Urine electrolytes and the urine anion and osmolar gaps,☆☆

https://doi.org/10.1016/S0022-2143(99)90190-7Get rights and content

Abstract

Urine ammonia concentration is crucial to understanding and quantifying the kidney's response to metabolic acidosis. This test is generally not performed by clinical laboratories. The urine anion gap and osmolar gaps have been proposed as surrogate measures of urine ammonia in patients with hyperchloremic acidosis. We measured ammonium and other electrolytes in the urine of patients attending our renal disease clinic who did not have severe metabolic acidosis and compared the results with those calculated by standard formulae for the anion and osmolar gaps. We found no correlation between measured ammonium values and the anion gap and attributed this lack of agreement to the presence in urine of substantial amounts of unmeasured inorganic anions, which the formula fails to consider. There was significant correlation between measured ammonium and the osmolar gap but not good agreement between the absolute values provided by the 2 methods. Solutes including sulfate and phosphate were quantified in 24-hour urine collections and showed great variability with respect to measured chloride and estimated protein catabolism. We conclude from these studies that there is no substitute for the direct determination of urine ammonium when an accurate concentration is desired. (J Lab Clin Med 1999;133:597-604)

Section snippets

Group I

One hundred fifty-nine randomly voided urine samples were collected from non-fasting patients at the Medical College of Virginia renal disease clinic. The male-to-female ratio was 61 to 98. Most of the patients were being followed for chronic renal failure secondary to hypertension and/or diabetes. These samples were analyzed for Na+, K+, Cl, creatinine, osmolality, and sulfate.

Group II

Thirty patients being seen in the renal clinic had routine blood chemistries performed as part of their regular

Results

The data from 159 randomly voided urine samples are shown in Table I.

. Urine electrolytes in 159 subjects

ParameterMeanSDRange95% CL
Na+ (mmol/L)72.037.09.0/187.066.0/78.0
K+ (mmol/L)28.019.03.0/147.025.0/31.0
Cl (mmol/L)96.050.012.0/257.088.0/104.0
SO4 2– (mmol/L)8.55.10.6/33.07.8/9.3
SO4 2– (mmol/g creatinine)9.74.50.5/21.09.0/10.4
Osmolality (mOsm/L)434.0144.0134.0/1019.0412.0/456.0
Na+ + K+ –Cl (mEq/L)3.730.0–113.0/124.0–0.9/8.3
Na++K+–Cl–SO4 2– (mEq/L)–13.030.0–132.0/88.0–18.0/–8.0
Whereas Na+ and

Discussion

Renal excretion of ammonium figures prominently in all discussions of the kidney's response to metabolic acidosis.1, 2, 3, 4 Therefore it is distressing that clinicians rarely have access to urinary ammonium levels that, at the very least, would be a useful measure of kidney function and might be diagnostic of the cause of the acidosis itself.9, 10, 11 Failure of the kidney to increase its output of ammonium in the setting of acidosis indicates that a defect in renal acidification may be

References (21)

There are more references available in the full text version of this article.

Cited by (0)

Reprint requests: Barry Kirschbaum, MD, Medical College of Virginia, Virginia Commonwealth University, P.O. Box 980160, Richmond, VA 23298.

☆☆

0022-2143/99 $8.00 + 0  5/1/97503

View full text