While evaluating a child with suspected polyuria, first step is to confirm polyuria by measuring the 24 hour urine output.
Once confirmed detailed history should be obtained about 24 hour fluid intake, type of fluids ingested, eating habits, drinking through day or night, weight gain/loss, episodes of dehydration, constipation etc.
Signs of failure to thrive, dehydration, rickets should be looked for in examination.
Next step is to do some basic investigations:
Blood tests for Serum Osmolality, Serum sodium, potassium, glucose, calcium, urea, creatinine.
Urinalysis (first morning urine): for osmolality, specific gravity and glucose
NORMAL OSMOLALITY RANGES
Serum osmolality 285 to 295 mOsm/kg
Random urine osmolality 300-900m mOsm/kg
Urine, after 12-14 hrs water deprivation > 850 mOsm/kg
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