2.2. Study participants and sampling
In this case-control study, a total of 237 volunteers between 18 and 65 years old were enrolled in the schizophrenia or the healthy control groups. One hundred seventeen patients with schizophrenia who applied to the psychiatry department of Istanbul University-Cerrahpaşa, Cerrahpaşa Medical Faculty in Istanbul were enrolled in the schizophrenia group of the study. Schizophrenia is diagnosed clinically according to the Diagnostic and Statistical Manual of Mental Disorders (DSM–5). Patients diagnosing with schizoaffective or schizophreniform disorders were excluded from this study. Also, possible suicide attempts of the patients with schizophrenia were determined with the Suicide Behaviors Questionnaire-Revised (SBQ-R) and clinical interviews. The healthy control group was consisted of 120 participants without any psychiatric disease or family history and having similar age and gender data with the schizophrenia group. Two venous blood samples from each volunteer were taken by venipuncture from a single cubital region and were collected into plain and containing dipotassium ethylenediaminetetraacetic acid (K2EDTA) vacuum tubes. The tubes were filled and K2EDTA tubes were immediately mixed with gentle 180° inversion. Serum samples were separated from plain blood tubes by centrifugation at 2500 rpm and stored at -80°C prior to T. gondii IgG analysis. Whole blood samples were also stored at -80°C for the assessment of T. gondii DNA by molecular methods.