Biological variation of plasma osmolality obtained with capillary versus venous blood

Background: Plasma osmolality (POsm) is a gold standard to assess hydration status but requires venipuncture. POsm obtained by lancing a digit, a source of capillary puncture blood (CAP), has not been validated. This study compared POsm from CAP versus venous blood (VEN) and validated its sensitivity to detect dehydration. Methods: Healthy young adults (Study A: n = 20 men, 22 women; Study B: n = 23 men, 23 women) participated. In Study A, CAP and VEN were compared under controlled euhydration meeting dietary reference intakes for water (3.7 L men, 2.7 L women). In Study B, CAP was assessed for sensitivity to detect dehydration with receiver operating characteristic analysis over two 24 h periods: euhydration for 24 h followed by water restriction over 24 h. POsm was measured using freezing point depression. Results: For all subjects, CAP POsm (283.0 +/- 3.9 mOsm/kg) was not significantly different (p = 0.07) from VEN (284.2 +/- 3.5) during euhydration and met analytical goals for individuality and heterogeneity. When outliers (n = 3) were eliminated, mean difference was -1.6 (+/- 3.2) lower (p < 0.01) with CAP. Fluid restriction increased (p < 0.001) CAP POsm (284.0 +/- 4.4 to 292.8 +/- 5.2 mOsm/kg), achieving excellent accuracy (0.92) and sensitivity (89.1%) to predict mild dehydration (2% body mass loss). Conclusions: POsm via CAP exhibited similar coefficients of variation and analytical goals compared to VEN combined with excellent accuracy and sensitivity to detect dehydration. Although CAP values were approximately 2 mOsm/kg lower than VEN, CAP appears an adequate substitute for tracking changes in non-clinical settings.