since 08/2105, when did patient 28443 receive the first sputum microbiology test?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 28443 ) and microbiologyevents.spec_type_desc = 'sputum' and strftime('%y-%m',microbiologyevents.charttime) >= '2105-08' order by microbiologyevents.charttime asc limit 1