how many patients have undergone a stool (received in transport system) test in 2105?
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select microbiologyevents.hadm_id from microbiologyevents where microbiologyevents.spec_type_desc = 'stool (received in transport system)' and strftime('%y',microbiologyevents.charttime) = '2105' )