title: Biological variation of plasma osmolality obtained with capillary versus venous blood.
abstract: 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.
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Wittbrodt
authors.forename: Sofia
authors.initials: S
authors.lastname: Espinoza
authors.forename: Mindy L
authors.initials: ML
authors.lastname: Millard-Stafford
mesh_terms: D002196:Capillaries / Q000378:metabolism; D003681:Dehydration / Q000097:blood*; D005260:Female; D006801:Humans; D008297:Male; D009994:Osmolar Concentration; D010949:Plasma / Q000737:chemistry*; D012372:ROC Curve; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014680:Veins / Q000378:metabolism; D055815:Young Adult