Validity of temperature, duration, and vessel seal on 24-hour urinary hydration markers

The purpose of this study was to examine the effect of storage temperature, duration, and storage vessel seal on 24 h urinary hydration markers. Twenty-one males (n = 8) and females (n = 13) (mean±SD; age, 24±5 y; body mass, 68.9±24.2 kg; height, 160.2±32.1 cm) without a history of renal disease or currently taking any medications or supplements known to affect the accuracy of urinary hydration markers were enrolled in this study. Participants provided a 24 h urine sample in a clean container with each urine sample being separate into four separate containers, two in each of the following temperatures: 7°C and 22°C. One specimen container at each temperature was either sealed using the manufacturers cap (single sealed) or the manufacturers cap plus laboratory wrapping film (double sealed). Each sample was analyzed after 1, 2, 3, 7 and 10 days. Urine samples were assessed for urine osmolality (U(OSMO)), urine specific gravity (U(SG)) and urine color (U(COL)). U(OSMO) was stable at 7°C for two days (mean difference [95% CI]; +1 mmol·kg(-1) [0+3], p>0.05) and three days (+1 mmol·kg(-1) [0, +3], p>0.05) for single sealed and double sealed containers, respectively. U(SG) measures were stable for singled sealed and double sealed for up to ten days when stored at 22°C. U(COL) measures were maintained for up to three days in all storage methods (p>0.05). In conclusion, if immediate analysis is unavailable, such as in the case of field based or longitudinal research, it is recommended that 24 h urine samples are stored in a refrigerated environment and hydration markers (U(OSMO) and U(COL)) be assessed within 48 h.