Reliability of a cryoscopic micro-osmometer using 15-µL plasma samples to measure hydration status in varied environmental conditions

Measurement of plasma osmolality (P osm ) remains popular for assessing hydration status in exercise science. However, a controlled reliability assessment of micro-osmometry using small sample volumes to measure P osm remains to be performed. This study aimed to examine the reliability of a cryoscopic micro-osmometer requiring 15-μL samples across varied environmental conditions. Thirteen young adults (26.4 ± 5.7 years) completed a 20-min steady-state cycling protocol on separate occasions in cool/dry conditions and hot/humid conditions. P osm was assessed using fingertip capillary samples taken in duplication prior to and following 20-min acclimatization and recovery periods around the cycling bout. Absolute (typical error of measurement =.62–.83 mOsmol·kg –1 ; coefficient of variation =.9% to 1.3%) and relative (intraclass correlation coefficient =.60–.85) reliability of this approach were supported. These findings suggest micro-osmometry requiring small plasma samples (15 μL) can be reliably used by practitioners to assess longitudinal changes and cross-sectional hydration status in varied environmental conditions. © 2017 Taylor & Francis.