what was the last value of patient 80555's laboratory test for ptt in the current hospital encounter?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 80555 and admissions.dischtime is null ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ptt' ) order by labevents.charttime desc limit 1