The Role Of Free Water Clearance In Evaluating Water Balance Disorders In Patients With Hyponatremia

Objective:Hyponatremia is more the result of a water balance disorder than a sodium balance disorder.Among them,hypotonic hyponatremia caused by increased Arginine vasopressin（AVP）resulting in reduced free water excretion is the most common in hyponatremia,but AVP is difficult to be measured clinically.Free-water Clearance(C_H2O)quantifies kidney concentration and dilution function,quantifies kidney drainage capacity,and is easy to calculate.Therefore,this study aims to analyze the role of water balance disorder factors in the pathogenesis of hyponatremia through free water clearance,and analyze the significance of blood and urine biochemical tests in the differential diagnosis of hyponatremia,so as to provide reference for the differential diagnosis and treatment of hyponatremia in clinical work.Methods:Fifty-nine patients with hyponatremia who were hospitalized in the Second Affiliated Hospital of Dalian Medical University from June 2020 to December2020 were selected.Records the general information of the selected patient’s age,gender,and primary disease,and collection of hyponatremia patients before treatment,blood electrolyte,plasma osmolality,urine osmolality,urine volume per unit time,urine electrolyte（urine sodium）,urine uric acid,urine urea,urine creatinine,urine routine,kidney function（creatinine,urea,uric acid,cystatin C）,and other indicators.Samples of plasma osmolality and urine osmolality were collected simultaneously.Urine sodium,and urine osmolality were obtained from the same urine sample.The free water clearance was calculated according to the plasma osmolality,urine osmolality and urine volume per unit time,and the fractional excretion of sodium,uric acid and urea were calculated according to the levels of serum sodium,urine sodium,uric acid,urine uric acid,urea,urine urea,creatinine and urine creatinine.First,according to the 2013 American guidelines for hyponatremia,all patients were divided into hypotonic hyponatremia group（<280m Osm/kg）and non-hypotonic hyponatremia group（≥280 m Osm/kg）according to the plasma osmolality,to compare the levels of free water clearance,urine osmolality,and fractional excretion of sodium between the two groups.According to the 2014 Clinical practice guideline on diagnosis and treatment of hyponatremia to hypotonic hyponatremia according to urine osmolality（100 m Osm/kg as the limit）,urine sodium（30 mmol/L as the limit）and the clinical evaluation of the extracellular fluid volume can be divided into hypervolemic hyponatremia group,hypovolemic hyponatremia group,and euvolemic hyponatremia group,compare the free water clearance,urine sodium,fractional excretion of sodium,fractional excretion of urea,and fractional excretion of uric acid if there is a statistically significant.The ROC curves of urine sodium,fractional excretion of sodium,fractional excretion of urea,and fractional excretion of uric acid were analyzed to discuss their diagnostic value in Syndrome of inappropriate antidiuresis（SIAD）.Results:1.Compared with the non-hypotonic hyponatremia group,the free water clearance of the hypotonic hyponatremia group was decreased（the negative value was larger）,and the difference was statistically significant（P<0.05）.There was no statistically significant difference in fractional excretion of sodium between the hypotonic hyponatremia group and the non-hypotonic hyponatremia group（P>0.05）.2.There was a negative correlation between free water clearance and urine osmolality（r=-0.66,P<0.01）.There was a positive correlation between urine osmolality and urine specific gravity（r=0.527,P<0.05）.3.The urine sodium,fractional excretion of sodium and fractional excretion of uric acid in the hypovolemic hyponatremia group and the hypervolemic hyponatremia group were lower than those in the euvolemic hyponatremia group,with statistical significance（P<0.05）.The uric acid in the euvolemic hyponatremia group was lower than that in the hypervolemic hyponatremia group,and the difference was statistically significant（P<0.05）.4.The areas under ROC curve of urine sodium,fractional excretion of sodium in SIAD were 0.895 and 0.863,respectively,with P values of 0.001 and 0.002（P<0.05）.The areas under ROC curve of fractional excretion of urea in SIAD was 0.726,and the P value was 0.048（P<0.05）.The areas under ROC curve of fractional excretion of uric acid in SIAD was 0.779,and the P value was 0.015（P<0.05）.Conclusion:1.In the pathogenesis of hyponatremia,free water clearance can be used as a reference index to evaluate the dominant role of water balance disorder in hypotonic hyponatremia.2.The decrease in free water clearance of hypotonic hyponatremia is related to the decrease of effective arterial blood volume（hypervolemia and hypovolemia）leading to the appropriate secretion of AVP and the inappropriate secretion of AVP when euvolemic.3.Urine sodium,fractional excretion of sodium,fractional excretion of uric acid,fractional excretion of urea have a certain reference value for the diagnosis of SIAD,and can be used as a reference index to distinguish the etiology of SIAD from other hyponatremia.