food fortification, reliable supplementation programs, and primary-care integration than from a campaign offering blood tests without a pathway for follow-up. Testing can identify a problem, but it does not remove the systemic barriers that produced the risk.

This is particularly important when considering nutritional equity. A testing program that reaches people with regular healthcare access may produce detailed data on the least disadvantaged part of a population, while those facing transport barriers, cost pressures, language barriers, or fragmented care remain underrepresented. Grassroots implementation, trusted community organizations, and accessible supplementation or fortified-food strategies may therefore be more consequential than simply expanding the number of tests.

A blood test can describe unequal exposure; it cannot, by itself, repair the conditions that make that exposure unequal.

How demographic context changes interpretation

Vitamin D deficiency risk is not distributed randomly, but neither is it explained by one demographic variable. Latitude can reduce the opportunity for cutaneous vitamin D synthesis during parts of the year, while clothing, indoor work, air pollution, mobility limitations, and sun-avoidance behavior can modify exposure further. Ageing skin is less efficient at producing vitamin D, and older adults may spend less time outdoors. Dietary intake may become more important when sunlight exposure is limited, but access to vitamin D-containing foods is itself shaped by income, geography, cultural practices, and food environments.

Ethnic differences in measured 25(OH)D status also require careful interpretation. Skin pigmentation can influence cutaneous vitamin D synthesis, but serum concentration is not a complete measure of biological effect, and the relationship between total 25(OH)D, vitamin D binding protein, and health outcomes is not fully resolved across all racial and ethnic groups. The exact diagnostic threshold for bioavailable or free 25(OH)D remains uncertain across groups with different vitamin D binding protein gene polymorphisms, and universal cutoffs for optimal non-skeletal outcomes have not been established for every demographic subgroup.

In practical terms, population monitoring should avoid turning a group-level pattern into an individual diagnosis. A lower average concentration in a population can justify nutritional policy, further investigation, or targeted support. It does not mean that every member of that group has the same deficiency, requires the same dose, or should be evaluated through the same assay.

Why 1,25(OH)2D is usually the wrong answer

One of the most persistent errors in vitamin D monitoring is the assumption that the active hormone must be the best marker of nutritional reserves. 1,25-dihydroxyvitamin D, or 1,25(OH)2D, is biologically active, but that does not make it the appropriate routine test for vitamin D deficiency.

The hormone circulates at picomolar concentrations and has a short serum half-life of approximately four to six hours. More importantly, during vitamin D deficiency, secondary hyperparathyroidism can stimulate the processes that maintain or increase 1,25(OH)2D. The result may remain normal or even become elevated while the body’s vitamin D stores are inadequate.

This creates a dangerous interpretive trap: a normal active-hormone result can appear reassuring even though it does not answer the question being asked. If the clinical question is whether a person has sufficient vitamin D reserves, total 25(OH)D is generally the relevant marker. 1,25(OH)2D belongs to a more specialized evaluation of calcium and phosphate metabolism and should be ordered for a specific clinical reason rather than as a general nutritional screen.

The distinction can be summarized simply:

• 25(OH)D asks about circulating vitamin D stores.
• 1,25(OH)2D asks about active hormonal regulation.
• A normal 1,25(OH)2D result does not rule out vitamin D deficiency.

This is also a matter of resource stewardship. Specialized testing can be more expensive, less available, and harder to interpret without the surrounding clinical information. Ordering the wrong marker does not add nuance; it adds noise.

Laboratory standardization: the part patients never see

Even a well-chosen analyte and an advanced instrument cannot guarantee useful monitoring if laboratory performance is inconsistent. This is where standardization programs become important, especially for epidemiological research and food-fortification policy.

The CDC Vitamin D Standardization and Certification Program uses unmodified single-donor human serum panels to reduce matrix effects and evaluate laboratory assay accuracy and precision. The use of human serum matters because artificial materials may not behave like real patient samples during analysis. A laboratory method that performs well on a simplified control material may behave differently in the presence of the proteins, metabolites, and other components found in clinical specimens.

Standardization is not the same as making every result identical. It creates a framework for assessing whether a method is sufficiently aligned with reference measurements and whether its performance remains stable. For researchers comparing vitamin D deficiency prevalence across regions or seasons, this distinction is essential. An apparent change in prevalence may reflect genuine variation in sunlight exposure or dietary intake, but it may also arise from differences in calibration, platform, sample handling, or the laboratory’s ability to separate metabolites.

A credible vitamin D monitoring program should therefore record more than the final concentration. At minimum, the research or clinical system should be able to identify:

• the assay platform and laboratory;
• whether the method measures total 25(OH)D and distinguishes D2 from D3;
• whether 3-epi-25(OH)D3 is resolved, particularly in infants;
• the laboratory’s quality-control and standardization status;
• the analytical range and lower limit of quantification;
• the timing of sampling in relation to season, supplementation, and relevant interventions;
• the demographic and geographic characteristics needed to interpret the result.

These details become especially important when evaluating food fortification. If a population-level intervention changes dietary intake, researchers need a measurement approach that can detect real changes without confusing analytical variation for biological response. Fortification policy may also affect the relative contribution of vitamin D2 and D3, depending on the foods and compounds used, which makes metabolite-specific measurement more informative in some research settings.

Choosing the method by population and purpose

There is no single best method for every demographic group, but there is a defensible route through the options.

For a healthy population covered by current empiric supplementation recommendations, routine baseline screening may add little value. The priority is accessible implementation: clear guidance, reliable delivery, and monitoring of program reach, particularly where systemic barriers make clinic-based testing uneven.

For adults aged 75 and older, testing should be guided by the clinical question rather than age alone. Reduced sunlight exposure, dietary limitations, mobility constraints, and coexisting illness may increase concern, but the decision still belongs within an individualized assessment. If measurement is needed, total 25(OH)D is generally more relevant than 1,25(OH)2D.

For children and adolescents, public health policy may favor supplementation without universal screening, while targeted clinical evaluation remains appropriate when symptoms, malabsorption, medication use, or other concerns change the context.

For pregnant individuals, the current guideline direction favors empiric supplementation rather than routine screening of every healthy patient. Where testing is clinically indicated, laboratories should use a validated 25(OH)D method and interpret the result within the broader maternal and nutritional picture.

For neonates and infants, assay selection deserves particular care because of the potential contribution of 3-epi-25(OH)D3. A method that cannot resolve the epimer may overestimate 25(OH)D3, and a laboratory should be able to explain how its assay handles this issue.

For epidemiological studies comparing regions, seasons, or demographic groups, LC-MS/MS with documented performance and appropriate standardization is the strongest option when resources allow. If immunoassays are used, the study design should preserve consistency and clearly describe the platform, calibration, and limitations rather than treating all measurements as directly comparable.

For fortification programs, measurement should be paired with implementation data. A change in laboratory values without information on food access, product coverage, dietary intake, adherence, season, and subgroup reach gives only a partial account of whether nutritional equity is improving.

The route forward is selective, standardized, and community-aware

Vitamin D monitoring is often framed as a contest between a sophisticated assay and a cheaper routine test. That is too narrow. The real comparison is between different ways of making a health decision: measuring an individual’s 25(OH)D, using a risk-based supplementation policy, conducting standardized population surveillance, or combining testing with food-based intervention.

LC-MS/MS remains the most analytically comprehensive method for total 25(OH)D because it can distinguish 25(OH)D2, 25(OH)D3, and relevant interfering metabolites. It is particularly valuable when neonatal samples, research comparisons, low concentrations, or complex clinical questions make specificity central. Routine immunoassays can have a place, but their performance must be understood in the population being measured, and results should not be assumed to be interchangeable across platforms.

The 2024 guideline shift is equally important: healthy populations should not be drawn into routine testing simply because vitamin D deficiency is common or politically visible. Where empiric supplementation is recommended, the more constructive public health question may be whether people can obtain and use the intervention, whether fortified foods reach those facing nutritional inequity, and whether grassroots implementation closes rather than widens access gaps.

Our task as public health practitioners is therefore not to maximize the number of vitamin D tests. It is to build a monitoring system that knows when a test will change care, when a standardized survey will change policy, and when the stronger intervention is to remove the barrier before it becomes another abnormal result.