Query using custom field to sort alphabetically

I’m sorting my posts in the category list alphabetically using

<?php
 $posts = query_posts($query_string .
'&orderby=title&order=asc');
?>

My post titles are names, so it obviously sorts using the first name. I’d like to sort using the last name and have created a custom field called “sortname” with the last name first. What does the query above need to be to sort using the custom field rather than the title?