what was the last value on the sodium lab test of patient 42956?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 42956 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'sodium' ) order by labevents.charttime desc limit 1