Medical Laboratory Scientist (MLS) ASCP Exam 2025 – 400 Free Practice Questions to Pass the Exam

TdT, or terminal deoxynucleotidyl transferase, is an enzyme involved in the synthesis of DNA and is particularly important in the context of hematopoiesis and lymphoid malignancies. This enzyme is predominantly expressed in the early stages of lymphoid development, specifically in lymphoblasts. When performing a TdT stain, lymphoblasts show positive staining because they are actively engaged in DNA synthesis, which is characteristic of immature lymphoid cells.

In contrast, myeloblasts, which are the precursors to granulocytes in the myeloid lineage, do not express TdT at significant levels. This means that during the staining process, they typically exhibit negative results for TdT. As a result, the TdT stain is frequently utilized as a marker to differentiate between lymphoid and myeloid malignancies in diagnostic pathology. The ability to identify lymphoblasts by their positive staining reaction is critical for diagnosing lymphoid leukemias, such as acute lymphoblastic leukemia (ALL).

Understanding the specific staining patterns of TdT helps laboratory scientists not only in the classification of leukemias but also in formulating appropriate treatment plans based on the underlying type of malignancy. Therefore, lymphoblasts being positive for TdT