instruction: in the last hospital encounter when patient 031-23724 last got a microbiology test?
response: SELECT microlab.culturetakentime FROM microlab WHERE microlab.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '031-23724' AND NOT patient.hospitaldischargetime IS NULL ORDER BY patient.hospitaladmittime DESC LIMIT 1)) ORDER BY microlab.culturetakentime DESC LIMIT 1