what was the first value of patient 83901's bilirubin, direct laboratory test in the previous month?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 83901 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'bilirubin, direct' ) and datetime(labevents.charttime,'start of month') = datetime(current_time,'start of month','-1 month') order by labevents.charttime asc limit 1