July 27, 2017
Water Deprivation Test 2: DI Not Strongly Suspected Minimize

Water deprivation test 2: DI not strongly suspected

This test is potentially very dangerous and must be undertaken with great care. Patients unable to conserve water may become critically dehydrated within a few hours of water restriction

Indication

If DI is not strongly suspected or if the level 1 test was uninterpretable ie not continued for long enough, or if there is already evidence that the kidneys have some concentrating ability eg random urine osmolality > plasma, then it is safe to proceed to the modified water deprivation test.

Contraindications

Other causes of polydipsia & polyuria eg diabetes mellitus, hypoadrenalism, hypercalcaemia, hypokalaemia, hypothyroidism, urinary infections, chronic renal failure and therapy with carbamazepine, chlorpropamide or lithium therapy.

If there is evidence for the ability to concentrate urine eg spot urine osmolality > 750 mOsm/kg.

Principle of test

Water restriction in the normal individual results in secretion of AVP by the posterior pituitary in order to reclaim water from the distal renal tubules. Failure of this mechanism results in a rise in plasma osmolality due to water loss, and a dilute urine of low osmolality. The two causes are a) a failure of AVP secretion and b) insensitivity of the renal tubules to AVP and they may be distinguished by the administration of DDAVP (synthetic AVP).

Side effects

Patients with true Diabetes insipidus may become severely water depleted during water deprivation and MUST be carefully monitored throughout the procedure.

Preparation

If DI is not strongly suspected, restrict food and drink after midnight prior to test.

Subjects should not smoke during the day of the test and should avoid alcohol and caffeine containing drinks on the previous evening (as alcohol caffeine and nicotine influence the action of AVP).

Requirements

  • Accurate weighing scales for weighing the patient
  • Volumetric flasks (200 mL) for measuring hourly urine volumes

Procedure

0000

Fluid restriction should commence

0800

weigh subject, measure urine volume and urine and plasma osmolality

0900

continue hourly with above urine measurements

FLUID RESTRICTION SHOULD BE STOPPED IF:

 

i) there is a fall in weight > 5%

ii) plasma osmolality > 300mOsm/kg.

This point should never be reached with careful observation of the subject.

Proceed to DDAVP test if urine osmolality rises < 30 mOsm/kg (in toto) over 3 successive urine samples. The test should be terminated if urine osmolality rises > 750 mOsm/kg

  

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