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Urine anion and osmolal gaps in metabolic acidosis

Michael Emmett, MD
Biff F Palmer, MD
Section Editor
Richard H Sterns, MD
Deputy Editor
John P Forman, MD, MSc


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".)


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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Literature review current through: Nov 2017. | This topic last updated: Feb 27, 2017.
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  1. Batlle DC, Hizon M, Cohen E, et al. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. N Engl J Med 1988; 318:594.
  2. Halperin ML, Vasuvattakul S, Bayoumi A. A modified classification of metabolic acidosis: a pathophysiologic approach. Nephron 1992; 60:129.
  3. Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders, 5th ed, McGraw-Hill, New York 2001. p.590.
  4. Inase N, Ozawa K, Sasaki S, Marumo F. Is the urine anion gap a reliable index of urine ammonium excretion in most situations? Nephron 1990; 54:180.
  5. Oh M, Carroll HJ. Value and determinants of urine anion gap. Nephron 2002; 90:252.
  6. Owen OE, Licht JH, Sapir DG. Renal function and effects of partial rehydration during diabetic ketoacidosis. Diabetes 1981; 30:510.
  7. CLARKE E, EVANS BM, MACINTYRE I, MILNE MD. Acidosis in experimental electrolyte depletion. Clin Sci 1955; 14:421.
  8. Tizianello A, Garibotto G, Robaudo C, et al. Renal ammoniagenesis in humans with chronic potassium depletion. Kidney Int 1991; 40:772.
  9. Han KH, Lee HW, Handlogten ME, et al. Effect of hypokalemia on renal expression of the ammonia transporter family members, Rh B Glycoprotein and Rh C Glycoprotein, in the rat kidney. Am J Physiol Renal Physiol 2011; 301:F823.
  10. Kim GH, Han JS, Kim YS, et al. Evaluation of urine acidification by urine anion gap and urine osmolal gap in chronic metabolic acidosis. Am J Kidney Dis 1996; 27:42.
  11. Dyck RF, Asthana S, Kalra J, et al. A modification of the urine osmolal gap: an improved method for estimating urine ammonium. Am J Nephrol 1990; 10:359.
  12. Carlisle EJ, Donnelly SM, Vasuvattakul S, et al. Glue-sniffing and distal renal tubular acidosis: sticking to the facts. J Am Soc Nephrol 1991; 1:1019.
  13. Sulyok E, Guignard JP. Relationship of urinary anion gap to urinary ammonium excretion in the neonate. Biol Neonate 1990; 57:98.
  14. Rolleman EJ, Hoorn EJ, Didden P, Zietse R. Guilty as charged: unmeasured urinary anions in a case of pyroglutamic acidosis. Neth J Med 2008; 66:351.
  15. Duewall JL, Fenves AZ, Richey DS, et al. 5-Oxoproline (pyroglutamic) acidosis associated with chronic acetaminophen use. Proc (Bayl Univ Med Cent) 2010; 23:19.
  16. 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.
  17. Meregalli P, Lüthy C, Oetliker OH, Bianchetti MG. Modified urine osmolal gap: an accurate method for estimating the urinary ammonium concentration? Nephron 1995; 69:98.