Renal tubular acidosis describes a group of transport defects in the kidney affecting the reabsorption of bicarbonate, the excretion of hydrogen ions or both.
Renal tubular acidosis results in
- Hyperchloraemia
- Metabolic acidosis
- Normal anion gap
- Normal/only slightly reduced eGFR
Renal tubular acidosis (RTA) are divided into:
Type 1 = Distal RTA
This is due to impaired secretion of hydrogen ions in the form of NH4+.
It is the most common form.
It is often associated with hypokalaemia.
Causes include:
- Primary – including a form associated with deafness
- Secondary
- Associated with genetic diseases including
- Sickle cell
- Ehlers Danlos
- Wilson disease
- Hypercalcaemia
- Autoimmune diseases including
- SLE
- Primary biliary cirrhosis
- Rheumatoid arthritis
- Drugs
- Lithium
- Toluene
- Amphotericin
- Trimethoprim
- Amiloride
Diagnosis is by oral acid load with ammonium chloride - the urine should become more acid, but in type 1 RTA the pH remains >5.5.
Treatment: oral bicarbonate, possibly thiazide diuretic.
Type 2 = Proximal RTA
Type 2 RTA is due to impaired bicarbonate reabsoption in the proximal tubule.
Causes include:
- Primary
- Secondary
- Fanconi syndrome (which can be caused by Wilson’s disease, cystinosis, myeloma…)
- Drugs
- Aminoglycoside
- Valproate
- Old tetracycline
- Acetazolamide
- Metals – lead, mercury
Diagnosis is by IV bicarbonate loading - normally fractional excretion is <5% of the filtered load, but in type 2 RTA it is around 15%.
Treatment: oral bicarbonate.
Type 3 RTA = a combination of type 1 and type 2
Type 4 RTA = Hyperkalaemic RTA
This is caused by hyperkalaemia impairing NH4+ formation
Causes are therefore any cause of hyperkalaemia, such as Addison’s or drug induced hyperkalaemia.
Treatment is to treat cause and control hyperkalaemia
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