tell me the first time that patient 87942 had a pt lab test in the last hospital visit?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'pt' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 87942 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) order by labevents.charttime asc limit 1