what was the last value of the calculated total co2 lab test for patient 56201 until 09/2103?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 56201 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'calculated total co2' ) and strftime('%y-%m',labevents.charttime) <= '2103-09' order by labevents.charttime desc limit 1