instruction: tell me the name of the organism found on the last sputum, expectorated microbiology test of patient 031-15417 until 20 months ago?
response: SELECT microlab.organism FROM microlab WHERE microlab.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '031-15417')) AND microlab.culturesite = 'sputum, expectorated' AND DATETIME(microlab.culturetakentime) <= DATETIME(CURRENT_TIME(), '-20 month') ORDER BY microlab.culturetakentime DESC LIMIT 1