how many patients have actually had a urea nitrogen, body fluid test until 3 years ago?
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select labevents.hadm_id from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'urea nitrogen, body fluid' ) and datetime(labevents.charttime) <= datetime(current_time,'-3 year') )