Categories: Nephrology

What is the urine anion gap?

Urine AG = Urine (Na + K – Cl)

NOTE: In the setting of Bicarbonate loss, kidneys will retain Bicarbonate and excrete Ammonium (NH4). We do not measure NH4(+) directly; it is measured indirectly by measuring the negative ion (Chloride) it is secreted with to maintain electroneutrality (NH4Cl). 

In the setting of non-anion gap metabolic acidosis:



  • Urine anion gap is negative; Neg GUT tive
  • Kidneys are functioning properly and attempting to retain Bicarbonate
  • Urine anion gap is positive
  • Kidneys are not functioning properly and unable to excrete ammonium normally



1. Cho KC. Chapter 21. Electrolyte & Acid-Base Disorders. McPhee SJ, Papadakis MA, Rabow MW, eds. CURRENT Medical Diagnosis & Treatment 2012. New York: McGraw-Hill; 2012.

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Dr. C Humphreys

Internal Medicine

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