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)