instruction: calculate the number of times patient 027-53836 had received a anion gap laboratory test until 4 years ago.
response: SELECT COUNT(*) FROM lab WHERE lab.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '027-53836')) AND lab.labname = 'anion gap' AND DATETIME(lab.labresulttime) <= DATETIME(CURRENT_TIME(), '-4 year')