Acetylesterase in lymphoblastic leukaemia associated with thymic enlargement.
Focal cytoplasmic acetylesterase activity was sought in the malignant cells of 91 consecutive children with acute lymphoblastic leukaemia, of whom 10 had an anterior mediatstinal mass at diagnosis. This subgroup with thymic disease (TD) was characterized by hyperleucocytosis, the total leucocyte count being greater than 200 X 10(9)/1 in 6 patients. Furthermore, there was a significant association (P less than 0.025) between TD and the presence of leukaemic blast cells expressing a thymic phenotype, in the form of rosette formation with sheep erythrocytes. Focal cytoplasmic acetylesterase activity identified TD with a sensitivity of 20%, a specificity of 91%, a positive predictive value of 25%, a negative predictive value of 88% and a diagnostic accuracy of 81%. The demonstration of this feature by a simple cytochemical technique can be a useful component of the profile of investigations which are employed in the classification of acute lymphoblastic leukaemia.