Other Tests in Polyuria	

The most precise way to diagnose the three basic causes of polyuria rests on the measurement of plasma vasopressin, plasma osmolality, and urine osmolality after osmotic stimulation and/or dehydration. It can be seen in Fig. 5.19a that after infusion of five per cent hypertonic saline, patients with cranial diabetes insipidus have values that fall to the right of the normal range, while those with primary polydipsia or nephrogenic diabetes insipidus remain in the normal range.