Renal tubular acidosis

Background

• Class of disorders characterized by impaired reabsorption of filtered bicarbonate or excretion of hydrogen ions
• Causes hyperchloremic metabolic acidosis, non-anion gap
• Diagnostic classification starts with low, normal, or high serum potassium
• Differentiate from uremia in which acidosis is anion gap

Etiologies

• Type II RTA
  • Multiple myeloma
  • Amyloidosis
  • Drugs - ifosfamide, acetazolamide
  • Heavy metals (Lead, mercury, copper/Wilson's)
  • Vitamin D deficiency, rickets, hypophosphatemia
  • Paroxysmal nocturnal hemoglobinuria
  • Renal transplant
  • Fanconi's syndrome
  • Familial
  • Primary metabolic (cystinosis, von Gierke's)
• Type I RTA
  • Autoimmune - Sjögren, SLE, RA
  • Drugs - Lithium, amphotericin B, ifosfamide
  • Sickle cell disease
  • Cirrhosis
  • Obstructive uropathy
  • Renal transplant
  • Familial (decreased H+/K+-ATPase, Na+/K+-ATPase activity)
• Type IV RTA
  • Aldosterone deficiency or resistance
  • Diabetic nephropathy (hyporenin/hypoaldosteronism)
  • Obstructive uropathy
  • Chronic tubulointerstitial disease
  • Potassium-sparing diuretics
  • Heparin induced adrenal insufficiency

Differential Diagnosis

• See metabolic acidosis

Workup

• ABG or VBG
• BMP, phos, mag
• Urine pH
• Urine electrolytes - Na, K, Cl, Ca, Cr
• Plasma aldosterone and cortisol for type IV RTA
• Note that when hyperkalemia is present, aldosterone should be elevated, so a "normal" aldosterone level may be relatively low

Features and Treatment

• Mild acidosis in adults with bicarb > 20 mEq/L may not require treatment
• Children should be treated due to impaired growth
• For type I and II RTA, treat all children and infants
  • Generally, 5-15 mEq/kg/day oral bicarbonate
  • May require IV bicarbonate for severe acidosis
  • PO potassium supplements
  • Treat adults with bicarb < 18-20 mEq/L
  • Treat underlying d/o (Vitamin D, etc.)

Proximal Tubular Acidosis

• Type II RTA - reduced bicarbonate reabsorption
  • Hypokalemia
  • Serum bicarb 12-20 mEq/L generally
  • Urine pH < 5.5, but can be variable
• Urine pH > 5.5 if undergoing alkali therapy

Distal Tubular Acidosis

• Type I RTA - impaired H+ and ammonium secretion
  • Hypokalemia
    • Normokalemia may be seen in chronic interstitial renal disease
  • Serum bicarb may be < 10 mEq/L
  • Urine pH > 5.5
  • Urinary anion gap > 50 mEq/L = Na + K - Cl
  • Nephrolithiasis, calcium stones from chronic acidosis causing bone resorption and hypercalciuria

Type IV RTA

• Also distal tubular acidosis
  • Hyperkalemia
  • Mild acidosis, with bicarb generally > 17 mEq/L
  • Urine pH < 5.5
• Serum aldosterone and cortisol levels to determine between:
  • Adrenal insufficiency, Addison's (low aldosterone, low cortisol)
  • Selective aldosterone deficiency (low aldosterone, normal cortisol)
  • Aldosterone resistance (nl to high aldosterone, nl to high cortisol)

References

• Department of Internal Medicine, Texas Tech University Health Sciences Center, Lubbock. Renal Tubular Acidosis Syndromes. South Med J. 2000;93(11). http://www.medscape.com/viewarticle/410658.
• UCSF Education. Renal Tubular Acidosis. Nov 2002. https://medicine.ucsf.edu/education/resed/Chiefs_cover_sheets/renal%20tubular%20acidosis.pdf.