instruction: find the number of patients categorized under blood gas lab test who were diagnosed with injury to femoral nerve.
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.long_title = "Injury to femoral nerve" AND lab."CATEGORY" = "Blood Gas"