Laboratory Evaluation for Pediatric Polydipsia
The initial laboratory work-up for a child presenting with polydipsia must include serum sodium, serum osmolality, plasma glucose, urine osmolality, and urinalysis to differentiate between diabetes mellitus, diabetes insipidus, and other causes of polyuria-polydipsia syndrome. 1
Essential First-Line Laboratory Tests
The following tests should be obtained simultaneously to establish the diagnosis:
- Plasma glucose is the most critical initial test to rule out diabetes mellitus, as hyperglycemia is a common and potentially life-threatening cause of polydipsia in children 1, 2
- Serum sodium and serum osmolality are essential to assess water balance and identify hyponatremia or hypernatremia 1, 2
- Urine osmolality must be measured to determine if the urine is inappropriately dilute (suggesting diabetes insipidus) or concentrated (suggesting osmotic diuresis or normal physiology) 1, 2
- Urinalysis with dipstick is mandatory to detect glucosuria (indicating diabetes mellitus) or proteinuria (suggesting kidney disease) 3, 1
Secondary Laboratory Evaluation Based on Initial Results
If Diabetes Mellitus is Suspected (Elevated Glucose):
- Hemoglobin A1c should be obtained to assess glycemic control 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
If Diabetes Insipidus is Suspected (Normal Glucose, Dilute Urine):
- Plasma copeptin levels should be measured, with levels <21.4 pmol/L prompting further testing for central diabetes insipidus 1, 4
- Blood urea nitrogen (BUN) and creatinine to assess renal function and identify chronic kidney disease 1
- Complete metabolic panel including electrolytes (sodium, potassium, chloride, bicarbonate) to detect metabolic abnormalities 1
- Serum calcium to exclude hypercalcemia as a cause of polyuria 1
Additional Diagnostic Considerations
- Thyroid function tests (TSH) should be performed to rule out thyroid dysfunction, particularly in patients with type 1 diabetes 1
- Genetic testing (AVPR2 and AQP2 genes) should be performed early in patients with suspected nephrogenic diabetes insipidus 1, 4
- 24-hour urine collection with measurement of urine volume and osmolality provides objective data to support the diagnosis and detect polyuria 3, 2
Critical Diagnostic Algorithm
The pattern of results guides the diagnosis:
- High glucose + glucosuria = Diabetes mellitus 1, 4
- Normal glucose + dilute urine (osmolality <200 mOsm/kg) + high-normal or elevated serum sodium = Diabetes insipidus 1, 4, 2
- Normal glucose + concentrated urine + normal serum sodium = Primary polydipsia or normal variant 2, 5
Important Clinical Caveats
A frequency-volume chart (bladder diary) is strongly recommended as it provides objective data on fluid intake and urine output, helps detect children with nocturnal polyuria, and identifies families with low adherence to instructions 3. This should be completed for at least 2 days with measured volumes, alongside documentation of symptoms for at least 1 week 3.
Daily monitoring may be required for newborns, infants, critically ill patients, or those at risk of refeeding syndrome, while clinically stable children may have measurements repeated 2-3 times per week 3, 1.
Do not proceed to water deprivation testing until basic laboratory evaluation is complete, as this invasive procedure should only be performed by a pediatric endocrinologist or nephrologist when the initial work-up fails to establish the diagnosis 5, 6.
The combination of urine osmolality <200 mOsm/kg with high-normal or elevated serum sodium is pathognomonic for diabetes insipidus and requires urgent subspecialty referral 4, 7, 2.