when was the last time patient 15986 got a lab test in this month for lactate?
select labevents.charttime from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'lactate' ) and labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15986 ) and datetime(labevents.charttime,'start of month') = datetime(current_time,'start of month','-0 month') order by labevents.charttime desc limit 1