Renal Tubular Acidosis

Aka: Renal Tubular Acidosis

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II. Pathophysiology

  1. Kidney unable to excrete daily acid load
  2. Results in Metabolic Acidosis

III. 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

IV. 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)

V. 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)

VI. References

  1. Morikawa (2025) Am Fam Physician 111(2): 148-55 [PubMed]

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