This diagnostic assessment estimates the concentration of unmeasured anions in urine. The process involves measuring urinary sodium, potassium, and chloride, and then applying a specific formula to derive the estimated anion concentration. For instance, if the urinary sodium is 100 mEq/L, potassium is 50 mEq/L, and chloride is 120 mEq/L, the calculated value would be 30 mEq/L (100 + 50 – 120 = 30).
The significance of this evaluation lies in its ability to differentiate between various causes of metabolic acidosis with a normal serum anion gap, particularly renal tubular acidosis and gastrointestinal bicarbonate loss. A negative result suggests increased ammonium excretion by the kidneys in response to metabolic acidosis, pointing towards bicarbonate loss from the gastrointestinal tract. Conversely, a positive result indicates impaired ammonium excretion, potentially indicative of distal renal tubular acidosis. Historically, this tool has aided clinicians in accurately diagnosing acid-base disorders when serum electrolytes alone provide insufficient information.
