Acute basophilic leukaemia: eight unsuspected new cases diagnosed by electron microscopy.

We report eight new patients with de novo acute basophilic leukaemia (ABL) diagnosed by electron microscopy (EM) in 184 patients with poorly differentiated AML who were selected for ultrastructural analysis between the years 1989 and 2002. Morphology by light microscopy, cytochemistry, immunophenotyping and cytogenetics did not enable an accurate diagnosis in any of these patients. In almost all the patients, the blasts showed reactivity for HLA-DR and CD34. EM studies demonstrated the presence of basophilic granules in the leukaemic blasts. These granules were membrane bound and their contents varied in appearance from uniformly electron dense to partially speckled or electron lucent. Theta granules were present in only three patients and no mast-cell type granules were observed. By light microscopy, the myeloperoxidase reaction was positive in three patients in an unusual coarse granular pattern. Ultrastructural demonstration of peroxidase in the granules, nuclear membrane and profiles of endoplasmic reticulum was observed in all eight patients. The reaction in the granules showed a particular speckled pattern. The outcome was unfavourable in six of our eight patients. As a definitive diagnosis of ABL may be made only by EM, we suggest including such studies as an integral part of the diagnostic work-up of acute leukaemia cases that lack differentiation markers.