Simple diagnosis of diabetes insipidus and antidiuretic hormone excess.

The value of information that may be obtained by measuring osmolality of body fluids is not generally appreciated by clinicians. Osmolality determination by freezing point depression is technically simple to perform and requires only 0.2 ml fluid. The simultaneous measurement of plasma and urine osmolality may yield useful information concerning alterations in water homeostasis (diabetes insipidus and antidiuretic hormone excess) and avoid uncomfortable and sometimes hazardous investigations of patients.