what was the first value of an mch lab test in patient 17953?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 17953 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'mch' ) order by labevents.charttime asc limit 1