How is urinary net change determined in the workup of hyperchloremic acidosis?

Updated: Oct 18, 2018
  • Author: Sai-Ching Jim Yeung, MD, PhD, FACP; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Answer

Urinary ammonium excretion (urine AG; urine net charge) is inferred from the urine AG, also known as the urine net charge, when direct measurement of ammonium is not possible.

The urine net charge is defined as follows: UNA+ + UK+ - UCl-. In this equation, UNA+ is the urinary concentration of sodium, UK+ is the urinary concentration of potassium, and UCl- is the urinary concentration of chloride. The urinary net charge and ammonium excretion have a linear relationship. When excretion of Cl- exceeds that of Na+ and K+, the urinary net charge is negative, and the assumption is that a substantial concentration of ammonium is present in the urine, which would be the case in metabolic acidosis of nonrenal origin.

Conversely, in hypokalemic and hyperkalemic dRTA, the urine concentration of ammonium is insufficient, excretion of Na+ and K+ exceed that of Cl-, and the urinary net charge is positive.

This method of analysis has potential pitfalls. A negative urine AG is also observed in patients whose acidosis is due to nonrenal causes but in whom maximal acidification fails because of decreased presentation of sodium to the distal nephron. In these cases, the urinary sodium concentration is very low. Urinary excretion of ketoanions secondary to systemic ketoacidosis can cause a positive AG despite adequate ammonium excretion. Thus, ketonuria should also be excluded in cases of metabolic acidosis in which the etiology is uncertain enough to warrant calculation of the urine AG.

The urine net charge is also less useful when large amounts of bicarbonate are present in the urine (pH >6).


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