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Myeloblasts may be distinguished from lym-phoblasts by three distinct ways: presence of Auer rods, reactivity with cytochemical stains, or reactivity with cell surface markers (for example, clusters of differentiation [CD] groups CD13, CD33) on blasts with specific monoclonal antibodies. The morphology of blasts can often be determined by an experienced morphologist; however, other supporting tests are always needed to confirm the initial designation. The features that can be used to differentiate a myeloblast from a lymphoblast are outlined in Figure 11-1. The chromatin material of a myeloblast is usually much finer than that of a lym-phoblast. A myeloblast often has more cytoplasm than a lymphoblast. Both size of the blast and number of nucleoli may not be helpful characteristics. Although a myeloblast is usually larger than a lymphoblast, sufficient variations are seen that this is not the best factor to consider. Along the same lines, the number of nucleoli that can be seen in a myeloblast is one to four, and a lymphoblast one or two, so when deciding lineage on a blast with two obvious nucleoli, either choice would be acceptable. Therefore, of the characteristic features listed in Figure 11.1, the most helpful is usually the chromatin staining pattern. As mentioned previously, other methods besides morphological examination must be used to confirm the type of blasts present, and often to quantify the number of blasts, particularly when two blast populations coexist in significant amounts in the leukemic bone marrow.

Other studies that can be used to diagnose the acute leukemias include chromosome analysis, molecular genetic studies, DNA flow cytometry, and electron microscopy.

From 5% to 10% of the AMLs have a preleukemic presentation termed "myelodysplastic syndrome." These patients are usually over the age of 50 and have anemia, thrombocytopenia, and monocytosis but with bone marrow blast percentages of less than 20% (see Chapter 14).

Cytochemical Stains

Cytochemical stains are very helpful in the diagnosis and classification of acute leukemias (Table 11.4). These stains are usually performed on bone marrow smears but may also be done on peripheral smears or bone marrow touch preps. The special stains are used to identify enzymes or lipids within the blast population of cells—hence, the reaction in mature cells is not of importance. The positive reactions that occur will be associated with a particular lineage, and with some of the stains (e.g., myeloperoxidase [MPO] and Sudan Black B [SBB]), the fine or coarse staining intensity is an indication of the lineage of blast cells. All of the cyto-chemical stains described below are negative in lym-phoid cells (with rare exceptions), so a positive result with any of these will most often rule out acute lym-phoblastic leukemia.

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