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USMLE Step 3 Hub

Nephrology & Electrolyte Emergencies

4-Step Acid-Base Master Algorithm

Every Step 3 test form contains 2–4 complex acid-base vignettes. Follow this strict sequence:

flowchart TD
    Step1["Step 1: Check Arterial pH\npH < 7.35 = Acidemia | pH > 7.45 = Alkalemia"] --> Step2["Step 2: Identify Primary Disturbance\nMetabolic (HCO3-) vs Respiratory (PaCO2)"]
    Step2 --> Step3["Step 3: Calculate Compensation\nMetabolic Acidosis: Winter's Formula: PaCO2 = (1.5 × HCO3-) + 8 ± 2"]
    Step3 --> Step4["Step 4: Check Serum Anion Gap\nAG = Na - (Cl + HCO3) | Normal = 10-12 mEq/L"]
    Step4 --> Step5["Step 5: If High Anion Gap, Calculate Delta-Delta\nDelta Ratio = (AG - 12) / (24 - HCO3-)"]

Interpretation of Delta-Delta (ΔΔ / Delta Ratio)

When high anion gap metabolic acidosis (HAGMA) is present:

  • Ratio < 0.8: Combined HAGMA + Normal Anion Gap Metabolic Acidosis (NAGMA) (e.g. DKA with severe diarrhea or renal tubular acidosis).
  • Ratio 0.8 – 2.0: Pure High Anion Gap Metabolic Acidosis (uncomplicated DKA, lactic acidosis).
  • Ratio > 2.0: Combined HAGMA + Concurrent Metabolic Alkalosis or Pre-existing Respiratory Acidosis (e.g. DKA with severe vomiting, or lactic acidosis in a patient on baseline loop diuretics).

Severe Electrolyte Management Protocols

1. Hyperkalemia (K⁺ > 6.5 mEq/L or ECG Changes)

Follow the 3-phase therapeutic strategy:

PhaseGoalMedication & DosingOnset
Phase 1: Membrane StabilizationPrevents fatal ventricular fibrillation / asystoleIV Calcium Gluconate 10% (10 mL over 2–3 min) (Use Calcium Chloride if in cardiac arrest)1–3 minutes (does not lower serum K+)
Phase 2: Intracellular ShiftTemporarily drives K+ from extracellular to intracellular fluid1. Regular Insulin 10 units IV + D50W 25 g
2. Albuterol nebulized (10–20 mg)
3. IV Sodium Bicarbonate (if concurrent metabolic acidosis)		15–30 minutes
Phase 3: Elimination from BodyPermanently removes K+ from the body1. Furosemide 40–80 mg IV (if adequate renal function)
2. Sodium Zirconium Cyclosilicate (Lokelma) or Patiromer
3. Emergent Hemodialysis (if refractory or ESRD)		Hours

2. Hyponatremia (Na⁺ < 135 mEq/L)

  • Severe Symptomatic Hyponatremia (Seizures, stupor, coma):
    • Administer 3% Hypertonic Saline 100 mL IV bolus over 10 minutes (repeat up to 2 times as needed).
    • Target: Raise serum sodium by 4–6 mEq/L rapidly to reverse cerebral edema and stop herniation.
  • Maximum Correction Rate Safety Limit:
    • Do NOT exceed ≤ 8 mEq/L in 24 hours (or ≤ 10–12 mEq/L in 48 hours).
    • Risk: Overly rapid correction in chronic hyponatremia causes Osmotic Demyelination Syndrome (Central Pontine Myelinolysis) leading to spastic quadriparesis, pseudobulbar palsy, and ‘locked-in’ syndrome.

Acute Kidney Injury: Prerenal vs Intrinsic ATN

Key Differential Contrast

Prerenal Azotemia vs Acute Tubular Necrosis (ATN)

Clinical Feature Prerenal Azotemia (Hypoperfusion) Acute Tubular Necrosis (Intrinsic AKI) Discriminating Clue
BUN / Serum Creatinine Ratio > 20:1 (Urea reabsorbed with water in proximal tubule) < 15:1 (Tubular reabsorptive capacity destroyed) BUN:Cr > 20:1 strongly favors prerenal
Fractional Excretion of Sodium (FeNa) < 1% (Kidneys avidly conserve sodium) > 2% (Damaged tubules cannot reabsorb sodium) FeNa < 1% is typical of prerenal
Fractional Excretion of Urea (FeUrea) < 35% (Accurate even if patient is on loop diuretics) > 50% Use FeUrea if patient received diuretics recently
Urine Sediment Microscopy Normal or bland; hyaline casts only Muddy brown granular casts, renal tubular epithelial cells Muddy brown granular casts are pathognomonic for ATN
Response to IV Fluid Challenge Rapid normalization of urine output and serum creatinine No immediate improvement; persistent oliguria or polyuria Fluid challenge is both diagnostic and therapeutic

Emergent Indications for Hemodialysis (The “A-E-I-O-U” Mnemonic)

  1. A - Acidosis: Severe metabolic acidosis refractory to medical therapy (pH < 7.15).
  2. E - Electrolytes: Severe hyperkalemia (K⁺ > 6.5 mEq/L) refractory to shifting agents or with progressive ECG changes.
  3. I - Ingestions / Toxins: Toxic alcohols (Methanol, Ethylene glycol), Lithium, Salicylates, Theophylline.
  4. O - Overload: Refractory pulmonary edema unresponsive to loop diuretics.
  5. U - Uremia: Uremic pericarditis (friction rub), uremic encephalopathy, or uremic bleeding diathesis.