Non anion gap acidosis
Background
- Non-anion gap metabolic acidosis (NAGMA) = metabolic acidosis with a normal anion gap (typically 8-12)
- Caused by loss of bicarbonate (GI or renal) or addition of chloride (hyperchloremic acidosis)
- Mnemonic: HARDUPS — Hyperalimentation, Acetazolamide, RTA, Diarrhea, Ureteral diversion, Pancreatic fistula, Saline infusion
- Distinguish from anion gap metabolic acidosis (AGMA) which has a different differential
Clinical Features
- Symptoms of underlying cause (diarrhea, polyuria)
- Kussmaul breathing (compensatory hyperventilation)
- May be asymptomatic if mild
Differential Diagnosis
- Hyperkalemia
- Resolving DKA
- Early uremic acidosis
- Early obstructive uropathy
- RTA Type IV
- Hypoaldosteronism
- K-sparing diuretics
- Hypokalemia
- RTA Type I
- RTA Type II
- Acetazolamide
- Acute diarrhea
- (May be assoc with gap if hypoperfusion -> lactic acidosis)
- CKD
- Intestinal, pancreatic, biliary fistula
- Hyperchloremic IVF infusions
- Hyperalimentation
GI Bicarbonate Loss
- Diarrhea (most common cause)
- Pancreatic fistula or drainage
- Ureteral diversion (ileal conduit, ureterosigmoidostomy)
- Cholestyramine
Renal Bicarbonate Loss / Impaired Acid Excretion
- Renal tubular acidosis (RTA)
- Type 1 (distal): impaired H+ secretion, urine pH >5.5, hypokalemia
- Type 2 (proximal): impaired HCO3 reabsorption, urine pH <5.5 after bicarb depleted
- Type 4 (hypoaldosteronism): hyperkalemia, urine pH <5.5
- Early renal failure
- Carbonic anhydrase inhibitors (acetazolamide, topiramate)
Iatrogenic
- Excessive normal saline infusion (dilutional/hyperchloremic)
- Hyperalimentation (TPN)
Evaluation
- BMP: calculate anion gap (Na - Cl - HCO3), check potassium
- Urine anion gap (Na + K - Cl) helps distinguish GI from renal cause:
- Negative urine AG = GI loss (kidneys appropriately excreting NH4+)
- Positive urine AG = renal cause (RTA — kidneys cannot excrete acid)
- Urine pH: >5.5 suggests Type 1 RTA
- Serum potassium: low (Type 1, 2 RTA, diarrhea), high (Type 4 RTA)
- Urine electrolytes
Management
- Treat underlying cause (e.g., volume replacement for diarrhea)
- Sodium bicarbonate for severe acidosis (pH <7.1) or symptomatic
- Correct potassium abnormalities
- Stop offending medications (acetazolamide, excessive NS)
- Type 4 RTA: treat hyperkalemia, consider fludrocortisone
Disposition
- Admit if severe acidosis, hemodynamically unstable, or significant electrolyte derangement
- Discharge if mild, correctable cause identified, and electrolytes stable