{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,10]],"date-time":"2026-04-10T09:15:56Z","timestamp":1775812556633,"version":"3.50.1"},"reference-count":18,"publisher":"MDPI AG","issue":"8","license":[{"start":{"date-parts":[[2026,4,10]],"date-time":"2026-04-10T00:00:00Z","timestamp":1775779200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Nutrients"],"abstract":"<jats:p>Background: Vitamin D deficiency in early childhood is commonly defined using fixed 25(OH)D thresholds, most often &lt;20 ng\/mL. However, inclusion of the C3-epimer (3-epi-25(OH)D3) in total 25(OH)D measurements may influence classification, particularly in children with borderline concentrations. Methods: In this cross-sectional study of 128 children aged 0\u201336 months, vitamin D metabolites were quantified using LC\u2013MS\/MS in dried blood spot (DBS) samples. Vitamin D deficiency was defined as &lt;20 ng\/mL. Total 25(OH)D was recalculated after subtraction of the C3-epimer to assess changes in deficiency classification. Agreement was evaluated using Cohen\u2019s \u03ba, and systematic differences were tested with McNemar\u2019s test. Diagnostic performance parameters were calculated using epimer-resolved 25(OH)D as the reference standard. Results: Mean 25(OH)D3 concentration was 20.3 \u00b1 6.2 ng\/mL, and 57% of children were classified as deficient. After epimer subtraction, deficiency classification changed in 22 of 128 children (17.2%). Agreement between classifications was substantial (\u03ba = 0.67), but McNemar\u2019s test demonstrated a significant systematic shift (p &lt; 0.001). Sensitivity of total 25(OH)D including the epimer for detecting deficiency was 70.3% (95% CI: 59.0\u201380.0%), with specificity of 100% (95% CI: 94.3\u2013100%). Reclassification was strongly concentrated among children with borderline 25(OH)D3 concentrations (18\u201322 ng\/mL), where 54.5% were reclassified compared with 4.2% outside this range. Reclassification was not associated with age. Conclusions: In young children, inclusion of the C3-epimer in total 25(OH)D measurement leads to potentially clinically relevant misclassification of vitamin D deficiency, particularly near diagnostic thresholds. Epimer-resolved assessment may improve diagnostic precision in cases with borderline vitamin D concentrations.<\/jats:p>","DOI":"10.3390\/nu18081188","type":"journal-article","created":{"date-parts":[[2026,4,10]],"date-time":"2026-04-10T07:55:12Z","timestamp":1775807712000},"page":"1188","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["C3-Epimer Inclusion Influences Vitamin D Deficiency Classification Near Diagnostic Thresholds in Early Childhood"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-0141-4155","authenticated-orcid":false,"given":"Tworowska","family":"Julia","sequence":"first","affiliation":[{"name":"Department of Paediatrics, Allergology and Gastroenterology, Ludwik Rydygier Collegium Medicum Bydgoszcz, Nicolaus Copernicus University Torun, 85-094 Bydgoszcz, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9976-9789","authenticated-orcid":false,"given":"Gory\u0144ski","family":"Krzysztof","sequence":"additional","affiliation":[{"name":"Faculty of Chemical Technology and Engineering, Bydgoszcz University of Science and Technology, Seminaryjna 3, 85-326 Bydgoszcz, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0616-9811","authenticated-orcid":false,"given":"Gory\u0144ska Paulina","family":"Zofia","sequence":"additional","affiliation":[{"name":"Faculty of Chemical Technology and Engineering, Bydgoszcz University of Science and Technology, Seminaryjna 3, 85-326 Bydgoszcz, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Stampor-Bednarska","family":"Justyna","sequence":"additional","affiliation":[{"name":"Department of Paediatrics, Allergology and Gastroenterology, Ludwik Rydygier Collegium Medicum Bydgoszcz, Nicolaus Copernicus University Torun, 85-094 Bydgoszcz, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1400-868X","authenticated-orcid":false,"given":"Sobieska-Poszwa","family":"Ola","sequence":"additional","affiliation":[{"name":"Department of Paediatrics, Allergology and Gastroenterology, Ludwik Rydygier Collegium Medicum Bydgoszcz, Nicolaus Copernicus University Torun, 85-094 Bydgoszcz, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5280-1876","authenticated-orcid":false,"given":"Krogulska","family":"Aneta","sequence":"additional","affiliation":[{"name":"Department of Paediatrics, Allergology and Gastroenterology, Ludwik Rydygier Collegium Medicum Bydgoszcz, Nicolaus Copernicus University Torun, 85-094 Bydgoszcz, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7374-9122","authenticated-orcid":false,"given":"Si\u00f3dmiak","family":"Joanna","sequence":"additional","affiliation":[{"name":"Department of Laboratory Medicine, Faculty of Pharmacy, Ludwik Rydygier Collegium Medicum Bydgoszcz, Nicolaus Copernicus University Torun, 85-094 Bydgoszcz, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Kowalski","family":"Konrad","sequence":"additional","affiliation":[{"name":"Masdiag Sp. z o.o., 01-882 Warsaw, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2026,4,10]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1907","DOI":"10.1210\/clinem\/dgae290","article-title":"Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline","volume":"109","author":"Demay","year":"2024","journal-title":"J. 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