whats the first value of anion gap until 61 months ago for patient 64538?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 64538 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'anion gap' ) and datetime(labevents.charttime) <= datetime(current_time,'-61 month') order by labevents.charttime asc limit 1