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Urine anion and osmolal gaps in metabolic acidosis

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INTRODUCTION

Measurement of the urine anion gap (AG) and urine osmolal gap may be helpful in the evaluation of patients with a normal AG (hyperchloremic) metabolic acidosis by providing an estimate of urinary ammonium excretion [1-3]. (See "Approach to the adult with metabolic acidosis".)

The clinical use of the urine anion and osmolal gaps will be reviewed here. Issues related to use of the plasma osmolal gap are discussed separately. (See "Plasma osmolal gap".)

URINE ANION GAP

The urine AG is calculated from the difference between the measured cations (Na and K) and the measured anion (Cl):

  Urine AG = Urine (Na + K - Cl)

The urine AG has a negative value in most patients with a normal AG metabolic acidosis due to the appropriate increase in urinary ammonium in an attempt to excrete the excess acid [1,2,4]. Ammonium is an unmeasured cation; as a result, an increase in its excretion as NH4Cl will lead to a rise in the urine Cl concentration and a negative urine AG, usually ranging from -20 to -50 meq/L.

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Last literature review version 18.2: May 2010
This topic last updated: May 15, 2008
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References
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  7. Sulyok, E, Guignard, JP. Relationship of urinary anion gap to urinary ammonium excretion in the neonate. Biol Neonate 1990; 57:98.
  8. Kamel, KS, Halperin, ML. An improved approach to the patient with metabolic acidosis: a need for four amendments. J Nephrol 2006; 19 Suppl 9:S76.
  9. Halperin, ML. Modified urine osmolal gap: An accurate method for estimating the urinary ammonium concentration. Nephron 1995; 69:100.
  10. Owen, OE, Licht, JH, Sapir, DG. Renal function and effects of partial rehydration during diabetic ketoacidosis. Diabetes 1981; 30:510.
Urine anion and osmolal gaps in metabolic acidosis