when was the first microbiological throat for strep test of patient 67480 performed in the first hospital visit?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 67480 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and microbiologyevents.spec_type_desc = 'throat for strep' order by microbiologyevents.charttime asc limit 1