What labs to order for an 18-month-old with polydipsia (excessive thirst) and possible polyuria (excessive urination)?

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, blood urea nitrogen, and creatinine, as suggested by the general evaluation of children with possible renal issues 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, as recommended for the initial evaluation of children with possible urinary tract issues 1. Measure serum calcium levels to rule out hypercalcemia. Consider thyroid function tests (TSH, free T4) as thyroid disorders can cause similar symptoms. A hemoglobin A1c and random blood glucose should be checked to screen for diabetes mellitus, as hyperglycemic crises can present with 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 crucial because polyuria and polydipsia in a toddler could indicate serious conditions like diabetes insipidus, diabetes mellitus, kidney disease, or electrolyte disorders, and early diagnosis is essential to prevent dehydration and electrolyte imbalances, which can be particularly dangerous in young children, as emphasized by the latest consensus on the diagnosis and management of congenital nephrogenic diabetes insipidus 1.

Some key points to consider when ordering these labs include:

• The importance of serum and urine osmolality in diagnosing diabetes insipidus 1
• The need for a comprehensive metabolic panel to assess electrolyte imbalances and renal function 1
• The role of urinalysis in detecting urinary tract infections or other abnormalities 1
• The potential for thyroid disorders to cause similar symptoms, necessitating thyroid function tests
• The importance of screening for diabetes mellitus, given its potential to cause hyperglycemic crises 1

From the Research

Laboratory Tests for Polydipsia and Polyuria in an 18-month-old

To investigate the cause of polydipsia and polyuria in an 18-month-old, several laboratory tests can be ordered. The following tests are relevant:

• Fasting plasma glucose level to rule out diabetes mellitus 2
• Urinalysis to check for signs of urinary tract infection, kidney disease, or diabetes 3, 4
• Electrolyte panel to evaluate sodium, potassium, and chloride levels 3
• Renal function tests, including serum creatinine and urea, to assess kidney function 3, 4
• Blood gas analysis to evaluate acid-base balance 3
• Aldosterone and renin levels to investigate possible renal artery stenosis or other renal disorders 3
• Vitamin D and calcium levels to rule out vitamin D intoxication or other metabolic disorders 5
• Fructosamine and glycosylated hemoglobin (HbA1c) levels to assess glucose control and detect diabetes or other glucose tolerance abnormalities 2, 6

Additional Tests

Depending on the results of the initial laboratory tests, additional tests may be ordered, such as:

• Abdominal ultrasound to evaluate kidney size and structure 3, 4
• Brain magnetic resonance imaging (MRI) or computed tomography (CT) scan to rule out cerebral ischemia or other neurological disorders 3
• Angiographic studies to investigate possible renal artery stenosis or moyamoya disease 3