QUESTION  I would be very grateful for your views on this water deprivation test
>59 years old lady non-diabetic, with recent diagnosis of adrenal insufficiency, onhydrocortisone. Symptoms of polyuria, polydipsia and thirst for 18 months with a urine output of over 4 litres in 24 hours. She underwent a water deprivation test. Results were as follows.
Baseline data:
weight 55.8 kg
plasma osm 285 mOsm/Kg
Urine osm 200 mOsm/Kg
After 8 hours, data were as follows:
Weight 53.6kg Plasma osm 294 mOsm/Kg, Urine osm 463 mOsm/Kg
Post DDAVP, data were as follows: Plasma osm 292 mOsm/Kg, Urine osm 796 mOsm/Kg

What do make of this result?     Yahya Mahgoub,MD <yahyamme@gmail.com>
Background-The water deprivation test (WDT) is an indirect measure of the production and action of AVP in response to dehydration. It has a number of limitations.

  1. Patient acceptability (low)
  2. Sensitivity
  3. Specificity
  4. Labour and resource-intense

Importantly, clinical utility is often limited by the high frequency of ‘grey area’ results: those falling outside the range in which the positive predictive value of DI is high.  This reflects, in part, the frequency of partial defects In addition to the problems in test performance.

A standard interpretation guide to diagnosing central diabetes insipidus (CDI) is given below.

  1. Urine Osmolality <300 mOsm/Kg and Plasma Osmolality >290 mOsm/Kg after dehydration
  2. Urine Osmolality >750 mOsm/Kg after DDAVP

A standard interpretation guide to diagnosing nephrogenic diabetes insipidus (NDI) is given below.

  1. Urine Osmolality <300 mOsm/Kg and Plasma Osmolality >290 mOsm/Kg after dehydration
  2. No response to DDAVP

Urine osmolality exceeding 750 mOsm/Kg during water deprivation phase of test excludes significant Diabetes Insipidus, particularly if plasma osmolality does not exceed 295 mOsm/kg. Hence DDAVP would not need to be administered as part of the test in such cases.

Interpretation & recommendation

During the WDT, the patient looses 2.2 kg of weight. This represents 2.2 litres of fluid. Baseline plasma osmolality is a little low for someone with DI, and would raise with me the suspicion of fluid loading overnight. Following dehydration, the patient fails to concentrate urine to the level at which we can exclude diabetes insipidus, meaning we should continue to consider the diagnosis given urine output of 4 litres in 24 hours. Response to DDAVP demonstrates normal renal response. This excludes NDI and remains consistent with CDI.

The coincidence of adrenal insufficiency and CDI raises the suspicion of a pituitary problem producing both. There may be a history of the polyuria and polydipsia getting worse after commencing hydrocortisone. This would reflect the requirement of cortisol to enable excretion of a free water load: absence of cortisol leading to AVP-independent renal AQP2 expression. A pituitary MRI may reveal a mass and/or the absence of a posterior pituitary bright spot. As CDI is uncommon with primary pituitary adenomas, a hypothalamic lesion or secondary deposit within the gland should be considered as a differential of any mass found.  Dr S Ball  26/5/15