what's the first value of the creatinine lab test of patient 12938 until 06/2105?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 12938 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'creatinine' ) and strftime('%y-%m',labevents.charttime) <= '2105-06' order by labevents.charttime asc limit 1