Increased free water clearance. Elevated plasma osmolality is more commonly associated with diabetes insipidus (DI) in which either production of AVP is lacking (central DI) or kidneys are insensitive to AVP (nephrogenic DI) [27]; each of which increases free water clearance and sufferers of DI are characteristically polyuric and polydipsic. This evidence concerns the gene AVP and Central diabetes insipidus.