instruction: give me the number of patients whose diagnoses icd9 code is 42831 and lab test fluid is other body fluid?
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.icd9_code = "42831" AND lab.fluid = "Other Body Fluid"