From the Research
A water deprivation test should be conducted under medical supervision, with a simplified and improved protocol, to diagnose conditions like diabetes insipidus, with a cut-off of < 400 mosmol/kg in urine and > 302 mosmol/kg in serum, achieving a sensitivity of 90% and specificity of 98%. The test involves restricting fluid intake for a duration that can be as short as 5 hours, while monitoring body weight, urine output, urine osmolality, and serum sodium levels 1. Begin by having the patient empty their bladder and record baseline weight, urine osmolality, and serum sodium. Then restrict all fluid intake while collecting hourly urine samples to measure volume and osmolality. Monitor weight every 2 hours, and stop the test if the patient loses more than 3% of body weight, serum sodium exceeds 150 mmol/L, or orthostatic hypotension develops. Some studies suggest that the water deprivation test can be combined with an overnight unsupervised period, which has been shown to be effective and safe in diagnosing patients with polyuria-polydipsia syndrome 2. However, the most recent and highest quality study recommends a simplified protocol with a shorter duration and a clear cut-off for diagnosis 1. After the water restriction period, administer desmopressin (DDAVP) 2 mcg subcutaneously or 10-20 mcg intranasally and collect additional urine samples at 1 and 2 hours post-administration. In central diabetes insipidus, urine osmolality will remain low during water restriction but increase significantly after desmopressin. In nephrogenic diabetes insipidus, urine osmolality remains low despite desmopressin. In primary polydipsia, urine osmolality increases appropriately during water restriction. This test works by stimulating the body's natural antidiuretic hormone (ADH) response to dehydration, which should concentrate urine unless there's a defect in ADH production or kidney response. Key points to consider when conducting a water deprivation test include:
- Monitoring patient's weight, urine output, and serum sodium levels closely
- Stopping the test if the patient loses more than 3% of body weight or develops orthostatic hypotension
- Using a clear cut-off for diagnosis, such as < 400 mosmol/kg in urine and > 302 mosmol/kg in serum
- Considering the use of desmopressin to differentiate between central and nephrogenic diabetes insipidus. It's also important to note that newer diagnostic tools, such as the measurement of copeptin, may improve the accuracy of diagnosis in the future 3.