what's the last test patient 64881 has had until 04/2104?
select d_labitems.label from d_labitems where d_labitems.itemid in ( select labevents.itemid from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 64881 ) and strftime('%y-%m',labevents.charttime) <= '2104-04' order by labevents.charttime desc limit 1 )