Department of Emergency Medicine, Inselspital, University of Bern, Bern, Switzerland.
Nephron Physiol. 2012;120(1):p1-5. doi: 10.1159/000336550. Epub 2012 Mar 6.
Calculation of electrolyte-free water clearance (EFWC) allows for quantification of renal losses of free water and was shown to be helpful in the differential diagnosis of dysnatremias and might help in the correction of the electrolyte disorders. A modified EFWC formula (MEFWC) was described to be more accurate than the conventional one; however, it has never been evaluated clinically.
In order to evaluate the performance of MEFWC compared to EFWC under clinical circumstances, we gathered data from a total of 912 patient days of 138 critically ill patients. EFWC and MEFWC were calculated on the basis of these data. Additionally, from data of critically ill patients, we calculated a prediction of serum sodium based on the Edelman equation using either EFWC or MEFWC and compared results.
Altogether, 343 normonatremic, 124 hyponatremic and 445 hypernatremic days were analyzed. Results for EFWC and MEFWC correlated significantly (R = 0.98). In patients with hyponatremia, the absolute difference between EFWC and MEFWC was significantly larger than in patients with normonatremia (437 vs. 256 ml, p < 0.01). The absolute difference between EFWC and MEFWC correlated significantly with the level of serum sodium (R = -0.41). The mean difference in the prediction of serum sodium change as calculated based on the Edelman equation between the formula using EFWC and the formula using MEFWC was 0.7 mmol/l (SD 0.68) and was highest in hyponatremia and lowest in hypernatremia.
Results of EFWC and MEFWC were comparable in critically ill patients. Under normal circumstances, the use of the more complicated MEFWC is not justified. In hyponatremia, the difference between EFWC and MEFWC is larger and thus might justify the use of the more complicated formula.
电解质自由水清除率（EFWC）的计算可量化自由水的肾丢失量，并有助于鉴别不同类型的电解质紊乱，且可能有助于纠正电解质异常。一种改良的 EFWC 公式（MEFWC）被描述为比传统公式更准确；然而，它从未在临床上进行过评估。
为了评估 MEFWC 在临床情况下与 EFWC 的性能，我们从 138 名危重症患者的 912 个患者日的数据中收集数据。根据这些数据计算 EFWC 和 MEFWC。此外，根据危重症患者的数据，我们使用 EFWC 或 MEFWC 基于 Edelman 方程计算血清钠的预测值，并比较结果。
共分析了 343 个正常血钠日、124 个低血钠日和 445 个高血钠日。EFWC 和 MEFWC 的结果显著相关（R = 0.98）。在低血钠患者中，EFWC 和 MEFWC 之间的绝对差异显著大于正常血钠患者（437 vs. 256 ml，p < 0.01）。EFWC 和 MEFWC 之间的绝对差异与血清钠水平显著相关（R = -0.41）。基于 Edelman 方程计算的血清钠变化预测值，EFWC 公式和 MEFWC 公式之间的平均差异为 0.7 mmol/L（SD 0.68），在低血钠时最高，在高血钠时最低。
EFWC 和 MEFWC 的结果在危重症患者中具有可比性。在正常情况下，使用更复杂的 MEFWC 是不合理的。在低血钠时，EFWC 和 MEFWC 之间的差异更大，因此可能有理由使用更复杂的公式。
