calculate the number of patients who underwent a laboratory test for cd4 cells, percent since 2100.
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select labevents.hadm_id from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'cd4 cells, percent' ) and strftime('%y',labevents.charttime) >= '2100' )