The Measurement of Oxygen Partial Pressure Determines the Accuracy of Calculated Oxygen Saturation by Blood Gas Analyzers

氧分壓的量測結果決定血氧飽和度計算值的準確性

Partial pressure of oxygen (pO_2) and oxygen saturation (sO_2) are parameters evaluating adequacy of oxygenation. While pO_2 can be directly measured, the sO_2 is usually yielded by calculation. Although inaccuracy of calculated sO_2 has been revealed, the mechanisms accounting for the inaccuracy of calculated sO_2 remains unexplored. 238 samples were analyzed simultaneously using Nova CCX and Siemens RP500 analyzers. The calculated sO_2 are compared to those measured by cooximeters, and the potential factors impairing the accuracy of calculated sO_2 are evaluated. The calculated sO_2 by Nova analyzer significantly differed from reference sO_2 by 2.9 (95% confidence interval [CI], 2.67~3.12) per 10% decrease of reference sO_2, while calculated sO_2 by Siemens analyzer was comparable to reference sO_2. The inaccuracy of calculated sO_2 by Nova analyzer can be corrected using pO_2 by Siemens analyzer for sO_2 calculation. The differences of pO_2 measurements by both analyzers reached 29.35±7.27 mmHg (95% CI, 27.79~30.91) or around 10% for samples with pO_2 less than 40 mmHg (n=87). The inaccuracy of pO_2 measurements is the main factor contributing to inaccuracy of calculated sO_2. Evaluating the accuracy of pO_2 measurement using venous blood samples is highly recommended, particularly for medical laboratories providing results of venous blood gas analyses.

氧分壓（pO_2）與血氧飽和度（sO_2）用於評估血液是否有充足的氧合狀態。氧分壓可直接被量測，而血氧飽和度通常是經由公式計算而得。儘管先前研究指出血氧飽和度計算值並不準確，但是造成血氧飽和度計算值偏差的機制並未被探討。我們利用Nova Stat Profile Critical Care Xpress（以下簡稱Nova分析儀）和Siemens Rapid Point 500（以下簡稱Siemens分析儀）血液氣體分析儀分析238支檢體，將得到的血氧飽和度計算值與血氧計（cooximeter）量測的結果進行比較，藉此探討造成血氧飽和度計算值偏差的可能原因。實驗結果發現每當血氧飽和度實測值減少10％，Nova分析儀的血氧飽和度計算值會偏離實測值2.9（95％信賴區間為2.76～3.12），然而，Siemens分析儀的血氧飽和度計算值與實測值卻不會有明顯的差異。我們進一步發現，當Nova分析儀的氧分壓被替換成Siemens的氧分壓時，Nova分析儀的血氧飽和度計算值的偏差會被修正。另外，兩台分析儀的氧分壓差異可達到29.35±7.27 mmHg（95％信賴區間為27.79～30.91），而且當氧分壓低於40 mmHg時，兩台儀器的氧分壓差異會達到10％。氧分壓量測的偏差是造成血氧飽和度計算值偏差的主因，因此本篇研究建議在量測靜脈血的氧分壓時，必須評估量測結果的準確性，尤其是提供靜脈血檢驗分析報告的實驗室。