What laboratory tests should be obtained in pediatric patients presenting with polydipsia (increased thirst)?

Laboratory Evaluation for Pediatric Polydipsia

In pediatric patients presenting with increased thirst (polydipsia), obtain serum sodium, serum osmolality, urine osmolality, plasma glucose, and urinalysis as the initial laboratory work-up to differentiate between diabetes mellitus, diabetes insipidus, and other causes of polyuria-polydipsia syndrome. 1

Initial Essential Laboratory Tests

First-Line Blood Tests

• Serum sodium and serum osmolality are critical initial measurements to assess water balance and identify hyponatremia or hypernatremia 1
• Plasma glucose must be measured immediately to rule out diabetes mellitus, as hyperglycemia is a common and serious cause of polydipsia in children 1
• Blood urea nitrogen (BUN) and creatinine to evaluate renal function and identify chronic kidney disease 1
• Complete metabolic panel including electrolytes (potassium, chloride, calcium, phosphorus, magnesium) to detect metabolic abnormalities 1

First-Line Urine Tests

• Urinalysis with urine osmolality is essential—inappropriately low urine osmolality (typically <200 mOsm/kg H₂O) in the presence of elevated serum osmolality suggests diabetes insipidus 1
• Urine specific gravity should be measured, as consistently low values (<1.035) indicate impaired urine concentrating ability 1
• Urine protein-to-creatinine ratio if proteinuria is detected on dipstick, as persistent proteinuria may indicate renal disease 1

Secondary Laboratory Evaluation

When Diabetes Mellitus is Suspected

• Hemoglobin A1c should be obtained if glucose is elevated or if the patient is obese 1
• Fasting lipid profile in children ≥10 years old once glycemic control is established 1
• Liver function tests in obese children to assess for metabolic syndrome 1

When Diabetes Insipidus is Suspected

• Plasma copeptin levels <21.4 pmol/L should prompt testing for arginine vasopressin (AVP) deficiency (central diabetes insipidus) 1, 2
• Response to desmopressin administration is diagnostic—central DI will respond with increased urine osmolality, while nephrogenic DI will not 1, 2
• Genetic testing should be performed early in patients with suspected nephrogenic diabetes insipidus, particularly testing AVPR2 and AQP2 genes in all symptomatic patients 1

When Renal Disease is Suspected

• Complete blood count with serum ferritin to assess for anemia and iron status 1
• Albumin and prealbumin to evaluate nutritional status, though these are affected by acute phase responses 1
• Urinary albumin excretion if not obtained within the previous 6 months, particularly in patients with diabetes or hypertension 1
• Renal ultrasound may be indicated if hematuria, infection, or renal insufficiency is present 1

Additional Considerations Based on Clinical Context

Screening for Secondary Causes

• Thyroid function tests (TSH) to rule out thyroid dysfunction, particularly in patients with type 1 diabetes who have increased risk of autoimmune conditions 1
• Serum calcium to exclude hypercalcemia as a cause of polyuria 1
• Alkaline phosphatase and specific vitamin levels (vitamin A, D) in patients with chronic conditions or on long-term medications 1

Monitoring Frequency

• Daily monitoring may be required for newborns, infants, critically ill patients, or those at risk of refeeding syndrome 1
• 2-3 times per week measurements are appropriate for clinically stable children 1
• Regular monitoring of serum electrolytes, urine volume and osmolality, and body weight is crucial once a diagnosis is established 1, 2

Critical Pitfalls to Avoid

• Do not rely solely on random glucose measurements—obtain fasting glucose and consider A1c for comprehensive diabetes screening 1
• Do not interpret low albumin as malnutrition in acutely ill children, as it reflects acute phase response and protein redistribution rather than nutritional status 1
• Do not delay genetic testing in suspected congenital nephrogenic diabetes insipidus—early testing using umbilical cord blood is recommended in male offspring of known female carriers 1
• Ensure proper correction of serum sodium for hyperglycemia (add 1.6 mEq/L to sodium for each 100 mg/dL glucose elevation) to avoid misinterpretation 1
• Recognize that normal laboratory values vary by age in pediatric patients—use age-appropriate reference ranges, particularly for creatinine and electrolytes 1