instruction: what was the last value of patient 80555's laboratory test for ptt in the current hospital encounter?
response: 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