Clinical Utility of Initial Terminal Deoxynucleotidyl Transferase Determinations in Childhood Acute Leukemias

Terminal deoxynucleotidyl transferase (TDT) activity was measured in bone marrow lymphoblasts obtained at diagnosis from 168 consecutive patients with childhood acute leukemia. Absolute concentrations of TDT were increased (>20 units/10⁸ blasts) in samples from 98 of 112 assessable patients with acute lymphocytic leukemia (ALL). The values ranged from <1 to 1502 units/10⁸ blasts with a median of 90 units contrasted with <1 to 219 units (median, 2.6 units) in studies of children without leukemia. Results of an immunofluorescence assay were in good agreement with enzymatic detection of the polymerase. Among 115 patients with adequate marrow smears, 105 had TDT-positive blasts. By contrast, in most children with acute myelogenous leukemia, TDT activity was either undetectable or less than 10 units/10⁸ blasts. Although the highest levels of TDT were found in blasts with the common ALL phenotype, quantitative determinations were not significantly related to the major immunological subtypes of ALL or to morphological features or periodic acid-Schiff reactivity of the lymphoblasts. The probability that a newly diagnosed case of leukemia would be ALL was 90% if TDT levels were greater than 20 units/10⁸ blasts. We conclude that absolute concentrations of TDT, as determined in this study, are of little value in identifying subclasses of ALL. The immunofluorescence assay, which is much less expensive and easier to perform than the enzyme assay, should prove useful for comfirming the diagnosis of ALL and for detecting extramedullary sites of leukemic infiltration.