Measurement of the urine anion gap and urine osmolal gap 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 Δanion gap/ΔHCO3 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 hydrogen ions (nonvolatile acid: ie, acids other than CO2) per day. To maintain acid balance, these hydrogen ions must be excreted in the urine in a process that includes the following (see "Chapter 11A: Renal hydrogen excretion"):
- Reabsorption of filtered bicarbonate, since bicarbonate loss is equivalent to the generation of hydrogen ions.
- Excretion of the daily nonvolatile acid load. Only trivial amounts of free hydrogen can be excreted since, at a urine pH of 4.5, the free hydrogen ion concentration is less than 0.04 meq/L. Thus, free hydrogen ions are either bound to buffers (such as phosphate) or to ammonia (NH3) to form ammonium (NH4).
- The main adaptive renal response to metabolic acidosis is to increase hydrogen ion excretion in the form of NH4. In patients with severe metabolic acidosis and normal renal function, NH4 excretion can increase from the normal value of 30 to 40 meq/day to as much as 200 to 300 meq/day [6,7]. NH4 is usually excreted with Cl but is sometimes excreted with other anions such as ketoacid anions, hippurate, or HCO3.
URINE ANION GAP
Calculation of the difference between the urine sodium (Na) plus potassium (K) concentration and the urine chloride (Cl) concentration can be helpful in patients with various acid-base disorders. This calculation has been called the urine anion gap (UAG):