The simplest way to prepare a bone marrow smear is to treat it similarly to a peripheral blood smear. A drop of well-mixed marrow sample is placed near one end of slide and a second slide is used to draw it the length of the slide until a feathered edge is produced. The slide should be air dried and labeled with the patients identification, date of collection, sample site or lab accession number (depending on the laboratory's standards).
If the marrow is rich in spicules or fragments, there will be "clumps" of marrow fragments at the end of the feathered edge where these fragments are deposited. Fragments, also known as spicules, are aggregates of bone marrow cells that are pulled from the bony matrix of the marrow space (trabecula) during the aspiration process. They represent what will be seen on the bone marrow biopsy between the bony trabecula, once the biopsy is decalcified and sectioned. Since these spicules are cohesive aggregates of cells, they will not spread well if a wedge smear technique is used, which may make it difficult to identify cell types present in the thicker parts of the spicules.
Another disadvantage of using the wedge technique to make smears of bone marrow is the fact that the distribution of cells will not be uniform or representative of how the cells are distributed in the marrow. This technique tends to skew the spread, based on the size of the cells present. Just as the larger bone marrow spicules wind up at the end of the feathered edge, so do cells like megakaryocytes and other aggregates, such as tumor clumps. However, since there tends to be more of a feathered edge in this type of preparation, it can be preferable to other methods when the marrow to be evaluated has no fragments.
Long slide prepared smears are particularly useful when evaluating leukemic patients at days 8 and 15 of therapy, when the cellularity is greatly reduced and clear morphology is needed to differentiate between treated lymphocytes and treated blasts. In this case, it is easier to distinguish blasts from lymphocytes when this smear technique is used.