Fluid & Electrolyte Disorders – Clinical Scenarios with Lab Data
You will be presented with 8 clinical scenarios of children with suspected fluid and electrolyte disorders.
For each, interpret the lab data and provide: 1) Diagnosis, 2) Any other test, 3) What to do next, 4) Follow-up plan.
Hyponatremia: ↓Na, low osmolalityHypernatremia: ↑Na, water deficitHyperkalemia: ↑K, peaked T wavesHypokalemia: ↓K, U waves
Case 1
A 6-month-old with diarrhea for 3 days, poor oral intake. Lethargic, sunken eyes, dry mucous membranes.
Serum Sodium
168 mEq/L (elevated)
Serum Potassium
4.2 mEq/L (normal)
Serum Osmolality
345 mOsm/kg (elevated)
Urine Osmolality
450 mOsm/kg (inappropriately dilute)
Urine Sodium
15 mEq/L (low)
Blood Pressure
80/50 mm Hg
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Hypernatremic dehydration (water deficit > sodium deficit) from diarrheal losses with inadequate water intake.
• Any other test: Serum calcium, magnesium, glucose; serum osmolality; urine electrolytes and osmolality (already done); renal function.
• What to do next: IV NS 20 mL/kg bolus for hemodynamic stability, then start D5 0.45% NS at 1.25-1.5x maintenance. Correct sodium slowly (≤10 mEq/L/24h) to avoid cerebral edema.
• Follow-up plan: Monitor serum Na q4-6h, urine output, weight, clinical signs. Re-evaluate deficit and adjust fluids. Target Na decrease 0.5 mEq/L/h.
Case 2
A 4-year-old with vomiting (gastroenteritis), lethargic, sunken fontanel, tachycardia (HR 150).
Serum Sodium
118 mEq/L (low)
Serum Potassium
3.8 mEq/L (normal)
Serum Osmolality
245 mOsm/kg (low)
Urine Osmolality
420 mOsm/kg (inappropriately high)
Urine Sodium
45 mEq/L (elevated)
Blood Pressure
95/60 mm Hg
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Hyponatremic dehydration (hypovolemic hyponatremia) – sodium loss > water loss from vomiting/diarrhea.
• Any other test: Serum cortisol, TSH (exclude adrenal/thyroid), serum potassium, magnesium, glucose. Check for acidosis (ABG).
• What to do next: IV NS 20 mL/kg bolus for volume resuscitation. Then D5 0.9% NS + 20 mEq/L KCl to correct deficit over 24h. Avoid rapid correction (>10 mEq/L/24h) to prevent ODS.
• Follow-up plan: Monitor serum Na q4-6h, urine output, weight. Correct dehydration slowly; re-check electrolytes after 24h. Identify and treat underlying cause.
Case 3
A 2-year-old with acute kidney injury (oliguria, fluid overload) and lethargy.
Serum Sodium
122 mEq/L (low)
Serum Potassium
6.8 mEq/L (elevated)
Serum Osmolality
260 mOsm/kg (low-normal)
Urine Osmolality
400 mOsm/kg
Urine Sodium
35 mEq/L
Blood Pressure
115/75 mm Hg (elevated)
ECG
Peaked T waves
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Hyperkalemic hyponatremia in acute kidney injury – impaired excretion of both water and potassium.
• Any other test: ABG (metabolic acidosis common), serum calcium (risk of hypocalcemia), BUN/creatinine, renal ultrasound.
• What to do next: Hyperkalemia emergency: IV calcium gluconate (100 mg/kg) for cardioprotection, then insulin+glucose, salbutamol. For hyponatremia: fluid restriction (50-70% maintenance), isotonic fluids. Avoid potassium.
• Follow-up plan: Monitor ECG, serum K+ and Na+ q4-6h. Consider dialysis if refractory hyperkalemia or uremia. Daily weight and strict I/O.
Case 4
A 14-year-old with weakness, muscle cramps, and vomiting. Taking diuretics.
Serum Sodium
135 mEq/L (normal)
Serum Potassium
2.2 mEq/L (low)
Serum Chloride
85 mEq/L (low)
Serum Bicarbonate
34 mEq/L (elevated)
Serum Magnesium
1.2 mg/dL (low-normal)
Blood Pressure
130/85 mm Hg (elevated)
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Hypokalemic metabolic alkalosis with hypochloremia – likely from vomiting and diuretic use (chloride depletion).
• Any other test: Urine chloride (<15 mEq/L suggests GI loss; >20 suggests renal loss), aldosterone/renin ratio, TSH, cortisol. Check for hypomagnesemia.
• What to do next: IV 0.9% NS + KCl (20-40 mEq/L) to correct volume deficit and chloride. Potassium replacement (oral/IV) – max 0.5 mEq/kg/h. If severe, admit for cardiac monitoring.
• Follow-up plan: Monitor K+, chloride, bicarbonate daily. Correct hypomagnesemia if present. Discontinue diuretics if possible. Psychiatric support if bulimia.
Case 5
A 3-year-old with fever, vomiting, and poor feeding. Irritable, doughy skin on exam.
Serum Sodium
172 mEq/L (elevated)
Serum Potassium
4.5 mEq/L (normal)
Serum Osmolality
358 mOsm/kg (elevated)
Urine Osmolality
120 mOsm/kg
Urine Sodium
8 mEq/L (low)
Blood Pressure
75/40 mm Hg
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Severe hypernatremic dehydration (water deficit) with inappropriately dilute urine – suggests central diabetes insipidus or inadequate free water intake.
• Any other test: Serum osmolality, copeptin level, water deprivation test (if stable), MRI brain, serum glucose, calcium.
• What to do next: IV NS 20 mL/kg bolus for hypotension, then D5 0.45% NS at 1.5x maintenance. Correct sodium slowly (≤10 mEq/L/24h). If DI confirmed, start desmopressin (DDAVP).
• Follow-up plan: Monitor Na q4-6h, urine output. Wean fluids gradually. Long-term DDAVP if central DI. Check for underlying CNS lesion (MRI).
Case 6
A 10-year-old with diarrhea (3 days), lethargy, and sunken eyes.
Serum Sodium
132 mEq/L (low-normal)
Serum Potassium
3.5 mEq/L (low-normal)
Serum Chloride
112 mEq/L (elevated)
Serum Bicarbonate
14 mEq/L (low)
Serum Osmolality
290 mOsm/kg (normal)
Blood Pressure
90/55 mm Hg
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Normal anion gap (hyperchloremic) metabolic acidosis – likely from diarrheal loss of bicarbonate (with chloride retention).
• Any other test: Anion gap = Na - (Cl + HCO3) = 132 - (112+14) = 6 (normal 8-12). Check lactate, urine electrolytes, stool culture. ABG to confirm.
• What to do next: IV 0.9% NS bolus (20 mL/kg) for dehydration, then D5 0.9% NS with 20 mEq/L KCl. Correct acidosis by restoring perfusion and replacing losses. Bicarbonate not routinely indicated.
• Follow-up plan: Monitor electrolytes, ABG, urine output. Treat underlying diarrheal cause (rehydration, ORS). Re-check bicarbonate and chloride in 24h.
Case 7
A 5-year-old with nephrotic syndrome, edema, and ascites.
Serum Sodium
125 mEq/L (low)
Serum Potassium
4.0 mEq/L (normal)
Serum Osmolality
265 mOsm/kg (low)
Urine Sodium
8 mEq/L (low)
Urine Osmolality
550 mOsm/kg (elevated)
Blood Pressure
110/70 mm Hg
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Hypervolemic hyponatremia (dilutional) in nephrotic syndrome – due to fluid retention and decreased effective circulating volume.
• Any other test: Serum albumin, urine protein/creatinine ratio, lipid profile, complement levels, renal biopsy if indicated.
• What to do next: Fluid and sodium restriction (1-2 mEq/kg/day sodium). Loop diuretics (furosemide) with albumin infusion to mobilize edema. Avoid hypotonic fluids.
• Follow-up plan: Monitor daily weight, electrolytes, urine output. Adjust diuretics. Treat underlying nephrotic syndrome with steroids. If refractory, consider kidney biopsy.
Case 8
A 12-year-old with polyuria (5 L/day), polydipsia, and nocturia. No fever, no vomiting.
Serum Sodium
155 mEq/L (elevated)
Serum Potassium
4.0 mEq/L (normal)
Serum Osmolality
320 mOsm/kg (elevated)
Urine Osmolality
150 mOsm/kg
Urine Sodium
60 mEq/L (elevated)
Blood Pressure
118/72 mm Hg
1️⃣ Diagnosis: (Write your answer below)
2️⃣ Any other test: (Write your answer below)
3️⃣ What to do next: (Write your answer below)
4️⃣ Follow-up plan: (Write your answer below)
✅ Model Answer:
• Diagnosis: Central diabetes insipidus (ADH deficiency) – polyuria, hypernatremia, dilute urine.
• Any other test: Water deprivation test (if stable), copeptin level, MRI brain (pituitary), serum calcium, glucose. Familial history.
• What to do next: Start desmopressin (DDAVP) 0.05-0.2 mg oral or intranasal. Replace free water deficit slowly. Monitor sodium closely.
• Follow-up plan: Monitor urine output, serum Na, weight. Adjust DDAVP dose. MRI brain if etiology unknown. Lifelong DDAVP if permanent DI.
⚠️ Key Concept: Fluid & Electrolyte Disorders
• Hyponatremia: Check volume status. Hypovolemic (urine Na <20): replace with NS. Hypervolemic (edema): fluid restriction + diuretics. Euvolemic (SIADH): fluid restriction.
• Hypernatremia: Water deficit > sodium deficit. Correct slowly (≤10 mEq/L/24h) to avoid cerebral edema.
• Hyperkalemia: ECG changes (peaked T waves, wide QRS). Calcium gluconate for cardioprotection, then insulin+glucose, salbutamol, kayexalate/dialysis.
• Hypokalemia: ECG: U waves, flattened T. Treat with oral/IV K+ (max 0.5 mEq/kg/h). Correct hypomagnesemia first.
• Metabolic acidosis: Calculate anion gap. Elevated AG: MUDPILES (DKA, lactic, toxins). Normal AG: diarrhea, RTA.
🎯 Examiner Scoring Checklist
• Correctly identifies hyponatremia vs hypernatremia and their volume status
• Recognizes hyperkalemia emergency and initiates cardioprotective treatment
• Identifies hypokalemia and metabolic alkalosis with appropriate replacement
• Recognizes normal anion gap acidosis in diarrhea
• Identifies diabetes insipidus and starts DDAVP
• Understands fluid restriction in hypervolemic hyponatremia
📌 Key Fluid & Electrolyte Interpretation:
• Hyponatremia: Volume status guides therapy. Hypovolemic → NS. Hypervolemic → fluid restriction. Euvolemic → SIADH.
• Hypernatremia: Water deficit. Correct slowly (≤10 mEq/L/24h). Risk: cerebral edema.
• Hyperkalemia: ECG first! Calcium gluconate → insulin/glucose → salbutamol → kayexalate/dialysis.
• Hypokalemia: Replace K+ (oral/IV). Correct Mg first if low.
• Metabolic acidosis: Anion gap = Na - (Cl+HCO3). Elevated → MUDPILES. Normal → diarrhea/RTA.
📋 Mock Test Feedback
💡 Examiner's note: Compare your answers with model answers. In real TOACS, you would discuss these with the examiner.