Diabetes insipidus presents with symptoms that are very similar to the much more common diabetes mellitus. Increases in thirst and urination are seen with both forms of diabetes.

Diagnosis of diabetes insipidus therefore involves ruling out diabetes mellitus and then establishing which type of diabetes insipidus a patient has. The treatment approach depends on whether the patient has cranial (or central) diabetes insipidus or nephrogenic diabetes insipidus.

The tests for diagnosing diabetes insipidus include:

However, if the injection of ADH fails to stop urine production, a diagnosis of nephrogenic diabetes insipidus is confirmed as adequate amounts of ADH are present but the kidneys are simply not responsive to it.

• Blood and urine is tested for levels of glucose, calcium and potassium. In cases of high sugar in the urine, diabetes mellitus may be suspected.

• The water deprivation test:

  This test involves depriving a patient of water for several hours. Individuals with diabetes insipidus pass large amounts of water in their urine despite this deprivation whereas a healthy individual would pass a small amount of concentrated urine.

• Vasopressin test:

  If the patient's urine is not concentrated enough, they may be administered an injection of the hormone vasopressin or antidiuretic hormone which regulates the body's fluid content. If the vasopressin is successful in stopping urine production, this indicates that the diabetes insipidus is caused by a vasopressin deficiency, meaning the patient has cranial diabetes insipidus. In the cranial form of the condition, brain structures that make and store vasopressin are affected.

• Imaging studies of the brain such as a CT scan or MRI scan are used to look for any structural damage to the hypothalamus (where ADH is made) or pituitary gland (where ADH is stored). Damage to these structures confirms the cranial form of diabetes insipidus.