Which initial investigations should I perform to confirm a diagnosis of diabetes insipidus in a patient presenting with polyuria and polydipsia?

To confirm a diagnosis of diabetes insipidus in a patient presenting with polyuria and polydipsia, initial investigations primarily involve laboratory tests to assess fluid balance and osmolality Feingold et al. 2000. These initial steps are crucial for distinguishing diabetes insipidus from other causes of polyuria, such as primary polydipsia Feingold et al. 2000. Key initial investigations include measuring plasma osmolality, urine osmolality, and serum sodium levels Feingold et al. 2000. Following these initial blood and urine tests, a water deprivation test is often necessary to confirm the diagnosis and differentiate between central and nephrogenic diabetes insipidus Feingold et al. 2000. During the water deprivation test, plasma vasopressin (antidiuretic hormone, ADH) levels may also be measured to aid in diagnosis Feingold et al. 2000.