Indication
The investigation of polyuria (Urine volume consistently >2.5L/24hr)
Contraindications
Firstly exclude patients with known chronic renal disease, diabetes mellitus or hypercalcaemia.
Thyroid & adrenal function must be normal or adequately replaced.
The test should not be performed if the patient is already dehydrated.
Patient preparation
If the patient can produce an EMU of >750mosmol/kg , diagnosis excluded.
If the patient is being treated with DDAVP this should be stopped 24 hours before the test. Lysine vasopressin may be used to control any polyuria but should be stopped 8 hours before the test.
Allow caffeine free fluids freely from midnight before the test.
An early light breakfast is permitted before the test, which should start at 0830hr.
Allow fluids freely before the test, which should start at 0830hr. A light breakfast is permitted.
No smoking permitted overnight or during the test.
Procedure
The patient is weighed before the test and after 4, 6, 7 and 8 hours. The test should be stopped if the body weight
falls by more than 3% of baseline weight, or if the patient feels unwell or looks dehydrated.
Arrange the procedure with the laboratory in advance, most samples require urgent analysis.
Measurements
At 0830h the bladder should be emptied and urine discarded. Urine is then passed hourly and the volume recorded.
The following urine samples should be saved.
| Urine 1: | First hour: | 0830 - 0930
|
| Urine 2: | third to fourth hour: | 1130 - 1230
|
| Urine 3: | sixth to seventh hour: | 1430 - 1530
|
| Urine 4: | seventh to eighth hour: | 1530 - 1630
|
At the mid point of each saved urine sample, 5mL blood (gold top with gel vial) should be
taken as follows for osmolality and sent to the laboratory, with its paired urine sample, without undue delay.
| Serum 1: | 0900hr
|
| Serum 2: | 1200hr
|
| Serum 3: | 1500hr
|
| Serum 4: | 1600hr
|
Interpretation guidelines
- If the result of Serum 1 (taken at 0900) is low (280mosmol/kgH2O), water depletion is unlikely and polyuria is probably an appropriate response to excessive water intake. The test may be discontinued at this point.
- If the result of Serum 1 is high (over 300mosmol/kgH2O) it may be dangerous to continue the test.
- A normal subject will produce urine with an osmolality of 600 mosmol/kgH2O or more and the plasma osmolality will not rise above 300mosmol/kgH2O because water will be retained to prevent dehydration.
- In diabetes insipidus the plasma becomes abnormally concentrated with an osmolality that may exceed 300mosmol/kgH2O whilst the urine osmolality remains less than that of the plasma.
- In order to differentiate between cranial and nephrogenic diabetes insipidus proceed to the
DDAVP test. (see below.)
- In compulsive water drinkers the plasma osmolality at the start of the test is normal or low and as a consequence of water deprivation the urine and plasma osmolality return to normal,
although a minor degree of impaired concentration, not as severe as in true diabetes insipidus,
may occur.
DDAVP TEST
INDICATION
The further investigation of diabetes insipidus
- The patient should now eat and drink normally
- At 1630hr give DDAVP 2µg IM.
- Collect urine: 2 hourly - i.e. 1630 - 1830, 1830 - 2030.
- Take blood for serum osmolality at 2030hr.
Interpretation guidelines
Cranial diabetes insipidus: serum osmolality returns to normal and urine osmolality increases to greater than 600mosmol/kgH2O.
Nephrogenic diabetes insipidus is suggested by a failure to respond.
| OSMOLALITY mosmol/kg | DIAGNOSIS
|
| BEFORE DDAVP | AFTER DDAVP |
|
| PLASMA | URINE | PLASMA | URINE |
|
| 280-300 | >600 | | | NORMAL
|
| >300 | <300 | FALLS | >600 | CRANIAL D.I.
|
| >300 | <300 | <300 | | NEPHROGENIC
|
| <300 | 300 - 750 | | | VARIOUS*
|
*VARIOUS DIAGNOSES
Partial Cranial diabetes insipidus
Partial Nephrogenic diabetes insipidus
Patient drinking before or during the test, possible compulsive water drinking.
WATER DEPRIVATION TEST
Please inform the laboratory (Ex 7704) when a test is going to take place.
| TIME | HOUR | URINE | BLOOD | WEIGH
|
| 0830 | 0 | | | 
|
|
|
| 0830 - 0930 | 1 | | |
|
|
|
| 0900 | | | 1 |
|
|
|
| 0930 - 1030 | 2 | | |
|
|
|
| 1030 - 1130 | 3 | | |
|
|
|
| 1130 - 1230 | 4 | | |
|
|
|
| 1200 | | | 2 |
|
|
|
| 1230 - 1330 | 5 | | |
|
|
|
| 1330 - 1430 | 6 | | |
|
|
|
| 1430 - 1530 | 7 | | |
|
|
|
| 1500 | | | 3 |
|
|
|
| 1530 - 1630 | 8 | | |
|
| 1600 | | | 4 |
|
All samples should be sent directly to the laboratory,
each request form should be marked
WATER DEPRIVATION TEST.
