BackgroundAutoimmune encephalitis (AE) can present as first episode psychosis (FEP) in children. An FEP diagnostic algorithm has been proposed, but how this algorithm applies to children is unknown. We assess the FEP diagnostic algorithm in children with FEP. MethodsThe FEP algorithm was applied to a retrospective cohort of children with FEP without other neurological symptoms. ResultsTwenty-four patients were included, with five AE (anti-NMDA receptor encephalitis) and 19 non-AE patients (12 primary psychiatric, two headaches, mycoplasma-related encephalitis, post-COVID-19 encephalitis, DRESS-drug reaction with eosinophilia and systemic symptoms-syndrome, cobalamin C deficiency, and two unknown). Some non-AE (5/19=26%) received immunotherapies, with symptom resolution in 1/5 (20%) with immunotherapy, and in 4/14 (29%) without immunotherapy.The FEP algorithm recommended CSF testing in all (5/5=100%) AE and in 6/19 (32%) non-AE patients, resulting in 100% sensitivity (95% confidence interval-CI:100–100%) and specificity 45.5% (95%CI: 16–75%), with a negative predictive value of 100% (95%CI: 100-100%). ConclusionFEP can occur in children from different causes, including AE and metabolic conditions. Evaluation of FEP should be broad, especially without CSF evidence of inflammation. The FEP algorithm is useful to assess patients who would benefit from CSF testing, and should be assessed in larger cohorts.