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Related Subjects: |Metabolic acidosis |Aspirin or Salicylates toxicity |Ethylene glycol toxicity |Renal Tubular Acidosis |Lactic acidosis

RTA 1,2 and 4
Lytes and gaps, we love it all
Kind request from kidney docs
Check pH before we talk
@DrCindyCooper

There is failure to excrete normally acidic urine so typical urinary pH is > 5.5 even with an acid load. Causes a normal Anion gap Metabolic acidosis as there is a basic failure to excrete an acid urine

About

• Through metabolism we are net acid producers
• Body removes 1 mmol/kg/day of H⁺ions from the body
• Plasma is slightly alkaline pH 7.40 (40 mmol/L)
• Excreted acidic urine pH 5-6 (1000-10,000 mmol/L) by

Aetiology

• Acidosis due to Failure to be able to
• H⁺ excretion in Distal tubule
• HCO₃^- reabsorption in Proximal tubule
• Due to defects in tubular transport
• Normal Anion Gap Metabolic acidosis

Types

• Distal (type 1) failure to excrete H⁺ ions
• Proximal (type 2) failure to absorb HCO₃-
• Type 3 is rare with features of both distal and proximal RT
• Hyperkalemic (type 4) RTA

Clinical

• Type 1: Growth delay in children affected bone and renal disease. Low K with polydipsia and polyuria due to impaired urinary concentration ability, as well as muscle weakness. Also nephrolithiasis and nephrocalcinosis due to hypercalciuria and hypocitraturia
• Type 2: Delayed growth in children. Rickets. Dental disease. May be seen with Fanconi syndrome (most common cause of acquired Fanconi syndrome in adults is multiple myeloma) and possible tubular defects, hypophosphatemia, hypouricemia, euglycemic glycosuria, and proteinuria and osteomalacia. Also seen with tenofovir, ifosfamide, sodium valproate, and topiramate
• Type 4: Patients with diabetes or interstitial nephritis. Often no symptoms may have a high K. Can be caused by obstructive uropathy and Lupus nephritis as well as drugs such as spironolactone, eplerenone, amiloride, trimethoprim and pentamidine, nonsteroidal anti-inflammatory drugs (NSAIDs), including cyclooxygenase-2 (COX-2) inhibitors, ACE inhibitors, and heparin or low molecular weight heparin, tacrolimus and ciclosporin

Investigations

• U&E: High K with Type 4, Low K with Type 1/2, High Chloride with all types.
• VBG: Hyperchloremic normal anion gap acidosis (the loss of serum HCO₃– leads to retention of Cl– so that the anion gap remains normal)
• Type 2 (proximal) may also have hypophosphatemia, hypouricemia
• Urine pH: Type 1 distal RTA unable to acidify their urine despite severe metabolic acidosis. Type 2 proximal are able to get urine pH < 5.5 and may have euglycemic glycosuria, and proteinuria proximal tubule defects

Diagnostic tests: may be considered

• Type 1: Measure urine-to-blood (U-B) pCO₂ gradient during an NaHCO₃ infusion can be used to diagnose H⁺-ATPase secretory defect.
• Type 2: NaHCO₃ loading test; during an IV NaHCO₃, the increase in serum HCO₃– above the reabsorption threshold will lead to fractional excretion of HCO₃– greater than 15% or urine pH > 7.5 in patients with proximal RTA. Patients with proximal RTA should also be evaluated for Fanconi syndrome by assessment of serum and urine samples for glycosuria, hypophosphatemia, and hypouricemia.
• Type 4: An NH4+ loading test is used to confirm distal RTA in patients with hypokalaemic, hyperchloremic metabolic acidosis and urine pH > 5.5; at 6 h after oral ingestion of NH4Cl 100 mg/kg, patients with distal RTA will develop a positive urine anion gap (UAG; i.e., urine Na⁺ + urine K⁺ - urine Cl–) [8]. As the NH4Cl test is associated with gastrointestinal adverse effects (e.g., nausea and vomiting), the furosemide–fludrocortisone test may be used as an alternative method of diagnosing distal RTA

Management

• Treatment options depend on the type of RTA diagnosed; alkali therapy may be used in patients with type 1 or 2 RTA.
• Hypokalaemic patients may need K supplements. Some Type 2 need Vitamin D supplements.
• Newer potassium binders may be beneficial for patients with type 4 RTA

References

• Renal Tubular Acidosis and Management Strategies: A Narrative Review Renal Tubular Acidosis