what's the last time patient 54825 had a minimum ptt?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 54825 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'ptt' ) order by labevents.valuenum asc, labevents.charttime desc limit 1