Low blast count myeloid disorders with Auer rods: a clinicopathologic analysis of 9 cases.

Auer rods are a hallmark of acute myeloid leukemia but occasionally are seen in myelodysplastic syndromes (MDSs) or chronic myelomonocytic leukemia, rarely in cases with fewer than 5% blasts. The significance of this finding is unclear. We report 9 cases of this unusual phenomenon. All patients had cytopenias, isolated to a single lineage in 4. Circulating blasts were present in 8 cases (rare to 2.5%). Bone marrow blasts ranged from 0.4 to 4.9%; 1% to 32% of blasts contained Auer rods. There were variable degrees of dysplasia; 1 case closely mimicked refractory anemia with ringed sideroblasts. Cytogenetic studies in 8 cases showed clonal changes in 4. In 5 patients, acute myelogenous leukemia (AML) developed 6, 6, 5, 13, and 24 months after diagnosis; the patients subsequently died. Three patients died at 1, 1, and 8 months without progression to AML, and only 1 was alive at 10 months. MDSs with fewer than 5% blasts and Auer rods seem to be a heterogeneous group, but rapid progression to death or AML in most cases suggests that Auer rods signify an aggressive biology in MDSs with a low blast count.