Calling get_currentuserinfo() from an external PHP file

I’m writing a shoutbox script for my blog (none of the available plugins provide the functionality/versatility i need), and I’ve run into a problem that has stumped me.
I have javascript code that uses AJAX to get message data from ‘shoutbox.php’ using POST.

$.ajax({
            type: "POST", url: "http://MYURL/chat/shoutbox.php", data: "action=update",
            complete: function(data){
                loading.fadeOut();
                messageList.html(data.responseText);
                messageList.fadeIn(2000);
            }
        });

I include wp-load.php at the top of my shoutbox.php file so that I can use wordpress functions in it. For instance, shoutbox.php contains this line:

$user_info = get_userdata($row['user']);
 echo $user_info->display_name

Which works perfectly to display the users name. I cannot, however, use the get_currentuserinfo() method. The following code

require('../nizz/wp-load.php');
global $current_user, $user_level;
get_currentuserinfo();
echo $user_level."a";
echo $current_user->user_level."b";

only prints “ab”.
How can I use the get_currentuserinfo() function to get the user level in this external file? Is there another method to get user level from this file?
I’d appreciate any help, thank you.