HDCN Article Review/Hyperlink

Kim GH, Han JS, Kim YS, Joo KW, Kim S, Lee JS

Evaluation of urine acidification by urine anion gap and urine osmolal gap in chronic metabolic acidosis

Am J Kidney Dis (Jan) 27:42-47 1996

Kim et al describe an evaluation of two urinary indices, the urine anion gap and the urine osmolal gap in two settings of chronic metabolic acidosis of renal origin, namely chronic renal failure (CRF) and distal renal tubular acidosis (DRTA). These two indices had been proposed originally by Halperin and his colleagues from Toronto, and had been evaluated by others as well. The reason for the introduction of these indices was to find a simple bedside estimate of urinary ammonia excretion as measurements in clinical labs remain largely unavailable.

Kim et al show a clear separation between the values of the urine anion gap measured in patients with chronic renal failure (CRF) or distal renal tubular acidosis (DRTA) compared to normal volunteers rendered acidotic by administration of ammonium chloride for 3 days. There was, however, no difference between the values in CRF and DRTA. The values for the urine osmolal gap also separated adequately between DRTA and normal acidotic controls. No values were obtained in CRF.

Kim et al enter the fray of the discussion of the urinary indices in metabolic acidosis (urine anion gap vs urine osmolal gap) with what appears to be a conciliatory set of observations. Both parameters in their hands appear to yield identical results. The broader issue, however, is whether their findings impact on the diagnostic approaches to chronic acidosis. Their CRF patients for example had a median creatinine of 10 mg/dl and none were on dialysis. It is difficult to conceive that the cause of the acidosis in this setting requires the determination of any urinary indices. Further, these tests could not differentiate between CRF and DRTA so their value in determining whether the patient has a pure CRF acodisis or one with an overlaping DRTA is negligible.

As the number of publications on these urinary indices increases, it is becoming apparent that their diagostic values gradually diminish. More and more confounding variables are identified and ultimately there will be a return to the gold standard of acid-base diagnosis: a physician with a keen mind! (Salim K. Mujais MD, Northwestern University, Chicago)