Is it reduced with desmopressin (dDAVP)?
Familial CDI, which seems to be the type that runs in your family, has no appreciable effect on GFR unless lack of treatment leads to permanent bladder distention, incomplete drainage, infections, and chronic pyelonephritis. This complication is uncommon, however, even in patients who are not treated for their CDI. Desmopressin has no direct effect on GFR that I am aware of. It may increase it indirectly by increasing total body water but this effect is probably quite small.
