instruction: when was the last time patient 027-91255 was tested for ph until 26 months ago?
response: SELECT lab.labresulttime FROM lab WHERE lab.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '027-91255')) AND lab.labname = 'ph' AND DATETIME(lab.labresulttime) <= DATETIME(CURRENT_TIME(), '-26 month') ORDER BY lab.labresulttime DESC LIMIT 1