324. Lack of Accuracy of the International Classification of Disease, Ninth (ICD-9) Codes in Identifying Patients With Encephalitis

Abstract Background ICD-9 codes have been widely used in studies utilizing large national databases that evaluate the clinical epidemiology of encephalitis in the United States. Many studies have showed that ICD-9 codes have poor accuracy in stroke, multiple sclerosis and pulmonary fibrosis but their utility in encephalitis is unknown. Methods Retrospective study of all adults with a discharge diagnosis of encephalitis by an ICD-9 code. The study was performed in 17 hospitals from the Memorial Hermann Hospital and Harris Health Hospital system in the Greater Houston area from March 2010 until July 2015. Medical records were reviewed and a case was considered accurately classified as encephalitis if they met the definition established by the international encephalitis consortium. Results A total of 1,241 cases were identified by a discharge diagnosis of ICD-9 code as having encephalitis. The most common cause identified was not having a central nervous system infection in 580 (46.7%) patients. A total of 244 (19.6%) patients were correctly identified as having encephalitis. Other causes identified were nosocomial meningitis (11.9%), community-acquired bacterial meningitis (8.1%), aseptic meningitis (5.8%), fungal meningitis (5.4%), tuberculosis (2.0%), and parasitic meningitis (0.2%). Conclusion ICD-9 codes have poor reliability in identifying patients with encephalitis questioning the accuracy of large nationwide studies that utilize them to identify patients. Table 1: Correct Clinical Diagnosis in 1,241 Patients with a Discharge Diagnosis of Encephalitis by ICD-9 Codes.DiagnosisNumber of Patients (%)Non-CNS infection580 (46.7)Encephalitis244 (19.6)Nosocomial meningitis148 (11.9)Bacterial meningitis101 (8.1)Aseptic meningitis72 (5.8)Fungal meningitis68 (5.4)Tuberculous meningitis25 (2.0)Parasitic meningitis3 (0.2) Abbreviation: ICD-9, an International Classification of Disease, Ninth; CNS, Central Nervous System. Disclosures All authors: No reported disclosures.