#150: NepHron Nuances: Approach to Diagnosing and Treating RTAs

August 27, 2025 | By Sam Masur

Audio

In this episode, returning guest Dr. Michelle Starr joins us to unpack the complexities of renal tubular acidosis. Together, we explore how to distinguish between RTA types and highlight the key diagnostic clues and treatment strategies most relevant to pediatric patients. Whether you’re managing a challenging case on the wards or brushing up for boards, this conversation will deepen your understanding and knowledge-base!

RTA Pearls

1. RTA is a cause of non-anion gap metabolic acidosis (NAGMA). Always consider it in pediatric patients with low bicarb and absence of GI losses. If you are concerned about this, check a VBG, followed by urine electrolytes after confirming acidosis.
2. If urine pH is high (> 5.5) in a patient with RTA, think Type I (distal) RTA.
3. In Type II RTA, urine acidification is initially impaired but can recover as serum bicarb drops.
4. Hypokalemia is common in Type I and Type II RTA, while Type IV RTA features hyperkalemia.
5. Nephrocalcinosis on imaging is a major clue – if you see it, suspect Type I RTA.
6. Treatment = Alkali therapy ± potassium depending on the type. Early diagnosis and adherence can normalize growth and development.

RTA Notes 

Overview

• Renal tubular acidoses are a group of disorders characterized by the kidneys’ inability to properly acidify the urine, leading to a normal anion gap metabolic acidosis. This is usually due to impaired renal hydrogen ion secretion, bicarbonate reabsorption, or both.
• RTA can develop due to various causes, including medications (such as topiramate or acetazolamide), inherited genetic conditions, urinary obstruction, infection, and autoimmune diseases such as SLE.
• The kidneys excrete acid in the form of hydrogen phosphate and ammonium (the latter being the primary mechanism).
• There are 3 main types of RTAs (Type I, Type II, and Type IV). Controversy exists as to whether there is also a Type III RTA (mixed proximal and distal RTA); this typically refers to tubular dysfunction leading to immature tubular acid handling, and is most often seen transiently in neonates due to high daily acid load, inadequate bicarbonate reabsorption, and poor acid tubular acid secretion.
• The most common unifying presentation is failure to thrive (height typically more affected with weight relatively preserved). Urine output is typically preserved. Depending on RTA type, you may also see nephrocalcinosis and/or hearing loss.
• Failure to appropriately recognize and treat RTAs can result in failure to thrive, poor neurodevelopmental outcomes, and other downstream manifestations such as rickets/osteomalacia and/or nephrocalcinosis depending on the underlying defect.

Workup

Initial Labs

• Blood gas (look for low pH, low bicarb, normal AG)
  • Remember the calculation for serum anion gap: AG = Na – (Cl + bicarb)
• Serum electrolytes (Na, K, Cl, bicarb)
• If you have a non-gap acidosis and have ruled-out other more common etiologies (see above), send urine electrolytes (urine Na, urine K, and urine Cl). This is used to calculate the urine anion gap (UAG). This helps us assess whether enough ammonium (i.e. acid) is being excreted
  • UAG = (urine Na + urine K) – urine Cl
  • Urine chloride acts as a surrogate marker for ammonium; this is the primary acid excreted by the kidneys
  • A positive UAG is suspicious for RTA, as it suggests the kidneys are not appropriately excreting acid
    • A negative UAG is indicative of a non-RTA cause of metabolic acidosis (i.e. GI losses)
• Urine pH (included in urinalysis)

Additional Workup to Consider

• Fractional excretion of bicarbonate (can help differentiate proximal vs distal defect)
• Workup for Fanconi syndrome if proximal tubule defect is suspected
• Renal ultrasound (check for nephrocalcinosis)

Differential Diagnosis

Non-anion-gap metabolic acidosis

• GI losses (i.e. diarrhea) – The most common cause! Usually accompanied by hypokalemia
  • Inappropriate reclamation/reabsorption of hydrogen ions is also a known cause in patients with urointestinal diversion
• Chloride intoxication – High amounts of chloride delivered via IV fluids leads to increased renal bicarbonate loss. Additionally, the pH of normal saline itself is acidic relative to serum and can contribute directly to the development of a metabolic acidosis
• Renal tubular acidosis – The kidney is unable to reabsorb bicarbonate and/or secrete hydrogen ions into the urine

Type I (distal) RTA

Pathophysiology

• Can be caused by any one of the following:
  • Impaired hydrogen ion secretion in the distal tubule leading to inability to appropriately acidify urine
  • Failure of the distal tubule to regenerate bicarbonate
  • Damaged distal tubule epithelial lining causing “leakiness,” or backflow of hydrogen ions
  • Disruption of the ENaC channel. Remember, the distal tubule assists in the reabsorption of sodium via the ENaC channel, which creates a gradient that allows the hydrogen ATPase to acidify the urine. Pathologic states that impair ENaC function can lead to a Type I RTA (think about things like urinary obstruction, UTI/pyelonephritis, and sickle cell disease)

Key Laboratory Features

• Urine pH > 5.5 despite systemic acidosis
• Hypokalemia, though hyperkalemia possible if the ENaC channel is implicated
• Nephrocalcinosis
• May have elevated urine Ca:Cr ratio

Presentation

• Polyuria, polydipsia
• Failure to thrive
• Constipation
• Rickets
• Growth delay

Treatment

• Alkali therapy, typically with sodium bicarbonate
  • Sodium bicarb comes in 650 mg or 325 mg tablets. The 650 mg pills are 8 mmol/pill while the 325 mg pills are 4 mmol/pill. Can be re-constituted in children who cannot take pills
  • For children too young to take pills, baking soda mixed with water is another great option. 1 teaspoon of baking soda equates to 60 mmol of bicarb
  • Starting dose for a Type I RTA is typically 2-4 mmol/kg titrated to effect
• The main difference in treatment between type I and type II RTAs is not what to give (bicarb), but how much to give. Type I RTA requires far less bicarb compared to a type II RTA
• Potassium supplementation if needed

Type II (proximal) RTA

Pathophysiology

• Defect in bicarbonate reabsorption in proximal tubule
  • Leads to a lower threshold of maximum reabsorption for bicarb (T_m). T_m refers to the plasma bicarbonate concentration above which the kidneys can no longer reabsorb all the filtered bicarbonate

Key Laboratory Features

• Low urine pH < 5.5. Initially high, but then falls to < 5.5 as serum bicarb drops and distal tubule acidifies urine via H⁺ secretion
• Hypokalemia
• May be part of Fanconi syndrome (look for glucosuria, phosphaturia, amino aciduria)

Presentation

• Failure to thrive
• Nephrocalcinosis (typically calcium phosphate stones)
• Rickets/osteomalacia due to bone turnover in an attempt to utilize calcium and phosphate as buffers
• Consider when additional proximal tubule defects are present

Treatment

• Higher doses of alkali required than in distal RTA
  • Starting dose typically 5-10 mmol/kg
• If medication-related, stop culprit medication (i.e. topiramate, acetazolamide). Patients will often still need bicarbonate for a period of time while awaiting re-equilibration
• May need potassium supplementation

Type IV (hyperkalemic) RTA

Pathophysiology

• Hypoaldosteronism or aldosterone resistance → impaired sodium delivery and decreased K⁺/H⁺ secretion
• Inability to conjugate ammonia to ammonium due to marked hyperkalemia

Key Laboratory Features

• Hyperkalemia
• Mild acidosis, often with urine pH < 5.5 (but may have normal urine pH)
• Associated with CKD, obstructive uropathy, or CAH

Presentation

• May be asymptomatic or found incidentally
• More common in older children or those with chronic kidney disease, diabetes
• Often seen in patients on RAAS inhibitors

Treatment

• Treat underlying cause
• Potassium-lowering medications as indicated
• Consider fludrocortisone in true hypoaldosteronism
• Alkali therapy as needed

Tips for counseling families

• Renal tubular acidoses are treatable. With adherence to therapy, kids grow and develop normally, and long-term outcomes are favorable.
• Patients should be managed by a nephrologist for acute/transient RTA (i.e. infection or medication-related) until the stimulus is removed, and then can transition to close monitoring with their pediatrician.
• Patients with underlying genetic defects or true Fanconi syndrome should be followed longitudinally by a nephrologist. These patients will often develop CKD.
• Frequency of labs and follow-up depend on the underlying RTA etiology as well as disease severity.

Goal

Listeners will explain the basic pathophysiology, diagnosis, and management of renal tubular acidosis to improve long-term outcomes.

Learning Objectives

1. Differentiate the major types of renal tubular acidosis by their pathophysiology, clinical features, and laboratory profiles.
   
2. Identify the diagnostic approach to non-anion gap metabolic acidosis, including the role of urine studies and the urine anion gap.
   
3. Recognize clinical clues such as failure to thrive, nephrocalcinosis, hearing loss, and Fanconi syndrome that point toward specific RTA subtypes.
   
4. Outline evidence-based management strategies for each RTA type and explain how early treatment improves growth and long-term outcomes.

Disclosures

Dr Michelle Starr reports no relevant financial disclosures. The Cribsiders report no relevant financial disclosures. 

Citation

Wilson S, Starr M, Berk J, Chiu C, Masur S. “#150: NepHron Nuances: Approach to Diagnosing and Treating RTAs”. The Cribsiders Pediatric Podcast. https:/www.thecribsiders.com/ 27th August, 2025.