bind parameter in existsnode function

I am trying to bind a parameter as the parameter to the existsnode() function.

     select extract(trans_details,'/Transaction/Details/ Key').getStringVal()

     from transaction_audit
     where existsnode(trans_details, :xml_value) = 1

the value I want to pass in (and that works when used in-line) is

     '//Key[@Value=''True''] and //Key[@Name=''Validation Status'']'

the xml snippet that exists inside the trans_details column is

     <Key Name="Validation Status" Value="True"/><Key Name="Authentication Status" Value="False"/> Received on Fri May 16 2008 - 09:29:21 CDT