9.8 Renal Tubular Acidosis & Nephrolithiasis

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)

$\text{Urine Anion Gap} = [\text{Urine Na}^+] + [\text{Urine K}^+] - [\text{Urine Cl}^-]$

• 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)

2. Nephrolithiasis: Stone Typology, Prevention & Acute Colic Triage

Physicochemical Stone Typology & 24-Hour Urine Prevention

1. 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.

2. 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.

3. 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.

4. 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.

5. 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