RTA Flashcards
What is Renal tubular Acidosis?
Group of hyperchloraemic metabolic acidoses
- Secondary to abnormality in urine acidification
- Impaired acid excretion
- Impaired bicarb reabsorption
- GFR preserved
Electrolytes show ↓HCO3 , ↑Cl , N Anion Gap
What is the equation for Anion Gap?
Cations – Anions = Anion Gap
(Na+ + K+) – (Cl- + HCO3-) = 10-20 mmol/L
What are classifications of Renal Tubular Acidosis?
- Type 1: Distal RTA : Inadequate H+ secretion (↓K)
- Type 2: Proximal RTA : Inadequate HCO3 reabsorption (↓K)
- Type 4: Distal RTA with ↑K. Mineralocorticoid deficiency
- Type 3: Rare (mixture of type 1 & 2) people die before birth though
What is the role of kidney in H+ homeostasis?
PROXIMAL TUBULE
- Reabsorption of HCO3 (80%)
- Reabsorption of glucose, amino acids, organic anions, phosphate
DISTAL TUBULE
- Regeneration of remaining HCO3 (20%)
- Secretion of H+
- NH4+ generation from glutamine
What are features of Type 1 (Distal RTA)?
- Failure to lower urine pH (<5.5)
- Impaired NH4+ excretion
- Intact HCO3 reabsorption
- Nephrocalcinosis, urolithiasis
What are features of Type 2 (Proximal RTA)?
- Impaired HCO3 reabsorption
- Isolated/generalised defect e.g as part of Fanconi’s Syndrome
- Bicarbonaturia (low threshold for bicarb reabsorption)
- Intact H+ secretion in distal cells
What are features of Type 3/mixed RTA?
- Initially used to describe transient & severe form of distal RTA in infants
- Currently used to describe a rare autosomal recessive syndrome resulting from carbonic anhydrase II deficiency
- Features of both proximal and distal RTA
- In addition to RTA, patients suffer osteoporosis, cerebral calcification & mental retardation
What are features of incomplete distal RTA?
- Persistently high urine pH (even after an acute acid load)
- Hypocitraturia
- Normal HCO3 (no metabolic acidosis) as able to maintain net H+ excretion
- Hypercalciuria + hypocitraturia = nephrolithiasis
What are features of Type 4 RTA (Hypoaldosteronism)?
Aldosterone deficiency or tubular resistance to action of aldosterone
Features
- Hyperkalaemia (impaired K secretion)
- Mild acidosis (impaired H+ secretion)
- Impaired secretion of NH4+
- Appropriately low urine pH (≤5.3 in presence of acidosis)
How is RTA diagnosed?
- Hyperchloraemic metabolic acidosis not explained by bicarbonate losses from the intestinal tract should raise the suspicion of RTA
- Plasma potassium (low in RTA 1 & 2, high in RTA 4)
- Presence of other features of Fanconi syndrome suggest type 2 RTA.
- Amount of bicarb required to correct acidosis can help distinguish between type 1 and type 2 RTA (type 2 requires much more bicarb)
What are some dynamic function tests for RTA?
- Early morning/first pass urine after overnight fast pH <5.5
- Urinary acidification test NH4Cl 100mg/kg body weight. Measure urine pH every hour for 8h
- NH4Cl can be unpleasant & is not used in patients with liver disease - Calcium chloride 1mmol/kg body weight
What will results for dynamic function tests for RTA?
- Normal: Urine pH will fall below 5.5 at least once
- Distal RTA (type 1) - urine pH fails to fall below 5.5 & often stays above 6.5
What is Fractional Excretion of HCO3?
Can be used to confirm Dx RTA type 2 (proximal)
([UHCO3/PHCO3] / [Ucreat/Pcreat]) x 100%
- Proximal RTA: >10-15%
- Distal RTA: <10%
- (but can only use if plasma HCO3 is >20mmol/L)
How can NH4 excretion be measured?
NH4 excretion can be measured but is estimated by calc anion gap
- UAG = Na + K – Cl
- In the presence of acidosis a negative UAG indicates pRTA and positive UAG indicates dRTA (type 1 or 4)
Osmolal gap
- UOG = Osmo – 2Na – 2K – urea – glu
- This also estimates NH4 excretion
How is Renal Tubular Acidosis managed?
- Type 1: Administer bicarb 1-2mmol/kg/day
- Type 2: Bicarb 5-15 mmol/kg/day (+K)
- Type 4: Fludrocortisone 0.1-0.2 mg/bds for mineralocorticoid deficiency OR diuretic therapy + low K diet if mineralocorticoid resistant
