🧪 Renal Tubular Acidosis (RTA) is characterized by the inability of the kidneys to excrete acidic urine, leading to a persistently high urinary pH >5.5 even under an acid load. It results in a normal anion gap (hyperchloremic) metabolic acidosis due to failure of renal acid excretion.

ℹ️ About

The body is a net acid producer from metabolism (≈1 mmol/kg/day H⁺ ions).
Normal plasma is slightly alkaline (pH ~7.40), while urine is normally acidic (pH 5–6).
RTA arises from failure of the tubules to handle H⁺ or HCO₃⁻ appropriately.

🧬 Aetiology

Defects in tubular transport mechanisms.
Failure in:
- Distal tubule H⁺ secretion (distal RTA).
- Proximal tubule HCO₃⁻ reabsorption (proximal RTA).
Leads to normal anion gap (hyperchloremic) metabolic acidosis.

📑 Types of RTA

Type 1 (Distal RTA): Failure to excrete H⁺ ions → urine cannot be acidified.
Type 2 (Proximal RTA): Failure to reabsorb HCO₃⁻.
Type 3: Rare, mixed features of Types 1 & 2.
Type 4: Hypoaldosteronism/hyporeninaemia → impaired H⁺ & K⁺ excretion → hyperkalemic RTA.

Type	Defect	Associations	Key Management
1 (Distal)	Failure to excrete H⁺ (↓ urine acidification)	SLE, Sjögren’s, sickle cell, Ehlers-Danlos, autoimmune disease, toxins (Toluene, Lithium, Amphotericin)	Oral bicarbonate; prevent renal stones
2 (Proximal)	Failure to reabsorb HCO₃⁻	Fanconi syndrome, Wilson’s disease, amyloidosis, myeloma	Oral bicarbonate ± Vitamin D; phosphate replacement
3 (Mixed)	Features of Type 1 + 2 (rare)	Renal insufficiency	Treat underlying cause
4 (Hyperkalemic)	Reduced aldosterone effect → ↓ H⁺ & K⁺ excretion	Diabetes, interstitial nephritis, ACEi/ARB, NSAIDs, K-sparing diuretics	Fludrocortisone, loop/thiazide diuretics, K⁺ binders, bicarbonate

🩺 Clinical Features

Type 1: Growth delay, nephrolithiasis, nephrocalcinosis, polydipsia, muscle weakness.
Type 2: Growth delay, rickets/osteomalacia, Fanconi syndrome (glycosuria, aminoaciduria, phosphate wasting).
Type 4: Often asymptomatic; may present with hyperkalemia in diabetics or interstitial nephritis.

🔎 Investigations

U&E: Hypokalaemia (Types 1 & 2), Hyperkalaemia (Type 4).
VBG: Normal anion gap metabolic acidosis (hyperchloremic).
Urine pH: >5.5 in Type 1 (inability to acidify), variable in Type 2, normal/low in Type 4.

🧪 Diagnostic Tests

Type 1: NaHCO₃ infusion → failure to generate urine–blood pCO₂ gradient.
Type 2: NaHCO₃ loading test; check for Fanconi features.
Type 4: NH₄⁺ loading test; confirms impaired distal acidification.

💊 Management

Alkali therapy (oral bicarbonate or citrate) for Types 1 & 2.
Potassium supplementation for hypokalaemic patients (Types 1 & 2).
Vitamin D and phosphate for osteomalacia/rickets (Type 2).
For Type 4: fludrocortisone (if hypoaldosteronism), loop/thiazide diuretics, potassium binders, alkali therapy.

💡 Exam Pearls:
• RTA = normal anion gap metabolic acidosis (hyperchloremic).
• Type 1 → renal stones/nephrocalcinosis, urine pH >5.5.
• Type 2 → Fanconi features, phosphate wasting.
• Type 4 → hyperkalaemia, common in diabetics on ACEi/ARB.

📚 References

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Renal Tubular Acidosis (RTA)