Sollanek Kurt J, Kenefick Robert W, Cheuvront Samuel N
Department of Kinesiology, Sonoma State University, Rohnert Park, California.
U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts.
J Clin Lab Anal. 2019 Mar;33(3):e22727. doi: 10.1002/jcla.22727. Epub 2018 Nov 25.
Small sample volumes may artificially elevate plasma osmolality (Posm) measured by freezing point depression. The purpose of this study was to compare two widely different sample volumes of measured Posm (mmol/kg) to each other, and to calculated osmolarity (mmol/L), across a physiological Posm range (~50 mmol/kg).
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 and 250 μL Posm samples (n = 126 pairs). Thirty-two volunteers were tested multiple times while EUH (n = 115) or DEH (n = 11) by -4.0% body mass. Protinol™ (240, 280, and 320 mmol/kg) and Clinitrol™ (290 mmol/kg) reference solutions were compared similarly (n = 282 pairs).
The 20 μL samples of plasma showed a 7 mmol/kg positive bias compared to 250 μL samples and displayed a nearly constant proportional error across the range tested (slope = 0.929). Calculated osmolarity was lower than 20 μL Posm by the same negative bias (-6.9 mmol/kg) but not different from 250 μL Posm (0.1 mmol/kg). The differences between 20 and 250 μL samples of Protinol™ were significantly higher than Clinitrol™.
These results demonstrate that Posm measured by freezing point depression will be ~7 mmol/kg higher when using 20 μL 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.
小样本体积可能会人为提高通过冰点降低法测量的血浆渗透压（Posm）。本研究的目的是在生理Posm范围（约50 mmol/kg）内，比较两种差异很大的测量Posm（mmol/kg）样本体积，并与计算的渗透压（mmol/L）进行比较。
使用冰点降低法测量Posm，并根据钠、葡萄糖和血尿素氮的测量值计算渗透压。通过比较20和250 μL的Posm样本（n = 126对）来研究样本体积的影响。32名志愿者在体重减轻4.0%达到等渗性脱水（EUH，n = 115）或低渗性脱水（DEH，n = 11）时接受多次测试。同样比较了Protinol™（240、280和320 mmol/kg）和Clinitrol™（290 mmol/kg）参考溶液（n = 282对）。
与250 μL样本相比，20 μL血浆样本显示出7 mmol/kg的正偏差，并且在测试范围内显示出几乎恒定的比例误差（斜率 = 0.929）。计算的渗透压比20 μL Posm低相同的负偏差（-6.9 mmol/kg），但与250 μL Posm无差异（0.1 mmol/kg）。Protinol™的20和250 μL样本之间的差异显著高于Clinitrol™。
这些结果表明，使用20 μL与250 μL样本体积时，通过冰点降低法测量的Posm将高出约7 mmol/kg。这种影响的大约一半可能归因于血浆蛋白。在计算渗透摩尔间隙或评估水合状态时，应仔细考虑Posm样本体积。
