Urine Anion Gap Calculator

What is the Urine Anion Gap (UAG)?

The Urine Anion Gap (UAG), also known as the urinary net charge, is a clinical calculation utilized in the evaluation of normal anion gap (hyperchloremic) metabolic acidosis (NAGMA). It provides an indirect clinical surrogate for the kidney's excretion rate of ammonium ($\text{NH}_4^+$), allowing physicians to differentiate between gastrointestinal bicarbonate loss and impaired renal acid secretion.

The Urine Anion Gap Formula

The calculation is based on the principle of urinary electroneutrality:

$$\text{UAG} = \left([\text{Na}^+] + [\text{K}^+]\right) - [\text{Cl}^-]$$

Where concentrations of urine sodium ($\text{Na}^+$), potassium ($\text{K}^+$), and chloride ($\text{Cl}^-$) are expressed in $\text{mEq/L}$ or $\text{mmol/L}$.

Physiological Basis of the Urinary Net Charge

In healthy kidneys responding to metabolic acidosis, the proximal and collecting tubules substantially upregulate ammonia synthesis ($\text{NH}_3$) and ammonium secretion ($\text{NH}_4^+$). Because ammonium carries a positive charge and is excreted largely with chloride ($\text{Cl}^-$) to maintain electroneutrality:

• When $\text{NH}_4^+$ excretion is appropriately elevated (e.g. up to $100\text{ to }200\text{ mEq/L}$ during diarrhea), urine $[\text{Cl}^-]$ dramatically exceeds $([\text{Na}^+] + [\text{K}^+])$, generating a negative UAG (typically $-20\text{ to }-50\text{ mEq/L}$).
• When renal tubular mechanisms fail to excrete sufficient $\text{NH}_4^+$ (as in distal Renal Tubular Acidosis or hypoaldosteronism), urine $[\text{Cl}^-]$ remains low, generating a positive UAG (typically $+10\text{ to }+40\text{ mEq/L}$).

Differential Diagnosis in Normal Anion Gap Metabolic Acidosis

Limitations and Pitfalls: When to Use the Urine Osmolal Gap

The Urine Anion Gap relies on the assumption that chloride is the sole major unmeasured urinary anion. If unmeasured anions are present in significant concentrations, they will be excreted with $\text{Na}^+$ or $\text{K}^+$, resulting in a falsely positive UAG despite high $\text{NH}_4^+$ levels. Such unmeasured anions include:

• Ketoacid anions (acetoacetate, $\beta$-hydroxybutyrate) in diabetic ketoacidosis or starvation ketosis.
• Drug metabolites such as penicillins (e.g. high-dose piperacillin or carbenicillin) or salicylates.
• Toluene inhalation resulting in hippurate excretion.

In these scenarios, or when urine $[\text{Na}^+]$ is extremely low ($< 20\text{ mEq/L}$), clinicians should calculate the Urine Osmolal Gap instead.

For related calculations, see our Anion Gap Calculator and TTKG Calculator.