when did patient 19894 get the last microbiological test until 08/2104?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 19894 ) and strftime('%y-%m',microbiologyevents.charttime) <= '2104-08' order by microbiologyevents.charttime desc limit 1