when did during the last hospital visit patient 86786 last get a sputum microbiology test?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 86786 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and microbiologyevents.spec_type_desc = 'sputum' order by microbiologyevents.charttime desc limit 1