We retrospectively selected patients admitted to our department between September 2010 and October 2018. In AE+ group we included 18 patients with clinical diagnosis of AE, accompanied by the positivity in serum and/or CSF for known neuronal antibodies (NMDAR: 4 patients, LGI-1: 9, GAD: 2, Ma2: 2, Hu: 1). In AE- group we included 14 patients, who fulfilled the following criteria: a) clinical, MRI and CSF data suggestive of autoimmune encephalitis, b) extensive neuronal antibodies (NMDAR, LGI- 1, CASPR2, GAD, Ma1, Ma2, Ro, Yo, Hu, Ri, CV2, GABAR, AMPAR) testing in serum and cerebrospinal fluid (CSF) resulted negative, c) reasonable exclusion of other causes. Between these two groups we compared the following data: demographics, signs and symptoms, association with malignancy, EEG, CSF, MRI and treatment administered. 

We did not find any significant differences between the two groups. Clinical picture was homogenous between AE+ and AE-, as well as MRI features, EEG, CSF data. 

AE- patients show similar features if compared to AE+ patients, except for autoantibody detection. As outlined by recent guidelines, our work suggests that, if clinical picture is highly suggestive for AE, antibody detection is not mandatory for diagnosis and, therefore, immunomodulatory treatment must not be delayed.