It is difficult to know whether the body has sufficient vitamin D based solely on sun exposure, dietary habits, or how a person feels. Total 25-hydroxyvitamin D [25(OH)D] in blood integrates contributions from cutaneous synthesis and external sources and is currently the primary biomarker used to assess vitamin D status. The value of testing is to turn a possible low status into measurable information and provide objective evidence for the assessment of bone health and calcium-phosphate metabolism.[1–4]

1. Why Test 25(OH)D?

Low vitamin D levels often cause no characteristic symptoms in the early stages. Fatigue, muscle weakness, or skeletal discomfort are not specific to vitamin D deficiency; living in a sunny region does not guarantee adequate vitamin D status for every individual. Latitude, season, skin pigmentation, age, outdoor activity, diet, obesity, absorption, and liver and kidney function can all affect 25(OH)D levels. A validated test can answer a question that lifestyle questionnaires and subjective symptoms cannot answer reliably: approximately what is the current vitamin D status?[2–4]

This objective information is particularly relevant for people with bone pain, muscle weakness, fragility fractures, metabolic bone disease, prolonged limited sun exposure, gastrointestinal malabsorption, or other relevant risk contexts. Testing is not equivalent to diagnosing a specific disease, but it can help identify clearly low levels and should be interpreted together with serum calcium, phosphate, parathyroid hormone, alkaline phosphatase, bone mineral density, and clinical findings.[2,4]

The value of testing depends on whether there is a clear clinical question. Professional guidelines emphasize that results are more likely to be used appropriately when there is a relevant risk context or a specific assessment purpose. In people with a defined clinical indication or known conditions that affect vitamin D absorption, metabolism, or bone mineral metabolism, 25(OH)D testing may have clearer clinical relevance. However, not every risk factor for low vitamin D status means that routine testing is necessary. For asymptomatic, generally healthy populations, evidence that universal screening improves health outcomes remains insufficient.[3,8,14,15]

How common are low levels?

A multicountry European study using standardized testing included 55,844 participants; 40.4% had 25(OH)D levels below 20 ng/mL and 13.0% below 12 ng/mL. Data from U.S. adults cited in the 2024 Endocrine Society guideline indicate that approximately 24% had levels below 20 ng/mL. These proportions vary with region, season, and the cutoff used, but they are sufficient to show that low 25(OH)D is not uncommon.[3,6]

Which Situations Are More Likely to Be Associated with Low Levels?

25(OH)D status is determined by cutaneous synthesis, external sources, absorption, and metabolism. Risk factors often overlap. For example, ultraviolet B exposure is reduced during winter at higher latitudes, while prolonged indoor living or extensive body covering reduces cutaneous synthesis. People with darker skin pigmentation generally require longer UV exposure to produce the same amount of vitamin D. Aging also reduces the skin's capacity for vitamin D synthesis.[2–4]

Life stage and living environment: Infants and young children, older adults, people living in residential care facilities, night-shift workers or those who spend most of their time indoors, and people with little outdoor activity are more likely to have low levels.

Absorption and metabolic factors: Inflammatory bowel disease, celiac disease, other forms of fat malabsorption, or a history of bariatric surgery may impair vitamin D absorption; chronic liver disease, kidney disease, and certain medications may also alter its conversion or metabolism.[2,4]

Body weight and distribution: People with obesity often have lower average serum 25(OH)D levels, partly because of factors related to vitamin D distribution within a larger volume of tissue. Testing provides more direct information about vitamin D status than relying solely on body size or lifestyle.[3,4]

25(OH)D levels also vary by season. A single result represents status at the time of sampling; when comparing results over time, the sampling season, specimen type, and testing platform should be consistent because each can directly affect interpretation.

2. What Happens with Long-Term Severe Deficiency?

Vitamin D helps the intestine absorb calcium and phosphate and supports normal bone mineralization. With persistent severe deficiency, calcium and phosphate absorption decreases, and the body may increase parathyroid hormone secretion to maintain serum calcium, thereby accelerating bone turnover. When osteoid tissue is inadequately mineralized, this can ultimately lead to skeletal diseases with a well-established causal relationship to severe vitamin D deficiency.[1,2,4,5]

Children: Nutritional rickets may occur, with manifestations including impaired growth, softened or deformed bones, delayed motor development, and muscle weakness; severe hypocalcemia may also cause spasms or seizures. Nutritional rickets may also be affected by inadequate calcium intake, and not all cases of rickets are caused by vitamin D deficiency.[4,5]

Adolescents and adults: Osteomalacia may occur, with diffuse bone pain, proximal muscle weakness, difficulty rising or walking, and increased skeletal fragility. Severe deficiency may also be accompanied by hypocalcemia and secondary hyperparathyroidism.[2,4]

Older adults: Low 25(OH)D levels may coexist with reduced muscle strength and increased risks of falls and fractures. However, these outcomes are influenced by multiple factors, including age, bone mineral density, muscle function, medications, and underlying diseases; fall or fracture risk cannot be determined from 25(OH)D levels alone.[1,2]

Low 25(OH)D levels are also commonly observed in people with cardiovascular disease, diabetes, cancer, infections, or autoimmune diseases, but such associations cannot generally be interpreted as direct causation. Therefore, 25(OH)D testing is used to assess vitamin D status, not to diagnose these diseases.[3,7]

3. Which Form of “Vitamin D” Is Actually Measured?

Vitamins D2 and D3 are precursors that enter the body. They are first converted in the liver to 25(OH)D2 and 25(OH)D3, respectively, and are then primarily converted by the kidneys to the hormonally active form, 1,25-dihydroxyvitamin D. Although the names are similar, these compounds have different physiological roles.[2,4]

4. Why Is 25(OH)D Difficult to Measure Accurately?

25(OH)D is a small, lipophilic molecule that is largely bound in blood to vitamin D-binding protein or albumin. Before measurement, it must be sufficiently released from these proteins. Total 25(OH)D testing must cover both 25(OH)D2 and 25(OH)D3, while blood also contains structurally similar metabolites such as 24,25-dihydroxyvitamin D and C3 epimers. Like “keys” with similar shapes, these compounds may be partially recognized together by some analytical systems or may affect results when separation is insufficient.[2,11–13]

As a result, results from different platforms may show bias: some methods respond differently to D2 and D3; some differ in their efficiency of releasing 25(OH)D from binding proteins; and some are affected by cross-reactivity with structurally similar metabolites. Differences in calibrators, reference materials, and quality-control systems can also cause the same sample to produce slightly different values across laboratories. NIH and international standardization studies have highlighted that 25(OH)D results may be biased high or low depending on the method and laboratory.[4,12,13]

5. What Is the Reference Measurement Procedure?

For analytical method evaluation, a standardized, traceable isotope-dilution liquid chromatography–tandem mass spectrometry (ID-LC-MS/MS) reference measurement procedure serves as a higher-order reference for the accuracy of 25(OH)D measurement. The CDC reference laboratory procedure first separates 25(OH)D2 and 25(OH)D3 from binding proteins, then uses chromatography to separate closely related components and tandem mass spectrometry to quantify them individually, and finally sums D2 and D3 to obtain total 25(OH)D. Results are calibrated using NIST standard reference materials and are traceable to the International System of Units.[9–11]

A more precise use of the term “gold standard”: an ID-LC-MS/MS method that is appropriately calibrated, quality-controlled, and traceable to a recognized reference measurement procedure can serve as a higher-order reference for evaluating the accuracy of serum total 25(OH)D measurements. Not every LC-MS/MS instrument or laboratory-developed method is automatically equivalent to a reference measurement procedure.

A reference-level workflow typically includes venous blood collection, serum separation, addition of an isotopically labeled internal standard, extraction, chromatographic separation, tandem mass spectrometric analysis, calibration, and quality control. It can better distinguish D2, D3, and some structurally similar compounds, but requires specialized equipment, trained personnel, and a comprehensive quality system and is not suitable for direct transfer to home or point-of-care testing settings.[9–11]

Reference-level laboratory workflow: Venous blood collection → Serum separation → Release of protein-bound 25(OH)D → Addition of an isotopically labeled internal standard and extraction → Chromatographic separation → Tandem mass spectrometric quantification → Calibration and quality control → Reporting of total 25(OH)D

Standardization Helps Different Platforms Use the Same “Ruler”

The CDC Vitamin D Standardization-Certification Program (VDSCP) uses traceable reference procedures and human serum samples to evaluate routine testing, helping laboratories and manufacturers identify systematic bias. Standardization does not eliminate all measurement uncertainty in every individual result, but it can reduce persistent platform-dependent high or low bias for the same sample, improving comparability across laboratories, reagent lots, and population studies.[9,10,12,13]

6. What Does POCT Immunochromatography Address?

The core value of POCT (point-of-care testing) is accessibility and timeliness. Some purpose-designed 25(OH)D POCT systems integrate sample processing, 25(OH)D release, immunological recognition, and signal generation into a single test device. When fingertip whole blood is used, venous blood collection, on-site serum separation, and large analytical instruments are not required, allowing an initial stratification result to be obtained within a relatively short time.[14,15]

How Does an Immunochromatographic Test Device Perform the Test?

A typical test device consists of a sample application area or blood-separation layer, a reagent-release area, a test line and control line on a nitrocellulose membrane, and an absorbent pad at the distal end. The sample moves forward by capillary action: red blood cells are blocked or separated, 25(OH)D is released from binding proteins and reacts with labeled recognition reagents, and a concentration-related color or fluorescence signal is then generated at the test line; the control line indicates whether chromatographic migration has proceeded as designed.[14,15]

Visual-read test devices typically provide semi-quantitative ranges, whereas reader-based devices convert color or fluorescence intensity into numerical values or ranges. A reader can reduce variation caused by lighting conditions and visual interpretation, but it must still be used together with the product-specific calibration curve, lot-specific quality control, and predefined interpretation rules.

For products using 20 and 30 ng/mL as two cutoff points, POCT provides an indication of “approximately which range the result falls into,” rather than a precise concentration at the level of a reference laboratory method. It is suitable for rapidly assessing vitamin D status; when a result is very close to a cutoff, is inconsistent with other information, or requires precise quantification, a standardized laboratory method provides a different level of information. The advantage of POCT is not that it is inherently equivalent to reference mass spectrometry, but that testing can be performed more quickly and accessibly.[9]

7. How Should 20 ng/mL and 30 ng/mL Be Interpreted?

First, convert the units: 1 ng/mL = 2.5 nmol/L; therefore, 20 ng/mL = 50 nmol/L and 30 ng/mL = 75 nmol/L. The 20 and 30 ng/mL values are not two globally uniform diagnostic thresholds for disease; they are commonly used status-stratification points arising from different public health and clinical frameworks.[1–4]

NASEM/NIH generally uses 20 ng/mL as a reference point for meeting the skeletal needs of most healthy people; EFSA uses 50 nmol/L as a target concentration when establishing dietary reference values. Some clinical frameworks have previously used 30 ng/mL. In 2024, the Endocrine Society no longer set a single target applicable to everyone for general disease prevention. The NASEM/NIH framework generally considers 20 ng/mL (50 nmol/L) a reference level for meeting the skeletal and overall health needs of most healthy people; EFSA uses 50 nmol/L as the target concentration when establishing dietary reference values for vitamin D. A level of 30 ng/mL (75 nmol/L) was previously used as a target by some clinical guidelines or expert frameworks, but the 2024 Endocrine Society guideline no longer endorses the uniform 30 ng/mL target from its previous guideline and no longer recommends using fixed 25(OH)D concentrations to uniformly define sufficiency, insufficiency, and deficiency across all populations.[1,3,4,16]

These two numbers are more like signposts than cliff edges. In a table, 19 and 21 ng/mL, or 29 and 31 ng/mL, fall into adjacent ranges, but this does not mean that physiological status changes abruptly at a cutoff. Interpretation should also consider analytical uncertainty, specimen type, and the purpose of testing.

Results also have a time dimension. Because 25(OH)D has a relatively long circulating half-life, it is useful for reflecting recent overall status, but season, outdoor activity, travel, dietary changes, and health status can still cause values to fluctuate over time. Therefore, the most reliable interpretation of a report is “which range did the person fall into at the time of this sampling,” rather than a permanent label that remains unchanged throughout life.[2,4,6]

8. Why Can POCT and Laboratory Results Differ?

Different specimen matrices: Authoritative 20/30 ng/mL frameworks are primarily based on total 25(OH)D in serum or plasma. Fingertip whole blood also contains red blood cells; hematocrit changes the proportion of plasma in a given sample volume and may also affect fluid flow through the test membrane. Whole-blood products therefore require their own calibration; serum-based cutoffs cannot simply be transferred unchanged.

Different recognition mechanisms: Immunoassays rely on antibodies or binding proteins to recognize 25(OH)D2, 25(OH)D3, and structurally similar metabolites, whereas mass spectrometry distinguishes analytes using chromatographic retention time and mass-to-charge ratio. Different analytical principles result in different selectivity.

Different signal direction in small-molecule competitive assays: Because 25(OH)D is a very small molecule, many immunochromatographic test devices use a competitive format. As the amount of 25(OH)D in the sample increases, less labeled material may bind at the test line, so the test line may become lighter as concentration increases. This differs from many sandwich-format lateral flow tests, in which a stronger line generally indicates a positive result. The specific direction of interpretation must follow the product's instructions for use.[14,15]

Operation and environmental conditions can also affect the signal: sample volume, buffer volume, reaction time, ambient temperature, lighting, color cards, and readers can all affect the color or fluorescence on the membrane. When a result is close to 20 or 30 ng/mL, even relatively small analytical variation may shift the result into an adjacent range.

Test-device design and manufacturing consistency: Membrane pore size and flow rate, conjugate-release efficiency, test-line coating amount, drying conditions, and lot-to-lot reagent consistency can all affect signal intensity. For products that have undergone design verification, lot release, and stability studies, these sources of variation need to be shown not to cause unacceptable classification bias.

Visual versus instrument-based interpretation: Visual results may be affected by ambient light, individual color perception, and the way a color card is compared. A reader can digitize the signal, but its optical system, algorithm, and calibration are also part of the analytical system. Both approaches should be used according to validated reading times and interpretation rules.

Calibration and traceability determine comparability: Methods calibrated against reference materials, standardized, and evaluated through external quality assessment are more likely to bring results from different laboratories and reagent lots onto a common scale. The CDC VDSCP was established precisely to improve this comparability for routine 25(OH)D testing.[9,10,12,13]

Choose the testing method according to the purpose. When a higher-order methodological reference is needed, consider a traceable ID-LC-MS/MS procedure; for routine quantitative testing, consider a validated and quality-controlled laboratory platform; when rapid range-based assessment is needed, a whole-blood-calibrated and performance-validated POCT can improve access to testing. Regardless of the method used, a 25(OH)D result describes vitamin D status; it is not, by itself, a diagnosis of a specific disease.

Key References

[1] EFSA Panel on Dietetic Products, Nutrition and Allergies. Dietary reference values for vitamin D. EFSA Journal. 2016;14(10):4547. DOI: 10.2903/j.efsa.2016.4547

[2] Giustina A, Bilezikian JP, Adler RA, et al. Consensus Statement on Vitamin D Status Assessment and Supplementation: Whys, Whens, and Hows. Endocrine Reviews. 2024;45(5):625–654. DOI: 10.1210/endrev/bnae009

[3] Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism. 2024;109(8):1907–1947. DOI: 10.1210/clinem/dgae290

[4] National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. Updated June 27, 2025. NIH ODS

[5] World Health Organization. Nutritional rickets: a review of disease burden, causes, diagnosis, prevention and treatment. Geneva: WHO; 2019. ISBN 978-92-4-151658-7. WHO Publications

[6] Cashman KD, Dowling KG, Škrabáková Z, et al. Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition. 2016;103(4):1033–1044. DOI: 10.3945/ajcn.115.120873

[7] Bouillon R, Manousaki D, Rosen C, et al. The health effects of vitamin D supplementation: evidence from human studies. Nature Reviews Endocrinology. 2022;18(2):96–110. DOI: 10.1038/s41574-021-00593-z

[8] US Preventive Services Task Force. Screening for Vitamin D Deficiency in Adults: Recommendation Statement. JAMA. 2021;325(14):1436–1442. DOI: 10.1001/jama.2021.3069

[9] Centers for Disease Control and Prevention. Vitamin D Reference Laboratory. Higher-order reference measurement procedures for total 25(OH)D. CDC official page

[10] Centers for Disease Control and Prevention. Improving the Performance of 25-Hydroxyvitamin D Tests: Vitamin D Standardization-Certification Program. Updated 2026. CDC VDSCP

[11] Mineva EM, Schleicher RL, Chaudhary-Webb M, et al. A candidate reference measurement procedure for quantifying serum concentrations of 25-hydroxyvitamin D3 and 25-hydroxyvitamin D2 using isotope-dilution LC-MS/MS. Analytical and Bioanalytical Chemistry. 2015;407:5615–5624. DOI: 10.1007/s00216-015-8733-z

[12] Wise SA, Camara JE, Burdette CQ, et al. VDSP Intercomparison Study 2—Part 1: LC-MS/MS assays and the impact of 3-epi-25-hydroxyvitamin D3. Analytical and Bioanalytical Chemistry. 2022;414:333–349. DOI: 10.1007/s00216-021-03576-1

[13] Wise SA, Camara JE, Burdette CQ, et al. VDSP Intercomparison Study 2—Part 2: ligand-binding assays and the impact of 25-hydroxyvitamin D2 and 24R,25-dihydroxyvitamin D3. Analytical and Bioanalytical Chemistry. 2022;414:351–366. DOI: 10.1007/s00216-021-03577-0

[14] Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. The Journal of Clinical Endocrinology & Metabolism. 2024;109:1907–1947. DOI: 10.1210/clinem/dgae290

[15] Krist AH, Davidson KW, Mangione CM, et al. Screening for Vitamin D Deficiency in Adults: US Preventive Services Task Force Recommendation Statement. JAMA. 2021;325(14):1436–1442. DOI: 10.1001/jama.2021.3069

[16] National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. National Institutes of Health. 2024. URL: https://ods.od.nih.gov/factsheets/Vitamind-HealthProfessional/