Urine anion and osmolal gaps in metabolic acidosis
- Michael Emmett, MD
Michael Emmett, MD
- Editor-in-Chief — Nephrology
- Section Editor — Fluid and Electrolytes
- Chief of Internal Medicine
- Baylor University Medical Center
- Biff F Palmer, MD
Biff F Palmer, MD
- Professor of Internal Medicine
- University of Texas Southwestern Medical Center
Measurement of the urine anion gap (UAG) and/or urine osmolal gap (UOG) may be helpful in the evaluation of patients with a normal anion gap (hyperchloremic) metabolic acidosis by providing an estimate of urinary ammonium (NH4) excretion (table 1) [1-5]. The normal renal response to metabolic acidosis is to increase urinary NH4 excretion. (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 serum anion and osmolal gaps are discussed separately. (See "The delta anion gap/delta HCO3 ratio in patients with a high anion gap metabolic acidosis" and "Serum osmolal gap".)
BRIEF OVERVIEW OF RENAL ACID EXCRETION
Ingestion of a typical Western diet generates approximately 50 to 100 meq of nonvolatile acid (ie, acids other than CO2) per day. To maintain acid balance, these nonvolatile acids must be excreted in the urine. The hydrogen ions are excreted via a process that includes the following:
●First, filtered bicarbonate must be reabsorbed since bicarbonate loss is equivalent to the generation of hydrogen ions.
●Next, the hydrogen ions associated with the nonvolatile acid that is generated must be excreted. Only trivial amounts of free hydrogen ions can be excreted since, at a urine pH of 4.5, the free hydrogen ion concentration is less than 0.04 meq/L. Thus, urine hydrogen ions must be bound to buffers such as phosphate (converting HPO4 to H2PO4) or to ammonia (converting NH3 to the ammonium ion, NH4) to be excreted in large amounts.
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