HDCN Article Review/Hyperlink

Kamel KS, Briceno LF, Sanchez MI, Brenes L, Yorgin P, Kooh SW, Balfe JW, Halperin ML

A new classification for renal defects in net acid excretion

Am J Kidney Dis (Jan) 29:136-146 1997

The authors discuss a new classification system for evaluation of hyperchloremic metabolic acidosis (HCMA), pointing out that the traditional classification schema of proximal, distal, and "Type IV" RTA poorly characterizes the pathophysiology of the disorder. Moreover, some patients have more global defects in renal acidification involving the entire nephron.

This revised classification of renal acidoses includes three categories:

  • excessive loss of bicarbonate
  • low ammonium excretion
  • excessive excretion of metabolizable organic anions (potential bicarbonate).
The urine anion gap or net charge ((Na + K) - Cl)) will be positive with any form of RTA due to decreased ammonium (and chloride) excretion; however, it may also be positive in some patients without RTA if there is excessive excretion of organic anions, e.g., ketoacid anions, because the excess ammonium excretion is accompanied by these unmeasured organic anions rather than chloride. Therefore, the authors suggest measuring the urine osmolal gap (urine osmolality - (2(Na + K) + urea + glucose)) in order to assess ammonium excretion, since urine NH4+ can be approximated by 0.5 x osmolal gap (assuming NH4+ and its accompanying anion are the major unmeasured osmotically active particles in urine). A low osmolal gap (e.g., < 100 mmol/kg) suggests low ammonium excretion and thus RTA. In such patients, a high urine pH (>6) suggests decreased distal H+ secretion and/or excessive distal delivery of HCO3- (i.e., "distal" and/or "proximal" RTA), whereas a low urine pH (<5) is suggestive of low GFR or hyperkalemia. A urine pH in the 5-6 range suggests interstitial disease.

A high osmolal gap where the majority of the urine anions are chloride suggests diarrhea or exogenous acid loading; however, if the osmolal gap is high and the urine chloride is low (+ urine net charge), increased excretion of an unmeasured anion such as hippurate (toluene exposure), ketoacid anions, or D-lactate should be considered. (David J. Leehey, M.D., Loyola University at Chicago)