{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,3]],"date-time":"2026-02-03T01:33:00Z","timestamp":1770082380141,"version":"3.49.0"},"reference-count":41,"publisher":"MDPI AG","issue":"3","license":[{"start":{"date-parts":[[2026,1,31]],"date-time":"2026-01-31T00:00:00Z","timestamp":1769817600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100004837","name":"Spanish Ministry of Science, Innovation and Universities","doi-asserted-by":"publisher","award":["FPU17\/00407"],"award-info":[{"award-number":["FPU17\/00407"]}],"id":[{"id":"10.13039\/501100004837","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Regional Department of Economy and Infrastructure of the Government of Extremadura","award":["GR18170"],"award-info":[{"award-number":["GR18170"]}]},{"name":"Regional Department of Economy and Infrastructure of the Government of Extremadura","award":["GR18003"],"award-info":[{"award-number":["GR18003"]}]},{"name":"Direcci\u00f3n General de Investigaci\u00f3n e Innovaci\u00f3n del Gobierno de Arag\u00f3n","award":["S53_23R"],"award-info":[{"award-number":["S53_23R"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Nutrients"],"abstract":"<jats:p>Background\/Objectives: Accurate hydration assessment is critical for optimizing performance and preventing heat-related complications in ultra-endurance athletes. This study evaluated the agreement and reliability between urine reagent strips and refractometry for field-based hydration assessment via urine-specific gravity (USG) in ultra-trail runners. Methods: Thirty-four ultra-trail runners (22 males, 12 females; mean age 43.71 \u00b1 11.50 years) participated during The Coastal Challenge, a 241-km multi-stage ultra-trail competition. Urine samples were collected before and after the first two stages (Stage 1: 41 km, 1071 m elevation; Stage 2: 40 km, 1828 m elevation). USG was measured using semi-quantitative urine reagent strips (Combur10Test M) and a handheld digital refractometer (Palm Abbe\u2122). Agreement was assessed via paired t-tests, Pearson and Spearman correlations, intraclass correlation coefficients, and Bland-Altman plots across four measurement time points. Results: Strong agreement existed between methods with correlation coefficients of 0.92\u20130.99 (p &lt; 0.01) within the hydration range typical of well-prepared ultra-endurance athletes (USG 1.010\u20131.020). No significant differences were found between devices at any time point (all p &gt; 0.05). Bland-Altman analyses revealed minimal mean bias (range: \u22120.002 to +0.001 g\/mL) and narrow limits of agreement, with fewer than 5% of values falling outside limits. Both methods detected significant increases in USG from pre- to post-stage (p &lt; 0.01), indicating exercise-induced hypohydration. Conclusions: Semi-quantitative urine reagent strips and handheld refractometers demonstrate strong agreement for hydration assessment in ultra-trail runners under field conditions when not severely hypohydrated, supporting their interchangeable use for practical monitoring.<\/jats:p>","DOI":"10.3390\/nu18030466","type":"journal-article","created":{"date-parts":[[2026,2,2]],"date-time":"2026-02-02T14:12:46Z","timestamp":1770041566000},"page":"466","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Agreement and Reliability Between Urine Reagent Strips and Refractometry for Field Assessment of Hydration in Ultra-Trail Runners"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-0717-8827","authenticated-orcid":false,"given":"Daniel","family":"Rojas-Valverde","sequence":"first","affiliation":[{"name":"Centro de Investigaci\u00f3n, Desarrollo e Innovaci\u00f3n en Salud y Deporte (CIDISAD), Escuela Ciencias del Movimiento Humano y Calidad de Vida (CIEMHCAVI), Universidad Nacional, Heredia 86-3000, Costa Rica"},{"name":"Cl\u00ednica de Lesiones Deportivas (Rehab & Readapt), Escuela Ciencias del Movimiento Humano y Calidad de Vida (CIEMHCAVI), Universidad Nacional, Heredia 86-3000, Costa Rica"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0074-3624","authenticated-orcid":false,"given":"Volker","family":"Scheer","sequence":"additional","affiliation":[{"name":"Ultra-Sports Science Foundation, 69310 Pierre-B\u00e9nite, France"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5711-7520","authenticated-orcid":false,"given":"Marcelo","family":"Tuesta","sequence":"additional","affiliation":[{"name":"Exercise and Rehabilitation Sciences Laboratory, School of Physical Therapy, Faculty of Rehabilitation Sciences, Universidad Andres Bello, Santiago 7550000, Chile"},{"name":"Laboratory of Sport Sciences, Centro de Medicina Deportiva Sports MD, Vi\u00f1a del Mar 2520000, Chile"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4084-8124","authenticated-orcid":false,"given":"Carlos D.","family":"G\u00f3mez-Carmona","sequence":"additional","affiliation":[{"name":"Research Group in Training, Physical Activity and Sports Performance (ENFYRED), University of Zaragoza, 44003 Teruel, Spain"},{"name":"Research Group in Optimization of Training and Sports Performance (GOERD), Faculty of Sport Science, University of Extremadura, 10005 Caceres, Spain"},{"name":"BioVetMed & SportSci Research Group, University of Murcia, 30100 Murcia, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2026,1,31]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"3","DOI":"10.1097\/JSA.0000000000000198","article-title":"Participation Trends of Ultra Endurance Events","volume":"27","author":"Scheer","year":"2019","journal-title":"Sports Med. 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