Measurement of the urinary electrolyte concentrations, osmolality, and pH play an important role in the diagnosis and management of a variety of disorders. This chapter will briefly review the meaning of these parameters and the settings in which they may be helpful (table 1). It is important to emphasize that there are no fixed normal values, since the kidney varies the rate of excretion to match net dietary intake and endogenous production. Thus, interpretation of a given test requires knowledge of the patient's clinical state. As an example, the urinary excretion of 125 meq of Na+ per day may be appropriate for a subject on a regular diet, but represents inappropriate renal Na+ wasting in a patient who is volume-depleted.
In addition to their clinical usefulness, these tests are simple to perform and are widely available. In most circumstances, a random urine specimen is sufficient, although a 24-h collection to determine the daily rate of solute excretion is occasionally indicated. When K+ depletion is due to extrarenal losses, for example, the urinary K+ excretion should fall below 25 meq/day. A random measurement, however, may be confusing in some patients. If the urine output is only 500 mL/day because of associated volume depletion, then the appropriate excretion of only 20 meq of K+ per day will be associated with an apparently high urine K+ concentration of 40 meq/L (20 meq/day ÷ 0.5 L/day = 40 meq/L).
The kidney varies the rate of Na+ excretion to maintain the effective circulating volume, a response that is mediated by a variety of factors including the renin-angiotensin-aldosterone system and perhaps atrial natriuretic peptide and related peptides. (See "Chapter 8C: Regulation of renal Na+ excretion".) As a result, the urine Na+ concentration can be used as an estimate of the patient's volume status. In particular, a urine Na+ concentration below 20 meq/L is generally indicative of hypovolemia. This finding is especially useful in the differential diagnosis of both hyponatremia and acute renal failure. The two major causes of hyponatremia are effective volume depletion and the syndrome of inappropriate antidiuretic hormone secretion (SIADH). The urine Na+ concentration should be low in the former, but greater than 40 meq/L in the SIADH, which is characterized by water retention but normal Na+ handling (ie, equal to intake). (See "Evaluation of the patient with hyponatremia".)
Similar considerations apply to acute renal failure which is most often due to volume depletion or acute tubular necrosis . The urine Na+ concentration usually exceeds 40 meq/L in the latter, in part because of the associated tubular damage and a consequent inability to maximally reabsorb Na+ [1-3]. Measuring the fractional excretion of Na+ and the urine osmolality also can help to differentiate between these conditions (see below).
In normal subjects, urinary Na+ excretion roughly equals average dietary intake. Thus, measurement of urinary Na+ excretion (by obtaining a 24-h collection) can be used to check dietary compliance in patients with primary hypertension (formerly called "essential" hypertension). Restriction of Na+ intake is frequently an important component of the therapeutic regimen [4,5], and adequate adherence should result in the excretion of less than 100 meq/day.