Reply to Jackson.

TO THE EDITOR—We appreciate Dr Jackson’s thoughtful comments regarding the diagnostic criteria for encephalitis proposed by the International Encephalitis Consortium (IEC) Working Group [1]. He raises concerns that (1) individual patients may have laboratory or pathologic evidence of encephalitis without meeting the clinical criteria specified in the IEC case definition, and (2) that patients with encephalitis and an additional cause for confusion or obtundation might be inappropriately excluded. The IEC case definition was developed as a tool for epidemiologic surveillance, research, and outbreak investigation. To identify patients prospectively, we believed that it was important to devise standardized criteria that relied on clinical aspects and initial laboratory testing, exclusive of results of diagnostic testing for specific etiologies. Furthermore, as no etiology is identified in approximately 50% of patients with encephalitis [2], the case definition was specifically formulated to be independent of causality. The case definition was not designed to be comprehensive and, as the article specifies, would not identify patients without altered mentation (eg, those with postvaricella cerebellar ataxia). Moreover, we stand by our emphasis on the importance of clinical manifestations of disease, as a positive test result may well represent a false positive or indicate prior or latent infection, in a patient who does not meet IEC case definition for encephalitis [3–9]. We agree with Dr Jackson that altered mental status is a common presentation for a number of conditions that mimic encephalitis. To maximize specificity, the IEC case definition excludes patients with alternative etiologies (eg, subarachnoid hemorrhage, urosepsis, and bacterial meningitis). This exclusionary criterion is only applicable when there is a clear-cut alternative diagnosis. Patients with depressed mentation and no alternative explanation, including those with metabolic