The Urine Anion Gap: Common Misconceptions

Area of Science:

• Nephrology
• Clinical Chemistry
• Internal Medicine

Background:

• Previous studies (1986, 1988) suggested a strong inverse correlation between urinary anion gap (UAG) and urine ammonia excretion (UNH4).
• This led to the widespread acceptance of UAG as an indirect measure of UNH4 in metabolic acidosis patients.
• This review critically examines this long-standing postulate.

Purpose of the Study:

• To review factors regulating UAG.
• To examine evidence challenging the postulate that UAG indirectly measures UNH4.
• To highlight errors in original study designs and interpretations.

Main Methods:

• Literature review of studies investigating UAG and UNH4.
• Theoretical analysis of electrolyte balance and acid excretion.
• Examination of factors influencing normal UAG values over time.

Main Results:

• In steady-state conditions, UAG primarily reflects sodium (Na), potassium (K), and chloride (Cl) intake.
• Discrepancies in UAG indicate extrarenal electrolyte losses or non-steady states, not ammonia excretion.
• Urine ammonia excretion (UNH4) depends mainly on the daily acid load, independent of UAG in the absence of renal dysfunction.
• Observed correlations between UAG and UNH4 are often fortuitous and not universally applicable.
• Normal UAG values have significantly increased due to dietary changes (increased K intake, use of non-chloride food additives).

Conclusions:

• The postulate linking UAG to UNH4 is flawed and lacks theoretical and empirical support.
• UAG is not a reliable indirect measure of UNH4 in metabolic acidosis.
• Clinicians must consider the increased normal UAG values and factors influencing them during interpretation.