Search Filter in Active Directory

Hi ,

 Can any body suggest me the search filter based on the ADGUID to
update fields in active directory

 I am using
search.Filter = String.Format(CultureInfo.InvariantCulture, "(&(GUID=
{0})(objectClass=person))", a);

 where a is the variable which contains the value of ADGUID

       Please reply ASAP,it's urgent