how much patient 84346 has changed in urea nitrogen last measured on the last hospital visit compared to the first value measured on the last hospital visit?
select ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 84346 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'urea nitrogen' ) order by labevents.charttime desc limit 1 ) - ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 84346 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'urea nitrogen' ) order by labevents.charttime asc limit 1 )