COMBINED FRACTIONAL EXCRETION OF SODIUM AND UREA BETTER PREDICTS RESPONSE TO SALINE IN HYPONATREMIA THAN DO USUAL CLINICAL AND BIOCHEMICAL PARAMETERS

BACKGROUND: The treatment of hyponatremic patients requires physicians to make a therapeutic choice between saline infusion and water restri ction. Therefore, they need readily available and reliable parameters to facilitate making that choice. This study was designed to determine whether the use of clearance ratios can help clinicians recognize sal ine-responding hyponatremic patients. PATIENTS AND METHODS: Thirty-fiv e nonedematous, hospitalized, hyponatremic patients were classified ac cording to their history and saline response into four groups: diureti c-taking patients, polydipsic patients, saline responders, and saline nonresponders. Within these four groups, clinical and biochemical volu me-related parameters, including clearance ratios, were prospectively evaluated before infusion of 2 L isotonic saline. Clearance ratios as well as usual clinical and biochemical parameters were tested for thei r accuracy in predicting saline responsiveness. RESULTS: Both positive (70%) and negative (54.5%) predictive values for hypovolemia were uns atisfactory; clinical prediction of hypovolemia was also characterized by low sensitivity (41.1%), but acceptable specificity (80%). In the polydipsia and saline-nonresponder groups, plasma urea and uric acid v alues tended to be lower than in the diuretic and saline-responder gro ups. However, the usefulness of these parameters was limited by too la rge an overlap among the different groups. In both polydipsic patients and saline responders, urinary sodium concentration was low. The comb ined amount of urinary sodium and potassium in relation to plasma sodi um did not discriminate among the different groups. Most helpful in di stinguishing among the groups was a combination of several clearance r atios (fractional excretions of sodium, potassium, urea, and uric acid ), since the predictive use of each parameter on its own was restricte d. The best indicator of saline responsiveness was a low fractional ex cretion of filtered sodium (< 0.5%) combined with a low fractional exc retion of urea (< 55%). CONCLUSION: The accuracy of clinical evaluatio n for predicting the state of extracellular fluid volume in hyponatrem ia is low. The combination oi: low fractional sodium excretion (< 0.5% ) and low fractional urea excretion (< 55%) is the best biochemical wa y to predict saline response, whereas high fractional potassium excret ion (> 20%) indicates diuretic intake.