July 23, 2017
Water Deprivation Test 1: Strongly Suspected Case of DI Minimize

Water deprivation test 1: strongly suspected case of DI

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

Investigation of suspected cranial or nephrogenic diabetes insipidus and primary polydipsia

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: Strongly suspected cases of DI

DO NOT RESTRICT FLUIDS UNTIL THE TEST COMMENCES

Requirements

Accurate weighing scales for weighing the patient

Volumetric flasks (200 mL) for measuring hourly urine volumes.

Procedure

DO NOT RESTRICT FLUIDS UNTIL THE TEST COMMENCES

0800

Weigh subject and begin fluid balance chart take samples of urine and plasma for osmolality.

 

Commence fluid restriction

0900

and hourly thereafter weigh the subject, measure urine volume and urine osmolality

FLUID RESTRICTION SHOULD BE STOPPED IF:

  • there is a fall in weight > 5%
  • 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.

Interpretation

Post-dehydration osmolality (mOsm/kg)

Post DDAVP osmolality (mOsm/kg)

Diagnosis

plasma

urine

urine

 

283-293

> 750

> 750

normal

> 293

< 300

< 300

nephrogenic diabetes insipidus

> 293

< 300

> 750

cranial diabetes insipidus

< 293

300-750

< 750

chronic polydipsia

< 293

300-750

< 750

partial nephrogenic DI or primary polydipsia

> 293

300-750

> 750

partial cranial DI

NB: chronic primary polydipsia can dissipate the renal medullary osmotic gradient, thereby reducing the renal response to endogenous and exogenous AVP. In cranial DI, maximal urinary concentration may be achieved only after repeated DDAVP.

Sensitivity and Specificity

When well performed, the water deprivation test has a sensitivity and specificity of 95% for diagnosing and differentiating severe cranial DI and nephrogenic DI. The incidence of false positive and false negative results for PP or partial CDI/NDI is 30-40% (investigate further).

References

  • Thompson CJ. Polyuric states in man. Clin Endocrinol Metab 1989;3:473-497
  • Miller M, Dalakos T, Moses AM, Fellerman H, Streeten DHP. Recognition of partial defects in antidiuretic hormone secretion. Arch Intern Med 1970;73:721-729.
  

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