An increase in serum osmolality normally results in a release of antidiuretic hormone (ADH). If hypertonic saline is given intravenously so that the plasma osmolality increases, the changes in serum ADH levels can be analyzed.
Patient Preparation: An intravenous infusion of 3% saline is given at 0.1 mL per kg per minute.
Specimen Collection: Two samples are drawn 15 minutes apart for baseline levels. Once the saline infusion is begun, specimens are drawn every 15 minutes over a 2 hour period.
Testing: Specimens are tested for osmolality and ADH levels.
Interpretation:
The resultant ADH levels are plotted vs the osmolality of the serum specimen. Different regions for patient response may be seen:
• regions 1 and 2: seen in normal patients or patients with psychogenic polydipsia
• region 3: seen in central diabetes insipidus (high osmolality, low vasopressin)
• region 4: seen in nephrogenic diabetes insipidus (high osmolality, high vasopressin)
• region 5: seen in syndrome of inappropriate ADH secretion (low osmolality)
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