count the number of patients who received their staph aureus screen microbiology test in the previous year.
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 datetime(microbiologyevents.charttime,'start of year') = datetime(current_time,'start of year','-1 year') )