This category collects articles on tumor marker tests used in oncology and clinical practice. Each article describes the specific marker (for example PSA, CA-125, CEA, AFP, CA19-9), why the test is ordered, how to prepare and collect the sample, typical reference ranges, factors that can raise or lower values, and the clinical situations where the marker is useful (screening, diagnosis, prognosis, treatment response, or surveillance). Clear cautions are included — tumor markers rarely confirm cancer on their own and always require clinical correlation and imaging/biopsy when needed. Content is written simply for patients, students, and healthcare professionals.