eddie reader on "pronlem with get_the_category"

I am trying to collect tracking data with regards to blog usage. I can collect everything I want except the category the individual blog belongs to,
I have tried the following
$cats = get_the_category(', ');
but I get nothing returned. The hack is in single.php where there is the following piece of code
<?php the_category(', ') ?>
to give the linked reference to a Category on the blog page.
Anybody any ideas?
Thansk Eddie Reader