Prevalence of anti-NMDA receptor antibodies among adult Chinese patients presenting with first-episode psychosis in Hong Kong

Background: Studies on the presence of anti-NMDA receptor antibodies were done primarily on the European population. In addition, most of the existing studies did not collect samples during the acute phase of the psychosis. The prevalence of anti-NMDA receptor antibodies among the Asian population presenting with first-onset psychosis in an acute medical setting is under-investigated. Method: We retrospectively recruited patients assessed by the consultation-liaison psychiatry team at the Prince of Wales Hospital, an acute teaching hospital in Hong Kong, for first-onset psychosis who were unmedicated at the accident and emergency department or the acute medical wards from January 2015 to March 2025. Patients aged < 18 or > 65 were excluded. The presence of anti-NMDA receptor antibodies was tested by cell-based indirect immunofluorescence assay (EUROIMMUN) on serum and/or cerebrospinal fluid samples. Results: We consecutively recruited 389 eligible patients. Among this population, 109 were tested for anti-NMDA receptor antibodies (Mean age = 36.0 years; 29.7% female; 97% ethnic Chinese, 82.6% diagnosed with primary psychotic disorders with no medical explanation). Cerebrospinal fluid (CSF) samples were taken from 60 of them. Among the tested patients, 2 (1.8%) were found to be positive for anti-NMDA autoantibodies (ages ranged from 29 to 37 years; 1 female). The antibodies were detectable in both blood and CSF samples. Both of the patients exhibited the classic features of autoimmune encephalitis with marked CSF pleocytosis. Conclusion: The prevalence of anti-NMDA receptor autoantibodies among our population presenting with first-episode psychosis is low, especially among patients with primary psychotic disorders. This prevalence is lower than that reported by studies mainly done among European-predominant populations. The prevalence of many known autoimmune diseases is known to vary among ethnic groups, and further studies are needed to explore the differences in anti-NMDA receptor positivity among these groups.