count the number of patients who were given a blood (cmv ab) microbiology test until 1 year ago.
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select microbiologyevents.hadm_id from microbiologyevents where microbiologyevents.spec_type_desc = 'blood (cmv ab)' and datetime(microbiologyevents.charttime) <= datetime(current_time,'-1 year') )