UAG

Q: Which one is the right formula for Urine An-ion Gap (UAG)?

A) Urine (Na + K - Cl)
B) Urine (Na + HCO3 - Cl)
C) Urine (Na - K + Cl)
D) Urine (Na + K + Cl)
E) Urine (Na + K - HCO3)


Answer: A

The UAG is not a perfect science but it helps in determining the cause of metabolic acidosis.

The UAG gets calculated as the difference between the sum of the urine sodium (Na) plus potassium (K) concentrations and the urine chloride (Cl) concentration (choices C & D wrong).

 UAG = Urine (Na + K - Cl)

The formula for the UAG is different from the formula to calculate the serum anion gap. The serum anion gap is the difference between the serum sodium and the sum of the serum chloride and bicarbonate concentrations (choices B & E are wrong).

The quantity of sodium and potassium absorbed by the gastrointestinal tract usually exceeds the quantity of absorbed chloride. Thus,  the UAG usually has a positive value between 20 and 90. The best utility to measure UAG is in large volume and/or chronic watery diarrhea, where the loss of sodium and potassium occurs in the stool without a parallel fall in chloride excretion. This results in negative UAG.

 If UAG stays positive with a hyperchloremic or nonanion gap metabolic acidosis, it indicates low or normal NH4 excretion, such as a distal RTA.

#acid-base-balance
#nephrology
#gastroenterology



References:


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. Oh M, Carroll HJ. Value and determinants of urine anion gap. Nephron 2002; 90:252. 

 3. 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.