ISAPI Filter Authentication

I have an ISAPI filter and I'm trying to get the user loggin name when NT
Challenge/Response is set. The problem is I can never get the loggin name
inside my "OnAuthenticate" function when I get the SF_NOTIFY_AUTHENTICATION from
IIS. The HTTP_FILTER_AUTHENT structure contains empty fields for username.
If Basic Authentication is used instead of Challenge/Response then everything is
fine. Also I tried to use the function "GetServerVariable" inside my "OnReadRawData" function to get the loggin name but I can only retrieve the
name once in a while and so it's not consistent. Doesn anyone else have this
kind of problem? Please help!