In a patient with non-anion gap metabolic acidosis, calculating the urine anion gap can help differentiate between diarrhea and renal tubular acidosis (RTA) as the etiology.
A. What is the urine anion gap and how is it calculated?
B. In the following case, please determine if the patient has a non-anion gap metabolic acidosis due to diarrhea or RTA:
A 24 year old male dancer comes to attention due to a low serum bicarbonate on routine blood work. Additional results include:
Serum: Na 140 meq/L, K 3 meq/L, HCO3 14 meq/L, Cl 116 meq/L
Urine: Na 50 meq/L, K 30 meq/L, Cl 40 meq/L.
Answere:
Serum: Na 140 meq/L, K 3 meq/L, HCO3 14 meq/L, Cl 116 meq/L
Urine: Na 50 meq/L, K 30 meq/L, Cl 40 meq/L.
Answere:
Urine anion gap is calculated by subtracting the urine concentration of chloride(anions) from the concentrations of sodium plus potassium (cations):
- = Na+ + K+ − Cl−
where the concentrations are expressed in units of milliequivalents/liter (mEq/L).
In contrast to the serum anion gap equation, the bicarbonate is excluded. This is because urine is acidic, so the bicarbonate level would be negligible.
Urine anion gap is an 'artificial' and calculated measure that is representative of the unmeasured ions in urine. Usually the most important unmeasured ion in urine isNH4+.
- A positive urine anion gap suggests a low urinary NH4+ (e.g. renal tubular acidosis).
- A negative urine anion gap suggests a high urinary NH4+ (e.g. diarrhea).
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