Observed Station · Polyuria & Diabetes Insipidus · Water Deprivation Test Interpretation
⏱️ TIME REMAINING
08:00
📋 Data Interpretation Station
Polyuria & Diabetes Insipidus – Water Deprivation Test Scenarios
You will be presented with 8 clinical scenarios of children with polyuria and polydipsia.
For each, interpret the lab data and provide: 1) Diagnosis, 2) Any other test, 3) What to do next, 4) Follow-up plan.
Central DI: ↑Na, ↓urine osm, DDAVP responsiveNephrogenic DI: ↑Na, ↓urine osm, no DDAVP responsePrimary polydipsia: urine concentrates normallyDM: hyperglycemia, glycosuria
Case 1
A 9-year-old boy with polyuria, polydipsia, and nocturia. No weight loss. Family history of autoimmune diseases.
Serum Sodium
156 mEq/L (elevated)
Serum Osmolality
315 mOsm/kg (elevated)
Urine Osmolality (random)
180 mOsm/kg (low)
Fasting Glucose
85 mg/dL (normal)
Water Deprivation Test
Urine osm remains 200 after 6h dehydration
After DDAVP (desmopressin)
Urine osm rises to 620 (rise >50%)
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) – hypernatremia, dilute urine, DDAVP-responsive (urine osmolality rises >50% after desmopressin).
• Any other test: MRI brain (pituitary/hypothalamus) to rule out tumor (craniopharyngioma, germinoma), histiocytosis, or structural lesion. Copeptin level (low in central DI).
• What to do next: Start desmopressin (DDAVP) – intranasal (10-20 mcg/day) or oral (0.05-0.2 mg/day). Monitor serum sodium and urine output.
• Follow-up plan: Monitor sodium, urine output, weight. Adjust DDAVP dose to maintain normal sodium. Annual MRI if lesion found. Counsel on risk of hyponatremia (over-treatment).
Case 2
A 6-year-old boy with polyuria, polydipsia, and poor weight gain. Family history of polyuria in maternal uncle.
Serum Sodium
152 mEq/L (elevated)
Serum Osmolality
310 mOsm/kg
Urine Osmolality (random)
150 mOsm/kg (low)
Fasting Glucose
80 mg/dL (normal)
Water Deprivation Test
Urine osm remains 160 after dehydration
After DDAVP (desmopressin)
Urine osm rises to 190 (rise <10%)
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: Nephrogenic Diabetes Insipidus (ADH resistance) – hypernatremia, dilute urine, no response to DDAVP. Family history suggests X-linked (AVPR2 mutation).
• Any other test: Genetic testing (AVPR2 for X-linked, AQP2 for autosomal). Serum calcium, potassium, lithium level (if exposure). Renal ultrasound (obstructive uropathy).
• What to do next: Thiazide diuretic (hydrochlorothiazide 1-2 mg/kg/day) + amiloride (if potassium-wasting) or indomethacin (2 mg/kg/day) to reduce urine output. Low-sodium diet.
• Follow-up plan: Monitor serum sodium, potassium, growth. Ensure adequate free water access. Genetic counseling for family. Avoid medications that worsen (lithium).
Case 3
A 10-year-old girl with polyuria, polydipsia, and normal sodium. She drinks >5 L of water daily. No weight loss.
Serum Sodium
138 mEq/L (normal)
Serum Osmolality
285 mOsm/kg (normal)
Urine Osmolality (random)
80 mOsm/kg (low)
Fasting Glucose
90 mg/dL (normal)
Water Deprivation Test
Urine osm rises to 850 after 6h dehydration
After DDAVP
No further rise (urine already concentrated)
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: Primary (psychogenic) polydipsia – excessive water intake suppresses ADH. Normal sodium, urine concentrates normally after water deprivation.
• Any other test: Psychiatric evaluation (anxiety, compulsive behavior). Rule out hypothalamic lesion (MRI brain if suspicion). Check for hyponatremia (if water intoxication).
• What to do next: Fluid restriction (reduce intake gradually). Behavioral therapy. Treat underlying psychiatric condition if present.
• Follow-up plan: Monitor sodium, urine output, weight. Education about risks of overhydration. Long-term psychiatric follow-up if needed.
Case 4
A 12-year-old boy with polyuria, polydipsia, weight loss, and fatigue for 3 weeks.
Serum Sodium
135 mEq/L (normal)
Serum Osmolality
290 mOsm/kg
Urine Osmolality (random)
400 mOsm/kg
Fasting Glucose
320 mg/dL (elevated)
Urine Ketones
Positive (3+)
HbA1c
10.2% (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: Type 1 Diabetes Mellitus – polyuria from glycosuria, hyperglycemia, ketonuria, weight loss.
• Any other test: C-peptide (low), autoantibodies (GAD, IA-2, ZnT8), anti-insulin antibodies. Basal metabolic panel (electrolytes, pH for DKA).
• What to do next: Start insulin therapy (basal-bolus regimen: long-acting glargine/detemir + rapid-acting aspart/lispro). Educate on carbohydrate counting, hypoglycemia management. Check electrolytes, pH for DKA.
• Follow-up plan: Frequent glucose monitoring (CGM), HbA1c every 3 months. Monitor for complications (retinopathy, nephropathy, neuropathy). Regular endocrinology follow-up.
Case 5
A 15-year-old with bipolar disorder on lithium therapy for 2 years presents with polyuria and polydipsia.
Serum Sodium
150 mEq/L (elevated)
Serum Osmolality
308 mOsm/kg
Urine Osmolality (random)
120 mOsm/kg (low)
Fasting Glucose
88 mg/dL (normal)
Water Deprivation Test
No concentration, urine osm 130 after dehydration
After DDAVP
No response (urine osm 150)
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: Lithium-induced nephrogenic diabetes insipidus – lithium blocks ADH effect on collecting duct. Dilute urine, hypernatremia, no DDAVP response.
• Any other test: Serum lithium level, renal function (creatinine, BUN), urine electrolytes, renal ultrasound (to assess for structural damage).
• What to do next: Reduce or stop lithium if possible (in consultation with psychiatry). Start amiloride (blocks lithium entry into collecting duct) or hydrochlorothiazide to reduce urine output.
• Follow-up plan: Monitor renal function, serum sodium, lithium levels. Consider alternative mood stabilizers. Avoid dehydration.
Case 6
A 4-year-old with polyuria, polydipsia, and constipation. No family history. On examination, no dysmorphism.
Serum Sodium
148 mEq/L (elevated)
Serum Osmolality
305 mOsm/kg
Urine Osmolality (random)
140 mOsm/kg (low)
Serum Calcium
12.5 mg/dL (elevated, normal 8.5-10.5)
Water Deprivation Test
No concentration, urine osm 160 after dehydration
After DDAVP
No response (urine osm 170)
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: Hypercalcemia-induced nephrogenic DI – high calcium impairs renal concentrating ability. Dilute urine, hypernatremia, no DDAVP response.
• Any other test: PTH level (hyperparathyroidism), vitamin D level, serum phosphate, urinary calcium/creatinine ratio. Renal ultrasound (nephrocalcinosis).
• What to do next: Treat hypercalcemia: IV fluids (normal saline), furosemide, calcitonin, bisphosphonates (if severe). Address underlying cause (parathyroid adenoma, vitamin D intoxication, sarcoidosis).
• Follow-up plan: Monitor serum calcium, sodium, renal function. Polyuria usually improves as calcium normalizes. Long-term management of underlying disease.
Case 7
A 12-year-old with sickle cell disease (HbSS) presents with polyuria, polydipsia, and enuresis.
Serum Sodium
146 mEq/L (elevated)
Serum Osmolality
300 mOsm/kg
Urine Osmolality (random)
180 mOsm/kg (low)
Fasting Glucose
85 mg/dL (normal)
Water Deprivation Test
No concentration, urine osm 190 after dehydration
After DDAVP
No response (urine osm 200)
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: Sickle cell nephropathy causing nephrogenic DI – medullary damage and papillary necrosis impair urine concentrating ability.
• Any other test: Renal function (creatinine, BUN), urinalysis (microalbuminuria), renal ultrasound (papillary necrosis, medullary echogenicity).
• What to do next: Optimize sickle cell disease management (hydroxyurea, hydration, prevent crises). Avoid NSAIDs. For DI: thiazide diuretic + amiloride to reduce urine output.
• Follow-up plan: Monitor renal function, urine output, blood pressure. Screen for chronic kidney disease. Regular hematology and nephrology follow-up.
Case 8
A 6-month-old boy with history of posterior urethral valves, post-valvotomy. Now has polyuria and polydipsia.
Serum Sodium
150 mEq/L (elevated)
Serum Osmolality
310 mOsm/kg
Urine Osmolality (random)
120 mOsm/kg (low)
Fasting Glucose
80 mg/dL (normal)
Water Deprivation Test
No concentration, urine osm 130 after dehydration
After DDAVP
No response (urine osm 140)
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: Post-obstructive nephrogenic DI – chronic obstruction causes tubular damage and impaired concentrating ability. No DDAVP response.
• Any other test: Renal ultrasound (hydronephrosis, cortical thinning), VCUG (if residual obstruction), renal function, urine electrolytes.
• What to do next: Ensure complete relief of obstruction. Manage DI with thiazide diuretic + amiloride. Low-sodium diet. Monitor hydration status.
• Follow-up plan: Monitor renal function, growth, blood pressure. Screen for chronic kidney disease. Urology and nephrology follow-up.
⚠️ Key Concept: Water Deprivation Test Interpretation
• Central DI: ↓urine osm (<300), ↑Na, DDAVP-responsive (>50% rise in urine osm).
• Nephrogenic DI: ↓urine osm (<300), ↑Na, no response to DDAVP (<50% rise).
• Primary Polydipsia: Normal Na, urine concentrates (>800) after dehydration.
• Diabetes Mellitus: Hyperglycemia, glycosuria, ketonuria.
🎯 Examiner Scoring Checklist
• Identifies central DI (DDAVP-responsive, hypernatremia, dilute urine)
• Recognizes nephrogenic DI (no DDAVP response, causes: X-linked, lithium, hypercalcemia, sickle cell, obstruction)
• Identifies primary polydipsia (urine concentrates after dehydration)