A 50 year old man consults his GP due to concerns around urinary frequency. Over the past 6 months, he has begun passing urine up to 20 times a day. He reports feeling thirsty and thus drinking a lot of water.

His blood tests, including a capillary blood glucose, are within normal limits. Random urine samples come back as very dilute (<300mosmol/kg). The clinician wishes to distinguish between central and nephrogenic diabetes insipidus.

Which of the following is the next most appropriate investigation to confirm the diagnosis?

Options:
explanation: This is the correct answer. Diabetes insipidus (DI) is a condition characterized by the reduced production or response to antidiuretic hormone (ADH), resulting in excessive urination and thirst. Causes can be central or nephrogenic. A water deprivation test is a diagnostic tool used to differentiate between central and nephrogenic diabetes insipidus. It involves depriving the patient of fluids and administering exogenous desmopressin (ADH) whilst monitoring urine osmolality and body weight changes.

A specific diagnosis must be obtained as treatment between the two types of diabetes insipidus differs. Note that although very dilute random urine samples of <300mosmol/kg make psychogenic polydipsia unlikely, a water deprivation test is also helpful to conclusively rule this out.
name: Conduct water deprivation test
label: a
answer: yes
explanation: A water deprivation test is a diagnostic tool used to differentiate between central and nephrogenic diabetes insipidus, and so is the next more appropriate investigation. An MRI brain may be indicated if the water deprivation test indicates a central diabetes insipidus, however it is not required at this time.
name: MRI brain
label: c
answer: no
explanation: A diagnosis of diabetes insipidus is normally not given before the water deprivation test. After checking that urine is not normally concentrated by taking random urine samples, the next most appropriate step is to conduct the water deprivation test. This is a diagnostic tool used to differentiate between central and nephrogenic diabetes insipidus. The specific diagnosis must be obtained as treatment between the two types of diabetes insipidus differs.
name: No further investigations needed: diabetes insipidus and treat accordingly
label: b
answer: no
explanation: Early morning urine samples are not necessary as the random urine samples are sufficient to show that the urine is never normally concentrated. The next most appropriate step is to conduct the water deprivation test. This is a diagnostic tool used to differentiate between central and nephrogenic diabetes inspidus. The specific diagnosis must be obtained as treatment between the two types of diabetes inspidus differs.
name: Early morning urine osmolarity
label: d
answer: no
explanation: A water deprivation test is a diagnostic tool used to differentiate between central and nephrogenic diabetes insipidus, and so is the next more appropriate investigation. A CT abdomen may be indicated if the water deprivation test indicates a nephrogenic diabetes insipidus, however it is not required at this time.
name: CT abdomen
label: e
answer: no

Correct answer: a