Nephrology

525 Urine Concentration Test

  • Urine Concentration Test  

    This test is performed on children suspected of having diabetes insipidus (because of polyuria/polydipsia). 

    Notify Biochemistry that a Urine Concentration/Water Deprivation Test will be performed on the next day.

    1. Ask the parent how long the child can go without a drink at home (= X hours)
       
    2. Commence test X hours before 8.00 am next day.  At commencement of the test weigh child and then give a small drink of water
       
    3. Calculate 3% of body weight and subtract this from starting weight to determine the weight at which the child will be 3% dehydrated
       
    4.  Withhold all fluids and weigh child 1 hourly.  The patient should be checked regularly by the registrar during the water deprivation.  In patients with Diabetes Insipidus, dehydration can occur in 3-4 hours
       
    5. Collect a sample (aliquot) of each urine specimen passed in separate containers
       
    6. When body weight falls 3% blood is taken for serum electrolytes and osmolality and the next urine passed is collected.  The results of the serum electrolytes and osmolality must be known within 30 minutes of taking the blood
       
    7. The test is now complete and the child may drink again.  Send all urine samples to Biochemistry for Osmolality

    Interpretation

    If body weight drops 3% and serum osmolality is > 290 mosm/kg H2O the test has been performed satisfactorily.  If serum osmolality is less than 290 or body weight has not dropped 3% the test is unsatisfactory and the urine osmolalities may be uninterpretable.

    If the test has achieved adequate dehydration (3% weight loss), urine osmolality >800 mosm/kg indicates normal urine concentrating capacity.  Maximum urine osmolality <250 mosm/kg indicates diabetes insipidus.  Maximum urine osmolality between 250 and 800 mosm/kg indicates either an unsatisfactory test, partial diabetes insipidus, chronic renal failure, diuretic administration or severe psychogenic polydipsia.

    DDAVP response Test

    A positive Urine Concentration Test (ie Failure to concentrate urine >250 mosm/kg) indicates a diagnosis of Diabetes Insipidus.  Response to DDAVP indicates whether the problem is due to deficiency of ADH or resistance to ADH (nephrogenic diabetes insipidus. 

    DDAVP response can be determined at the end of a Urine Concentration Test or can be determined on a separate occasion.

    Intranasal DDAVP, O.25 ml (25 µg) is given and urine collected in separate aliquots for 6 hours.  The patient may drink ad lib.  Urines are sent for osmolality (if no increase in osmolality, ADH resistance is present).