What is the best way to distinguish between partial CDI and dipsogenic DI or another form of primary polydipsia?
One of the biggest problems in the differential diagnosis of DI is distinguishing partial CDI from dipsogenic DI or other forms of primary polydipsia. There are two ways to make this distinction. One way is to measure plasma AVP during a suitable osmotic stimulus such as fluid deprivation with or without hypertonic saline infusion. The decision on whether or not to use hypertonic saline depends on how the patient responds to fluid deprivation. If the latter results in concentration of the urine before plasma osmolality and sodium concentration reach the level necessary to unambiguously differentiate a normal from a subnormal rise in plasma AVP (Posm 295 to 300, Pna 143 to 146 ) then hypertonic saline should be given until the desired endpoint is reach at which time the measurement of plasma AVP is repeated. The other way to make this distinction is give a closely monitored therapeutic trial of dDAVP at doses sufficient to completely normal urine osmolality and volume for 24 to 48 hours
