what was the last value on the creatine kinase (ck) lab test of patient 54894 this month?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 54894 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'creatine kinase (ck)' ) and datetime(labevents.charttime,'start of month') = datetime(current_time,'start of month','-0 month') order by labevents.charttime desc limit 1