If the history and physical examination are not revealing, a urinalysis should be obtained In almost all cases of polydipsia, the urine-specific gravity will be low (less than 1.010) A specific gravity greater than 1.020 usually represents appropriate thirst If the urinalysis is abnormal, DM (glucosuria, possibly ketonuria, and pseudo-hypersthenuria), sickle cell disease (isosthenuric), or an intrinsic renal disorder (cellular elements and sediment) should be suspected If the urinalysis is normal, electrolytes, calcium, and renal function tests may reveal conditions associated with electrolyte imbalances Patients with poorly controlled DM, DI, or nephrogenic DI may have hypernatremia if they are examined when dehydrated A hemoglobin electrophoresis may be needed to determine whether the patient has sickle cell disease However, patients with sickle cell disease usually have the diagnosis confirmed before the development of tubular dysfunction and polydipsia Because of the high resolution required to diagnose most intracranial causes, magnetic resonance imaging scan is usually necessary TABLE 64.1 CAUSES OF POLYDIPSIA Diabetes mellitus Electrolyte imbalances Hypercalcemia Hypokalemia Bartter syndrome Catecholamine excess Pheochromocytoma Neuroblastoma Ganglioneuroma Cystinosis Diabetes insipidus (antidiuretic hormone deficient) Craniopharyngioma Pituitary adenoma Langerhans cell histiocytosis Head trauma Sarcoidosis Leukemia Infection Aneurysm Intraventricular hemorrhage Hereditary Drugs Methylxanthines Amphotericin B Diuretics Lithium Renal causes Renal tubular acidosis Nephrogenic diabetes insipidus Sickle cell anemia Interstitial nephritis Obstructive uropathy Primary polydipsia Psychogenic polydipsia Neurogenic polydipsia FIGURE 64.1 Diagnostic approach to a child with polydipsia UA, urinalysis; BUN, blood urea nitrogen; Cr, creatinine; Ca, calcium TABLE 64.2 COMMON CAUSES OF POLYDIPSIA Diabetes mellitus Sickle cell anemia Diabetes insipidus (antidiuretic hormone deficient) Patients suspected of having primary polydipsia, DI, and nephrogenic DI require further testing that can be dangerous Because these tests need to be performed in a closely supervised, controlled setting, these patients should be admitted for evaluation Patients with primary polydipsia should respond to a water deprivation test by increasing their urine-specific gravity and osmolality Patients with DI and nephrogenic DI should have rapid weight loss while continuing to excrete urine