Excretion of electrolytes in the urine shows balance between anions (negatively charged) and cations (positively charged). Commonly measured electrolytes are sodium, potassium and chloride. The difference between measured cations (potassium and sodium) and anions (chloride) is termed the anion gap which is usually explained by ammonium (cation). In patients with acidosis the urine anion gap can indicate whether the acidosis is due to renal or nonrenal causes by reflecting ammonium excretion.
The excreted electrolytes along with glucose and urea are responsible for the urine osmolality. The difference between measured osmolality and an estimate based on sodium, potassium, urea and glucose is termed the osmolal gap. This also can be used to estimate ammonium excretion.
urine anion gap =
= (sodium in mEq/L) + (potassium in mEq/L) - (chloride in mEq/L)
estimated urine osmalality =
= (2 * (sodium in mEq/L)) + (2 * (potassium in mEq/L)) + (urea in mmol/L) + (glucose in mmol/L))
urine osmolal gap =
= (measured urine osmolality) - (estimated urine osmolality)
urine ammonium =
= 0.5 * (urine osmolal gap)
where:
• Conversion of glucose from mg/dL to mmol/L involves multiplying by 0.0555 (equivalent to 1/18)
• Conversion of urea from mg/dL to mmol/L involves multiplying by 0.357 (equivalent to 1/2.3)
Interpretation:
• Negative values for the urine anion gap indicate normal renal ammonium excretion. In a patient with acidosis this suggests a nonrenal cause for the acidosis.
• Positive values for the urine anion gap indicate impaired renal ammonium excretion. In a patient with acidosis this suggests a renal cause for the acidosis.
Limitations:
• Concurrent volume depletion due to GI losses can result in a false positive anion gap since distal acidification of the urine may be impaired due to a decrease in distal sodium delivery
• With an elevated serum anion gap acidosis (lactic acidosis, ketoacidosis, etc.), unmeasured anions may also be present in the urine, causing a positive urine anion gap despite a nonrenal cause of the acidosis.
