Etiology

• Type I (distal H⁺ secretion defect): low serum K⁺, urine pH >5.5, associated with autoimmune disease, hypercalcemia
• Type II (proximal HCO₃^- reabosrbtion defect): low serum K⁺, urine pH < 5.5, associated with multiple myeloma, drugs (eg, sulfa)
• Type III (rare): normal serum K⁺, urine pH < 5.5, associated with renal insufficiency
• Type IV (hyporeninemic hypoaldosteronism): high serum K⁺, urine pH < 5.5, associated with diabetes mellitus, drugs (eg, NSAIDs)

DDx

Other causes of normal gap metabolic acidosis

• Gastrointestinal loss of HCO₃^-, eg, diarrhea, pancreatic ileostomy or ileal loop bladder
• Renal tubular acidosis
• Recovery from diabetic ketoacidosis
• Dilutional acidosis from rapid administration of 0.9% NaCl
• Carbonic anhydrase inhibitors
• Chloride retention or administration of HCl equivalent or NH₄Cl

See related DDx

• Hypokalemia