Normal anion gap acidosis

In renal physiology, normal anion gap acidosis, and less precisely non-anion gap acidosis, is an acidosis that is not accompanied by an abnormally increased anion gap.

The most common etiology of normal anion gap acidosis is diarrhea with a renal tubular acidosis being a distant second.

Differential diagnosis
The differential diagnosis of normal anion gap acidosis is relatively short (when compared to the differential diagnosis of acidosis):
 * Hyperalimentation
 * Acetazolamide and other carbonic anhydrase inhibitors
 * Renal tubular acidosis
 * Diarrhea: due to a loss of bicarbonate. This is compensated by an increase in chloride concentration, thus leading to a normal anion gap, or hyperchloremic, metabolic acidosis. The pathophysiology of increased chloride concentration is the following: fluid secreted into the gut lumen contains higher amounts of Na+ than Cl−; large losses of these fluids, particularly if volume is replaced with fluids containing equal amounts of Na+ and Cl−, results in a decrease in the plasma Na+ concentration relative to the Cl−concentration. This scenario can be avoided if formulations such as lactated Ringer’s solution are used instead of normal saline to replace GI losses.
 * Ureteroenteric fistula - an abnormal connection (fistula) between a ureter and the gastrointestinal tract
 * Pancreaticoduodenal fistula - an abnormal connection between the pancreas and duodenum
 * Spironolactone

It can be remembered with the mnemonic HARD-UPS.

As opposed to high anion gap acidosis (which involves increased organic acid production), normal anion gap acidosis involves either increased production of chloride (hyperchloremic acidosis) or increased excretion of bicarbonate.

One more commonly used older mnemonic is "USED CARP":
 * U - ureterosigmoidostomy
 * S - saline administration (in the face of renal dysfunction)
 * E - endocrine (Addisons, spironolactone, triamterene, amiloride, primary hyperparathyroidism)
 * D - diarrhea
 * C - carbonic anhydrase inhibitors
 * A - ammonium chloride
 * R - renal tubular acidosis
 * P - pancreatitis