instruction: when did patient 64519 receive his or her last microbiology test until 09/2105?
response: SELECT microbiologyevents.charttime FROM microbiologyevents WHERE microbiologyevents.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 64519) AND STRFTIME('%y-%m', microbiologyevents.charttime) <= '2105-09' ORDER BY microbiologyevents.charttime DESC LIMIT 1