Bit · Renal
RTA Type 1 vs Type 2 vs Type 4
Three flavours of renal tubular acidosis. All cause non-anion-gap metabolic acidosis. The pivot is urine pH and serum potassium.
Mechanism
All three are inherited or acquired defects in tubular handling of acid or aldosterone. The metabolic acidosis they produce is normal-anion-gap (hyperchloremic), with the renal compensation broken at a specific tubular site:
- Type 1 (Distal) RTA — α-intercalated cells of the collecting duct can't secrete H⁺. Urine cannot be acidified. Urine pH > 5.5 even with severe acidosis. Hypokalemia (more K⁺ excreted to compensate). Causes Ca²⁺/phosphate stones (calcium phosphate, alkaline urine) and rickets/osteomalacia. Associations: amphotericin B, Sjögren, SLE.
- Type 2 (Proximal) RTA — proximal tubule can't reabsorb HCO₃⁻. Serum HCO₃⁻ falls until a new lower steady state where filtered load matches reduced reabsorption. Urine pH initially > 5.5, then falls to < 5.5 once steady state is reached. Hypokalemia. Often part of Fanconi syndrome (also glycosuria, aminoaciduria, phosphaturia). Causes: multiple myeloma, Wilson disease, lead, acetazolamide, ifosfamide.
- Type 4 (Hyperkalemic) RTA — aldosterone deficiency or resistance → impaired Na⁺/K⁺/H⁺ exchange in collecting duct. Hyperkalemia (the pivot — opposite of types 1 and 2). High K⁺ suppresses ammoniagenesis → impaired acid excretion. Urine pH usually < 5.5. Causes: diabetic nephropathy, ACE inhibitors, ARBs, spironolactone, heparin, NSAIDs, Addison's.
Differentiator Table
| Type 1 (Distal) | Type 2 (Proximal) | Type 4 (Hyperkalemic) | |
| Defect | Collecting duct can't secrete H⁺ | PCT can't reabsorb HCO₃⁻ | ↓ Aldosterone (or resistance) |
| Urine pH | > 5.5 (cannot acidify) | > 5.5 early, < 5.5 at steady state | < 5.5 (usually) |
| Serum K⁺ | Low | Low | HIGH |
| Serum HCO₃⁻ | Low (often severe) | Low (moderate, plateaus) | Mildly low |
| Stones / bones | Calcium phosphate stones, rickets | Hypophosphatemic rickets (Fanconi) | None |
| Causes | Sjögren, SLE, amphotericin B, lithium, hereditary | Multiple myeloma, Wilson, lead, acetazolamide, ifosfamide, Fanconi syndrome | Diabetic nephropathy, ACEi/ARB, spironolactone, NSAIDs, Addison |
| Treatment | Oral bicarbonate; correct hypokalemia | Oral bicarbonate (large doses) + K⁺ | Treat cause; fludrocortisone if Addison; low K⁺ diet |
The Pivot
Two questions decide it:
- Is the serum K⁺ high or low? High → Type 4. Low → Type 1 or 2.
- If low K⁺ — can the urine be acidified? Urine pH stays > 5.5 → Type 1. Urine pH falls to < 5.5 once HCO₃⁻ is low enough → Type 2.
Stones help: calcium phosphate stones in alkaline urine = Type 1.
NBME-Style Stem
A 38-year-old woman with Sjögren syndrome is found on routine labs to have serum HCO₃⁻ 16 mEq/L, K⁺ 3.1 mEq/L, Cl⁻ 110 mEq/L. Anion gap is normal. Urine pH is 6.4 despite severe acidemia. Renal ultrasound shows bilateral nephrocalcinosis. Which type of renal tubular acidosis is most likely?
Concept Anchor
All three RTAs make blood acidic at the tubule; the difference is where the leak is. Type 1 can't push acid out (alkaline urine), Type 2 can't reabsorb base, and Type 4 has no aldosterone (high K⁺) — and the K⁺ direction tells you the type before any urine test.