In a suspected case of diabetes insipidus, what laboratory finding is consistent with central diabetes insipidus?

In cases of suspected diabetes insipidus, the response to vasopressin (also known as antidiuretic hormone) is a critical factor in differentiating between central and nephrogenic diabetes insipidus. In central diabetes insipidus, there is a deficiency of vasopressin due to a problem with the production in the hypothalamus or its release from the posterior pituitary.

When vasopressin is administered to a patient who has central diabetes insipidus, their kidneys respond appropriately by reabsorbing more water, which results in an increased urine osmolality. This means that the kidneys become more water-permeable, allowing for a greater concentration of urine as more water is retained in the body. Therefore, an increase in urine osmolality after administration of vasopressin is a hallmark finding that supports the diagnosis of central diabetes insipidus.

In contrast, in nephrogenic diabetes insipidus, the kidneys are unable to respond to vasopressin, which would result in little to no increase in urine osmolality following administration of the hormone. Other findings such as changes in plasma osmolality during fluid deprivation can provide additional context, but they do not directly indicate a response to