SELECT based on function result

I have a function that tells me if a record is positive and negative
based on several field values. I use it in select statements:

ohc=> SELECT sample_id, is_wipe_positive(tblleadwipe.sample_id) AS
positive FROM tblleadwipe WHERE hud_building_id IS NOT NULL;
 sample_id | positive
-----------+----------
        73 | f
        81 | t
(2 rows)

I see that I cannot change my WHERE statement to WHERE positive = 't'
because the column positive does not exist. Now I am looking for the
best way to return all records that are found positive or negative using
a query. Can anyone offer any guidance to how I can return all the
positives (or negatvies)? Or do I need to write another function that
does that?

--
Robert