what's patient 51675's maximum value of monocytes until 40 months ago?
select max(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 51675 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'monocytes' ) and datetime(labevents.charttime) <= datetime(current_time,'-40 month')