TdT in adult acute lymphoblastic leukemia: Relationship with blast cell count

Terminal deoxynucleotidyl transferase (TdT) was determined by a biochemical assay in peripheral blood cells of 27 adult patients with acute lymphoblastic leukemia (ALL). Eighteen of 19 patients presenting with a blast cell count higher than 0.5x10³/μl were TdT⁺. Many such cases remained TdT⁺ when they were restudied during induction therapy or at early relapse, with a blast cell count lower than 0.5x10³/μl. In contrast, 8 of 8 patients presenting with 'oligoblastic' leukemia (i.e. with a blast cell count lower than 0.5x10³/μl), were TdT^-. These results suggested that leukemic cells of patients presenting with a very low blast cell count may have no or very low TdT enzyme activity. Such patients who were disregarded in prior studies, account for about 20% of all ALL cases, and may have a better prognosis.