Approach to normal anion gap metabolic acidosis

The approach to metabolic acidosis with a normal anion gap is shown in Fig 1. Note that it is first important to ensure that the anion gap is indeed normal and not spurious (i.e. not greater than normal when corrected for albumin). Note also that urinary electrolyte determinations in the intensive care unit (ICU) are often confounded by concomitant administration of medications, particularly diuretics.

Tubular Acidosis Anion Gaps

Fig. 1 Approach to normal anion gap metabolic acidosis. The mnemonic for normal anion gap (AG) acidosis is 'if No GO P, you're DeHD'. If the serum AG is normal and there are no features suggesting a cause of a low serum AG, the urinary AG should be calculated from urinary electrolytes. If the urine AG is positive, it is the result of either diminishing NH 4+ secretion or the presence of osmotically active ketoacids. This can be distinguished by determining the urinary osmolar gap (OG): OG = (measured urinary osmolality) - {2([Na+]u + [K+]u) + (urinary urea) + (urinary glucose)}. The clinical situation and/or plasma [K +] determine probable final diagnosis, but special tests may be necessary to confirm this or to differentiate further various forms of renal tubular acidosis (RTA).

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