Importance Of Sample Volume To The Measurement And Interpretation Of Plasma Osmolality

Abstract

Small sample volumes may artificially elevate plasma osmolality (Posm) measured by freezing point depression. PURPOSE: To compare different sample volumes of measured Posm (mmol/kg) to each other, and to calculated osmolarity (mmol/L) in euhydrated (EUH) and dehydrated (DEH) volunteers. METHODS: Posm was measured using freezing point depression and osmolarity calculated from measures of sodium, glucose, and blood urea nitrogen. The influence of sample volume was investigated by comparing 20 μL and 250 μL Posm samples (n = 158 pairs). Protinol (240, 280, 320 mmol/kg) and Clinitrol (290 mmol/kg) reference solutions were compared similarly (n = 246 pairs). Twenty-one volunteers were tested multiple times while EUH (n = 72) or DEH (n = 7) by - 4.0% body mass. RESULTS: The 20 μL samples of Protinol, but not Clinitrol, were significantly higher by 3 mmol/kg when compared to 250 μL samples. The 20 μL samples of plasma were 7 mmol/kg higher than 250 μL samples with a nearly constant systematic error across the range tested (slope = 0.917). Calculated osmolarity was significantly lower than 20 μL Posm (- 6.6 mmol) but not different from 250 μL Posm (<1.0 mmol). When using common criteria for EUH (<290 mmol/kg), only 19/72 EUH volunteers were correctly classified using 20 μL Posm, however, 65/72 were correctly classified using 250 μL Posm. DEH volunteers met the common > 295 mmol/kg DEH criteria in 7/7 (20 μL Posm) and 6/7 (250 μL Posm) cases studied. The average change in Posm from EUH to DEH was similar for 20 μL (11 mmol/kg) and 250 μL (10 mmol/kg) samples. CONCLUSIONS: These results demonstrate that Posm measured by freezing point depression will be ∼7 mmol/kg higher when using 20 vs. 250 μL sample volumes. Approximately half of this effect may be due to plasma proteins. Posm sample volume should be carefully considered when calculating the osmole gap or assessing hydration status. The opinions or assertions contained herein should not to be construed as official or reflecting the views of the Army or the DoD.