Following successful preparation of the bone marrow aspirate smears in the laboratory, the remaining bone marrow aspirate particles (and possible clot) are filtered and wrapped in lens paper, placed in fixative, embedded in paraffin, and sectioned. This is commonly referred to as the "clot." The clot section includes the remaining particles from the aspirate as well as red blood cells. Usually, three levels cut at 3 microns thick are stained with hematoxylin and eosin (H&E). Thin sectioning allows for appreciation of cellular detail. Additionally, a clot section is stained with Prussian blue to evaluate for iron storage. The clot section is used to evaluate bone marrow cellularity. The clot section is often optimal for immunohistochemical (IHC) staining as it has not been exposed to decalcification chemicals.
Often, the blood clot is friable upon sectioning (after paraffin processing). One way to provide better, non-friable sections is to soak the faced block in a solution of very dilute ammonium hydroxide prior to sectioning. Once the block has soaked in ammonium hydroxide, rinse in water and return to your ice.
Image 1 (right): To filter the aspirate, place a funnel lined with lens paper into a beaker. Pour the remaining bone marrow aspirate sample from the EDTA tube into the funnel.
Image 2 (below): Once the bone marrow aspirate has settled through the funnel, remove the lens paper and securely wrap the remaining particles and/or clot. Place the sample into the cassette-in formalin, which is ready for processing.