Immunodiagnostics techniques are applied in clinical immunology to evaluate the disease state. These tests target disease-specific markers to detect the antigen levels corresponding to disease status. The antigens are uniquely expressed by tissue cells or are shed in body fluids such as blood, serum, and plasma. Many immunodiagnostic assays are currently used in clinical applications, and many are being developed and continue to be created for rapid tests. The goal is to develop these tests for rapid detection of antigens using a single-use kit for various diseases (e.g., COVID-19 and cancer) with high sensitivity and specificity. The critical aspects of immunodiagnostic tests are the unique antigens targeted to detect the disease reliably. Ideally, the desired characteristics of tumor antigens are that they should be highly expressed or released only by tumor tissues, unique for a given tumor type, detectable at low levels, able to identify the same tumor type across patients, and directly relate to tumor size. Traditional cancer biomarkers (e.g., CEA, PSA) often exhibit low positive predictive values when applied in mass cancer screening tests. This book chapter discusses the specificity, sensitivity, and stability of tumor-specific (TSA) and tumor-associated antigens (TAA).