instruction: count the number of patients whose drug code is thym25i and lab test fluid is cerebrospinal fluid (csf).
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 = "THYM25I" AND lab.fluid = "Cerebrospinal Fluid (CSF)"