Polyuria - Diagnosis Tests| Water Deprivation Test, Vasopressin (Pitressin Test)
POLYURIA
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Last Updated : 10/1/2014
Kumud P Mehta
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Diagnostic tests should be done in a stepwise manner and practical approach is as follows:
- Establish presence of polyuria
- Morning sample of urine to be tested for sugar (Diabetes Mellitus), Specific Gravity (less than 1005 - Diabetes insipidus {DI}, 1010 - chronic renal failure).
- Plasma and urinary osmolality are important investigations to decide further work up: If urine/plasma osmolality is less than 1: water deprivation test should be done to differentiate polydipsia from diabetes insipidus. If urine/plasma osmolality is more than 1.5 - solute diuresis is considered.
- IV Urinary calcium, sodium chloride excretion, BUN, S. creatinine, arterial blood PH and TCo2 or bicarbonate for diagnosis of chronic renal failure and renal tubular acidosis and Fanconi's syndrome.

Water deprivation test: A 12 hours overnight fast is kept during which water is withheld (8pm - 8am). 8am urine sample is collected for quantity, specific gravity and osmolality. Weight loss up to 5% may be found. Hourly urine sample is tested for specific gravity & osmolality and serum sodium and plasma osmolality estimated after 2-3 hours.

Interpretation
* If urinary osmolality increases 2 times after water deprivation i.e. >750 mosm/kg- polydipsia
* If urinary osmolality <750 mosm/kg - D.I. Pitressin or vasopressin test should be done to differentiate central from nephrogenic DI.
* If urine Osmolality increases to 500 mosm/kg and urine/plasma osmolality is or more- polydipsia
Serum sodium levels may reach up to 160-170 meq/l in diabetes insipidus and dehydration may occur, then the test should be stopped.

Vasopressin or pitressin test to differentiate Central from Nephrogenic DI.
Inj Pitressin in oil 10mcg for infants and 20 mcg for children is given subcutaneously or by nasal insufflation.
- In central DI: urinary osmolality and specific gravity increases within 2-4 hours and urine volume falls.
- In nephrogenic DI: urinary osmolality, specific gravity and output does not show dramatic improvement.

Special imaging studies
- CT/MRI of brain for signal in post pituitary, to detect cysts, tumors, hydrocephalus craniopharyngiomas, histiocytosis as secondary causes of central DI.
- Ultrasonography of KUB with MCU for diagnosis of obstructive uropathy, reflux nephropathy, hydronephrosis, cystic kidney disease etc.

Genetic studies for diagnosis of inherited types of nephrogenic DI, distal RTA, nephronophthisis, cystinosis etc.



Contributor Information and Disclosures

Kumud P Mehta
Consultant Pediatrician & Pediatric Nephrologist, Jaslok Hospital & Research Centre, Bai Jerbai Wadia Hospital for children, Mumbai, India.


First Created : 1/9/2002
Last Updated : 10/1/2014

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