instruction: when has patient 46590 first had the ptt minimum value during their first hospital encounter?
response: SELECT labevents.charttime FROM labevents WHERE labevents.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 46590 AND NOT admissions.dischtime IS NULL ORDER BY admissions.admittime LIMIT 1) AND labevents.itemid IN (SELECT d_labitems.itemid FROM d_labitems WHERE d_labitems.label = 'ptt') ORDER BY labevents.valuenum, labevents.charttime LIMIT 1