From the Guidelines
For an 18-month-old child presenting with polyuria and polydipsia, I recommend ordering serum glucose, electrolytes (sodium, potassium, chloride, bicarbonate), blood urea nitrogen (BUN), creatinine, calcium, urinalysis, urine specific gravity, and urine osmolality, as these tests are crucial in differentiating between the main causes of polyuria and polydipsia in toddlers, including diabetes mellitus, diabetes insipidus (central or nephrogenic), urinary tract infection, or psychogenic polydipsia, as suggested by the most recent expert consensus statement 1.
The initial evaluation of a child with polyuria and polydipsia should focus on identifying the underlying cause, which can be achieved through a combination of laboratory tests and clinical assessment.
- Serum glucose and urinalysis can help diagnose diabetes mellitus, which would show elevated serum glucose and glucosuria 1.
- Electrolytes, BUN, and creatinine can help assess renal function and identify any electrolyte imbalances.
- Urine specific gravity and osmolality can help differentiate between diabetes insipidus and other causes of polyuria, as diabetes insipidus presents with dilute urine despite elevated serum osmolality 1.
- Calcium levels are important as hypercalcemia can cause nephrogenic diabetes insipidus.
A first morning urine sample should be collected to assess concentration ability, and if diabetes insipidus is suspected, serum osmolality and antidiuretic hormone (ADH) levels should be added to the initial tests.
- Consider a water deprivation test under careful supervision if initial tests are inconclusive, as this can help confirm the diagnosis of diabetes insipidus 1.
- It is essential to note that early diagnosis is crucial, as some conditions like diabetes mellitus require immediate intervention to prevent complications, and delayed diagnosis can lead to severe consequences, including hypertonic dehydration and failure to thrive 1.
In contrast to other studies that may suggest different diagnostic approaches, the most recent expert consensus statement 1 provides the strongest evidence for the recommended laboratory tests, and its findings should be prioritized in clinical practice.
From the Research
Laboratory Tests for Polyuria and Polydipsia
The following laboratory tests are indicated for an 18-month-old child presenting with polyuria and polydipsia:
- Urine osmolality test to determine if the polyuria is due to a water or solute diuresis 2
- Serum osmolality test to help differentiate between primary polydipsia and antidiuretic hormone (ADH) deficiency or insensitivity 2
- Water deprivation test to establish the cause of polyuria if the initial evaluation is inconclusive 2, 3
- Vasopressin test to differentiate between neurogenic and nephrogenic diabetes insipidus (DI) 2
- Copeptin measurement, which may be a safer and faster biomarker for etiological diagnosis of polyuria-polydipsia syndrome in children 4, 5
- Estimation of free water clearance and measurement of electrolytes in blood and urine may also be useful in the diagnostic approach 6
Specific Tests for Diabetes Insipidus
For children presenting with polyuria-polydipsia syndrome, the following tests may be useful in diagnosing diabetes insipidus:
- Water deprivation test with assessment of ADH activity, which is currently the gold standard for differential diagnosis 4
- Copeptin assay, which may be valuable in the differential diagnosis of central diabetes insipidus, nephrogenic diabetes insipidus, and primary polydipsia 5
- Desmopressin test to assess responsiveness to desmopressin injection, which can help diagnose central diabetes insipidus 3