instruction: how many patients are diagnosed with peritonitis nos and tested under category blood gas in lab?
response: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN diagnoses ON demographic.hadm_id = diagnoses.hadm_id INNER JOIN lab ON demographic.hadm_id = lab.hadm_id WHERE diagnoses.short_title = "Peritonitis NOS" AND lab."CATEGORY" = "Blood Gas"