Urine anion gap is the difference between the sodium and chloride ions in the urine. To put it simply
Urine Anion Gap = Urine Sodium minus Urine Chloride
Urine Anion Gap = Urine Na – Urine Cl
Normal urine anion gap is usually less than 10 meq/L.
The utility of calculating the urine anion gap is in differentiating between Renal Tubular Acidosis (RTA) and Diarrhea specifically as the causes of metabolic acidosis which has a normal anion gap.
Anion gap is calculated both for serum and urine. The conventional anion gap and more commonly advised and investigated is the serum anion gap. This anion gap is calculated as the difference between sodium ions minus chloride and bicarbonate ions combined.
Serum is the blood plasma component of blood which lacks fibrinogen.
Serum Anion Gap = Sodium minus (Chloride plus Bicarbonate)
Serum Anion Gap = Na – ( Cl + HCO3 )
Normal serum anion gap is (6 – 12).
The urine anion gap is positive in case of renal tubular acidosis compared to diarrhea which is a negative value.
Acid is excreted in the form of ammonium chloride by the kidneys. In renal tubular acidosis , the acid secretion and excretion into the urine is impaired and defective. Chloride in the urine is a representative of the acid. Meaning, the more acidic the urine, the more will it contain chloride. Similarly, the less acidic the urine, the less it will contain chloride. So this renal tubular acidosis leads to a decreased chloride level in the urine and hence a positive UAG.
Diarrhea leads to a metabolic acidosis. the kidney as a compensatory measure tries to get rid of this acid by increasing its excretion causing an acidic urine to be formed and excreted. As we discussed earlier that more chloride is synonymous with more acid. Hence the excess chloride excreted in urine causes the URINE ANION GAP to become a negative value.
Diarrhea and Renal tubular acidosis both are associated with a normal serum anion gap and UAG helps to differentiate between these two disorders and determines the diagnosis.
RTA = Positive Urine Anion Gap
Diarrhea = Negative Anion Gap