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[@[EMAIL PROTECTED]
''True''] and //Key[@[EMAIL PROTECTED]
''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"/>