The invention relates to diagnostics, namely to a reagent kit, a rapid method and a device for detecting the fact of chronic, ischemia-linked brain pathology. A special feature of the invention is the use of an immunoactive hybrid peptide produced as a product of two fragments of the NMDA neuroreceptor subunits. A device is described that allows quick and convenient testing of autoantibodies in the patient's blood that recognize the hybrid peptide. The method of detection of autoantibodies is based on the principle of lateral flow immunochromatography. The invention can be used for prophylactic medical examination (screening of chronic ischemia-linked brain lesions), to detect decompensated chronic cerebral ischemia at the prehospital stage by general practitioners or neurologists, as well as in neurosurgery and sports medicine for diagnostics of delayed cerebral ischemia in persons with craniocerebral injury.