Clinical Use
1 Investigation of equivocal osmolality values in patients with suspected diabetes insipidus.
2 Detection of inappropriate vasopressin secretion.
Applications
1 Diabetes insipidus.
High quality osmometry of plasma and urine samples obtained during strictly controlled water deprivation and after DDAVP administration is the preferred means of diagnosing diabetes insipidus and differentiation of the pituitary and nephrogenic forms of the disease. Measurement of plasma vasopressin will be required only in exceptional circumstances.
2 Inappropriate vasopressin secretion.
This syndrome is best diagnosed by osmometry. Measurement of plasma vasopressin will be required only in exceptional circumstances.
Patient Preparation
Drug (hypnotics, diuretics, opiates) therapy and alcohol ingestion must be discontinued if possible.
1 Investigation of diabetes insipidus.
The patient may have a light breakfast but a small amount of water to drink is permitted. Take blood (5 mL) at t0, and 10, 14 and 18h later whilst the patient receives nothing to eat or drink. Collect a urine specimen coincident with each blood sample. The test should be discontinued if the patient’s body weight falls by 5% or more.
For an 8h deprivation test, samples are taken at 0, 2, 4 and 8h. When either 8h have elapsed or the plateau of urine concentration is reached, DDAVP may be given in a dose of 2 or 20 mg intranasally and further urine samples collected for 12h. After administration of DDAVP the patient may have a light meal but drink no more than twice the volume of urine passed during fluid restriction.
Hypertonic saline infusion:
The patient is fasted overnight and only allowed to drink water before the test. A basal blood sample is obtained and then 0.855M NaCl (5% saline) is infused at a rate of 0.06 mL/kg/min for 2h. Blood pressure is recorded at 15 min intervals and blood samples taken at intervals with a final sample 15 min after the end of the infusion. If the patient experiences severe thirst they may suck a few chips of ice, but should avoid rapid ingestion of large volumes of fluid after the test.
2 Inappropriate vasopressin secretion.
With the patient recumbent, preferably in the morning, take blood (5 mL) and collect a urine specimen at this time.
Sample Preparation
Transfer the blood (5 mL) to a lithium heparin tube (preferably plastic). Separate the plasma promptly at 4C. Send a portion with the corresponding urine sample to the local laboratory for measurement of plasma and urine osmolality. Freeze the remainder of the plasma (-20C) immediately (in a plastic tube). Send frozen plasma samples (2 mL) to the SAS laboratory. Ensure the samples remain frozen during transport. Record plasma and urine osmolality values on the SAS request form.
Reference Range
Please contact the SAS laboratory.
Centre offering this assay