Renal tubular acidosis - types and causes (type 1, 2, 4)
Core concept
- Normal anion gap (hyperchloraemic) metabolic acidosis with preserved or near-preserved GFR
- First fork: urine anion gap = (UNa + UK) - UCl, a surrogate for urinary NH4+
- Negative ("neGUTive") = GI bicarbonate loss - diarrhoea, ileostomy
- Positive = RTA - kidney cannot generate NH4+
- Unreliable when other unmeasured anions are present (ketones, hippurate) - use the urine osmolal gap
| Type 1 distal | Type 2 proximal | Type 4 | |
|---|---|---|---|
| Defect | Impaired distal H+ secretion (alpha-intercalated cell H+-ATPase) | Impaired proximal HCO3 reclamation (NHE3, CA, NBCe1) | Aldosterone deficiency or tubular resistance |
| Serum K | Low | Low | HIGH |
| Urine pH | >5.5 always | Variable - falls <5.5 once serum HCO3 drops below the reduced threshold | Usually <5.5 |
| Serum HCO3 | Can be <10 | Plateaus 12-18 | Mild, 16-22 |
| Stones | Nephrocalcinosis, calcium phosphate stones | No - osteomalacia/rickets | No |
| FE HCO3 on load | <5% | >15% | - |
| Alkali requirement | 1-2 mmol/kg/day | 10-15 mmol/kg/day | Often none - treat the K |
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