A serological test detects autoantibodies directed against ribosomal proteins P (phosphoproteins P0, P1, and P2). These antibodies are highly specific for systemic lupus erythematosus (SLE), appearing in approximately 10–20% of patients. Their presence is strongly associated with the neuropsychiatric manifestations of SLE (NPSLE), particularly psychosis and severe depression, and may also play a role in lupus nephritis.