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Renal Tubular Acidosis

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Renal Tubular Acidosis

  • See Also
  1. Type I Renal Tubular Acidosis: Distal RTA
  2. Type II Renal Tubular Acidosis: Proximal RTA
  3. Type III Renal Tubular Acidosis
  4. Type IV Renal Tubular Acidosis
  • Pathophysiology
  1. Kidney unable to excrete daily acid load
  2. Results in Metabolic Acidosis
  • Types
  1. Type I Renal Tubular Acidosis: Distal RTA (Classic RTA)
  2. Type II Renal Tubular Acidosis: Proximal RTA
  3. Type III Renal Tubular Acidosis
  4. Type IV Renal Tubular Acidosis: Hyperkalemic RTA
  • Labs
  1. Chemistry Panel
    1. Serum Chloride elevated
    2. Serum bicarbonate decreased
    3. Anion Gap normal
  2. Arterial Blood Gas (ABG)
    1. Non-Anion Gap Metabolic Acidosis
  3. Fractional Excretion of Bicarbonate
    1. FE-HCO3 <5%: Distal RTA
    2. FE-HCO3 >15%: Proximal RTA
      1. Assumes serum bicarbonate >20 meq/L
  4. Urine Anion Gap (obtain urine Electrolytes)
    1. Urine Anion Gap = Urine Sodium + Urine Potassium - Urine Chloride
    2. Decreased or Normal Urine Anion Gap <-10
      1. Extrarenal (e.g. Diarrhea, TPN) Non-Anion Gap Metabolic Acidosis
      2. Appropriate renal ammonia excretion
    3. Increased Urine Anion Gap >+10
      1. Renal Non-Anion Gap Metabolic Acidosis
      2. Impaired renal ammonia excretion (e.g. Renal Tubular Acidosis)
  • Evaluation
  1. Step 1: Lab findings suggestive of Renal Tubular Acidosis (RTA)
    1. Non-Anion Gap Metabolic Acidosis AND
    2. Urine Anion Gap Positive
      1. Negative Urine Anion Gap suggests gastrointestinal losses
  2. Step 2: Serum Potassium
    1. Low or normal: Go to Step 3
    2. High Serum Potassium
      1. Type IV Renal Tubular Acidosis
      2. Aldosterone Deficiency
  3. Step 3: Urine pH
    1. High Urine pH >5.3
      1. Type I Renal Tubular Acidosis (Distal RTA, Classic RTA)
    2. Low or normal Urine pH (or variable Urine pH)
      1. Type II Renal Tubular Acidosis (Proximal RTA)
  • References
  1. Morikawa (2025) Am Fam Physician 111(2): 148-55 [PubMed]