9.8 Renal Tubular Acidosis & Nephrolithiasis

ABIM Internal Medicine Exam

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9.8 Renal Tubular Acidosis & Nephrolithiasis

Key Takeaways Explain This Topic AI

Nephrolithiasis is a separately enumerated blueprint subsection under Nephrology and Urology.

A positive urine anion gap in normal anion gap acidosis indicates impaired renal ammonium excretion and therefore a renal tubular acidosis.

Distal type 1 renal tubular acidosis causes urine pH above 5.5 with hypokalemia and calcium phosphate stones.

Proximal type 2 renal tubular acidosis is often part of Fanconi syndrome with glycosuria, phosphaturia and aminoaciduria.

Increased fluid intake to achieve high urine output is the single most effective intervention for preventing recurrent stones of any composition.

Last updated: August 2026

1. Normal Anion Gap Metabolic Acidosis & Renal Tubular Acidosis (RTA)

Normal Anion Gap Metabolic Acidosis (NAGMA / Hyperchloremic Acidosis, AG 8–12 mEq/L) represents either gastrointestinal loss of HCO3- or impaired renal acidification.

The Urine Anion Gap (UAG) Urine Anion Gap = [ Urine Na + ] + [ Urine K + ] − [ Urine Cl − ] \text{Urine Anion Gap} = [\text{Urine Na}^+] + [\text{Urine K}^+] - [\text{Urine Cl}^-] Urine Anion Gap = [ Urine Na + ] + [ Urine K + ] − [ Urine Cl − ]

Negative UAG (typically -20 to -50 mEq/L): Indicates intact renal ammonium (NH4+) excretion (unmeasured NH4+ is paired with Cl-, driving urine Cl- higher than Na + K). Confirms Gastrointestinal Bicarbonate Loss (severe diarrhea, VIPoma, ileostomy) .

Positive UAG (typically +10 to +40 mEq/L): Indicates impaired renal excretion of NH4+. Confirms Renal Tubular Acidosis (RTA) .

Differential Diagnosis of Renal Tubular Acidosis (RTA)

Feature Type 1 (Distal) RTA Type 2 (Proximal) RTA Type 4 (Hyperkalemic) RTA

Primary Defect Impaired H+ secretion by alpha-intercalated cells in cortical collecting duct Impaired HCO3- reabsorption in proximal convoluted tubule Aldosterone deficiency or aldosterone resistance in collecting duct

Serum Potassium Hypokalemia (Low K+) Hypokalemia (Low K+) HYPERKALEMIA (High K+)

Urine pH during Acidemia Invariable Urine pH > 5.5 (inability to acidify urine) Variable (initially >5.5; falls to <5.5 once serum HCO3 drops below threshold) Urine pH < 5.5

Nephrolithiasis / Nephrocalcinosis Common (Bilateral Calcium Phosphate stones due to alkaline urine and hypocitraturia) Absent Absent

Key Associations Autoimmune: Sjögren Syndrome , SLE, Rheumatoid Arthritis; Amphotericin B, Ifosfamide, genetic Fanconi Syndrome (glucosuria with normal blood sugar, phosphaturia, aminoaciduria, uricosuria); Multiple Myeloma , Tenofovir, Cisplatin Diabetic Nephropathy (Hyporeninemic Hypoaldosteronism), NSAIDs, ACEi/ARBs, MRAs, Trimethoprim, Calcineurin inhibitors

Treatment Oral Sodium Bicarbonate or Potassium Citrate (provides base and citraturia to dissolve/prevent stones) Treat underlying etiology; high-dose sodium bicarbonate (caution: worsens hypokalemia) Low-potassium diet, Loop / Thiazide diuretics (Furosemide) ; oral Fludrocortisone (if primary adrenal insufficiency)

• 2. Nephrolithiasis: Stone Typology, Prevention & Acute Colic Triage
• Physicochemical Stone Typology & 24-Hour Urine Prevention

Calcium Oxalate Stones (75–80% of all stones):

Microscopy / Imaging: Radiopaque; envelope-shaped (dihydrate) or dumbbell-shaped (monohydrate) crystals.

Etiologies: Hypercalciuria (most common), Hyperoxaluria (dietary, primary, or enteric hyperoxaluria due to fat malabsorption in Crohn disease, celiac disease, Roux-en-Y gastric bypass: unabsorbed free fatty acids bind intestinal calcium, leaving unbound oxalate to be hyper-absorbed in the colon), and Hypocitraturia (citrate normally chelates calcium; low in chronic metabolic acidosis and chronic diarrhea).

Prevention Strategy:

High fluid intake (>2.5–3.0 L/day to maintain urine volume >2.0–2.5 L/day).

Normal Dietary Calcium (1000–1200 mg/day): CRITICAL BOARD RULE: NEVER restrict dietary calcium! Calcium restriction increases intestinal absorption of free oxalate, paradoxical worsening of stone formation.

Low sodium diet (<2000 mg/day; reduces renal calcium excretion) and low animal protein.

Thiazide Diuretics (Chlorthalidone 25–50 mg or HCTZ 25–50 mg daily): Enhances renal calcium reabsorption, lowering urinary calcium.

Potassium Citrate (30–60 mEq/day): Alkalinizes urine and provides urinary citrate.

Calcium Phosphate Stones (10–15%):

Microscopy / Imaging: Radiopaque; amorphous or wedge/rosette crystals; forms in alkaline urine ( urine pH > 6.5–7.0 ); strongly associated with Type 1 Distal RTA and Primary Hyperparathyroidism .

Uric Acid Stones (5–10%):

Microscopy / Imaging: Radiolucent on plain abdominal radiography (KUB) (invisible on X-ray, visible on non-contrast CT); diamond or rhomboid crystals in acidic urine ( urine pH < 5.5 ).

Etiologies: Persistent low urine pH, gout, myeloproliferative disorders, tumor lysis syndrome, high-purine diet.

Prevention / Treatment: Urinary alkalinization with oral Potassium Citrate (target urine pH 6.5 to 7.0) dissolves uric acid stones. Add Allopurinol if hyperuricosuria persists.

Struvite Stones / Magnesium Ammonium Phosphate (10–15%):

Microscopy / Imaging: Radiopaque; "coffin-lid" prism crystals; rapidly forms massive staghorn calculi filling the renal pelvis and calyces.

Pathogenesis: Infection with urease-producing bacteria ( Proteus mirabilis , Klebsiella pneumoniae , Morganella , Pseudomonas ; NOT E. coli ). Urease hydrolyzes urea into ammonium and hydroxide, producing markedly alkaline urine pH > 7.2–8.0 .

Treatment: Complete surgical stone removal (Percutaneous Nephrolithotomy [PCNL]); antibiotics alone cannot sterilize bacteria trapped inside the stone matrix.

Cystine Stones (1–2%):

Microscopy / Imaging: Faintly radiopaque ("ground-glass"); pathognomonic hexagonal crystals ; positive sodium nitroprusside cyanide test.

Pathogenesis: Autosomal recessive defect in proximal tubular transport of dibasic amino acids ( COLA : Cystine, Ornithine, Lysine, Arginine).

Prevention / Treatment: Massive hydration (>3–4 L/day), intensive urinary alkalinization (potassium citrate to target pH >7.0–7.5), and thiol-chelating agents ( Tiopronin or D-penicillamine).

• Acute Renal Colic Triage & Obstructing Urosepsis Emergency ┌───────────────────────────────────────────────┐
• │ Acute Renal Colic / Suspected Nephrolithiasis│
• └───────────────────────┬───────────────────────┘
• │
• Non-Contrast Helical CT Abdomen/Pelvis
• (Renal Ultrasound if Pregnant / Pediatric)
• │
• ┌─────────────────────────────────┴─────────────────────────────────┐
• ▼ ▼
• ┌───────────────────────────────────────────┐ ┌───────────────────────────────────┐
• │ OBSTRUCTING STONE + FEVER / UROSEPSIS? │ │ NO INFECTION / SEPSIS PRESENT │
• │ (WBCs in urine, Temp >38°C, Hypotension) │ └─────────────────┬─────────────────┘
• └─────────────────────┬─────────────────────┘ │
• │ ┌──────────────────┴──────────────────┐
• ▼ ▼ ▼
• SURGICAL UROLOGIC EMERGENCY Stone < 5 mm Stone 5 to 10 mm
• Immediate Emergent Decompression • >85% spontaneous passage • ~50% spontaneous passage

(Retrograde Ureteral Stent or PCN Tube) • Outpatient hydration & NSAIDs • Medical Expulsive Therapy:

• IV Broad-Spectrum Antibiotics • Strain urine for analysis Tamsulosin 0.4 mg PO daily
• NO ESWL OR URETEROSCOPY IN ACUTE INFECTION! │
• ┌─────────────────────┴─────────────────────┐
• ▼ ▼
• Stone > 10 mm Failure to pass <4-6 weeks
• Extracorporeal Shock Wave or intractable pain / AKI
• Lithotripsy (ESWL) • Ureteroscopy (URS) +
• Percutaneous Nephrolithotomy laser lithotripsy
• (PCNL for >20 mm / Staghorn) • Double-J Stent