The Use (and Misuse) of Urinary Sodium and Chloride Measurements

MEASUREMENT of urinary sodium and chloride concentrations is useful whenever knowledge of whether the kidney is retaining salt will narrow the differential diagnosis. Any concentration of sodium or chloride in the urine is normal if it favors maintenance of normal systemic fluid and electrolyte status. Only when a patient's fluid and electrolyte status is abnormal and one can predict what a normal kidney's protective response would be can study of the urine electrolytes be of value. Detection of a Salt-Retaining State Urinary sodium and urinary chloride concentrations should both be determined whenever the concentration of one or the other electrolyte is sought. If the kidney is in a saltretaining state, both urinary sodium and chloride concentrations usually are less than 20 mEq/L. It is, however, not unusual for one or the other electrolyte level to be deceptively high due to the need for electroneutrality in the urine. For example, in