what is the last time patient 73913 had the microbiology test in the last hospital encounter?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 73913 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) order by microbiologyevents.charttime desc limit 1