instruction: how many patients whose drug code is metr500pm and lab test fluid is ascites?
response: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN prescriptions ON demographic.hadm_id = prescriptions.hadm_id INNER JOIN lab ON demographic.hadm_id = lab.hadm_id WHERE prescriptions.formulary_drug_cd = "METR500PM" AND lab.fluid = "Ascites"