From the Guidelines
For an 18-month-old child presenting with polyuria and polydipsia, I recommend ordering a comprehensive panel of laboratory tests to evaluate for diabetes insipidus and other causes of increased urination and thirst. Start with a basic metabolic panel to assess electrolytes, glucose, BUN, and creatinine, as recommended by recent guidelines 1. Include serum osmolality and paired urine osmolality to evaluate the kidney's concentrating ability, which is crucial in diagnosing diabetes insipidus 1. Order a urinalysis to check for glucose, protein, and specific gravity. Measure serum calcium levels to rule out hypercalcemia. Consider thyroid function tests (TSH, free T4) as hyperthyroidism can cause similar symptoms. A hemoglobin A1c and random blood glucose should be checked to screen for diabetes mellitus, as hyperglycemia can lead to polyuria and polydipsia 1. If the initial workup suggests diabetes insipidus, further testing may include a water deprivation test (under close supervision) and measurement of antidiuretic hormone (ADH) levels. These tests are essential because polyuria and polydipsia in a toddler could indicate serious conditions like diabetes insipidus, diabetes mellitus, kidney disease, or electrolyte disorders, which require prompt intervention to prevent dehydration and electrolyte imbalances, particularly in young children 1.
Some key points to consider when evaluating these laboratory results include:
- Serum osmolality is usually >300 mOsm/kg H2O owing to hypernatraemia in cases of diabetes insipidus, whereas urine is inappropriately diluted, with osmolality usually <200 mOsm/kg H2O 1
- Urine osmolality can be higher than 200 mOsm/kg H2O in milder cases of diabetes insipidus 1
- Infants are particularly at risk of dehydration as they have no free access to fluid, and the intake of large fluid volumes can cause gastro-oesophageal reflux and vomiting 1
- Early diagnosis is critical as some causes require prompt intervention to prevent dehydration and electrolyte imbalances, which can be particularly dangerous in young children 1
Given the potential severity of these conditions, it is crucial to prioritize the most recent and highest quality evidence when making diagnostic and treatment decisions, such as the international expert consensus statement on the diagnosis and management of congenital nephrogenic diabetes insipidus 1.
From the Research
Laboratory Tests for Polydipsia and Polyuria in an 18-Month-Old Child
To approach the diagnosis of polydipsia and polyuria in an 18-month-old child, several laboratory tests can be considered based on the potential underlying causes.
- Urinalysis: To check for the presence of glucose, protein, or other abnormalities that could indicate diabetes mellitus or renal issues 2.
- Blood Glucose Levels: Fasting plasma glucose or a random plasma glucose test to diagnose diabetes mellitus, which is a common cause of polydipsia and polyuria 2.
- Electrolyte Panel: To evaluate for any electrolyte imbalances, such as hyponatremia or hypokalemia, which could be related to renal tubular dysfunction or other conditions 3.
- Renal Function Tests: Including serum creatinine and urea to assess kidney function, especially if there's suspicion of acute kidney injury or chronic kidney disease 4.
- Vitamin Levels: Particularly vitamin D and vitamin A, as excessive intake can lead to intoxication, which may present with symptoms like hypercalcemia and could indirectly relate to polyuria 5.
- Hormonal Assays: Such as aldosterone and renin levels, which could be abnormal in cases of renovascular hypertension or other endocrine disorders affecting fluid balance 3.
Additional Diagnostic Considerations
- Abdominal Ultrasound: Useful for evaluating kidney size and structure, as well as detecting any obstructions or abnormalities in the urinary tract 3, 6.
- Imaging Studies of the Brain: If there's suspicion of a neurological cause for the symptoms, such as moyamoya disease, which can have systemic effects including on renal function 3.
It's crucial to tailor the diagnostic approach to the individual child's presentation and history, considering a wide range of potential causes for polydipsia and polyuria.