calculate the number of patients that had a staph aureus screen microbiology test since 2102.
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select microbiologyevents.hadm_id from microbiologyevents where microbiologyevents.spec_type_desc = 'staph aureus screen' and strftime('%y',microbiologyevents.charttime) >= '2102' )