The test with the restriction of liquids allows you to establish the diagnosis of diabetes insipidus and to conduct a differential diagnosis between nephrogenic and central non-diabetes mellitus.
The principle of this test is that dehydration increases the osmolarity of blood plasma. Osmolarity of more than 300 washes / kg is a powerful stimulus for the secretion of vasopressin, which in turn leads to a maximum concentration of urine. Since in most medical institutions of Russia, the study of the osmolarity of blood and urine is not carried out, the equivalent of this is the determination of serum sodium level and urine specific gravity.
Method of conducting. After an overnight fast, the patient is weighed, after which he stops taking fluids, usually from 8.00 in the morning. A basal laboratory test includes determining serum sodium and urea levels in the blood serum; specific weight and sodium level in urine. The duration of water deprivation ranges from 6-8 hours (in younger children and with significant polyuria) to 24 hours, depending on tolerance. Usually dehydration, stimulating the release of vasopressin, occurs after 6-9 hours. Every hour of the test, the patient’s weight is measured, the level of sodium in the blood serum, the volume and specific gravity of urine are examined. If an increase in serum sodium level of more than 3 mmol / l or a decrease in patient weight by 5% is noted, the sample should be discontinued. This confirms the presence of diabetes insipidus due to the absence of antidiuretic hormone.With sufficient secretion of antidiuretic hormone (healthy people or neurogenic polydipsia) during the test, a significant decrease in urine output, no increase in serum sodium, and no weight loss are observed. It is necessary during the test to strictly monitor the absence of fluid intake!
Further, for the differential diagnosis of central and nephrogenic non-diabetes mellitus, Minirin is prescribed in a dose: children 50 µg 2 times a day, adults 100 µg 2 times a day and Zimnitsky test is determined before and against Minirin. With central diabetes mellitus with Minirin intake, the volume decreases and the proportion of urine increases. The test is contraindicated in the presence of hypernatremia.
Vasopressin blood plasma
The test with restriction of liquids is sufficient for the diagnosis of diabetes insipidus and differential diagnosis between its central and nephrogenic form in most patients. In a number of clinics, vasopressin in the blood plasma is also determined during the test, although this indicator is rarely used in diagnostic tests. The value of this indicator refers primarily to the differential diagnosis between partial central and nephrogenic diabetes insipidus, while the hormone level is low in the first case and high in the second.