The assessment of bone marrow cellularity is semi-quantitative, i.e. the average estimate is performed at at 100-fold magnification. A 5-stage classification is used.
- empty bone marrow (aplastic bone marrow)
- decreased cellularity (hypocellular bone marrow)
- normal cellularity (normocellular bone marrow)
- increased cellularity (hypercellular bone marrow)
- full marrow (packed marrow)
Some hematologists consider the latter two to be equivalent.
The cellularity is indicated in percent as ratio of blood-forming cells to fat tissue (through fixation of detached "lacunae"). The cellularity is age dependet.It can be calculated using a rule of thumb as 100% minus age. In a 15-years old, cellualarity is normal with 85%, in a 70-years old, a cellularity around 30% is normal. If there is a hypocellular bone marrow, the blood-forming cells are clearly decreased, but still detected. In the case of an aplastic bone marrow, only very few blood-forming cells can be found. The remaining elements containing a nucleus are stromal cells. This picture is often observed, for example, in aplastic anemia or after myelosuppressive chemotherapy. The blood-forming marrow predominates in a hypercellular bone marrow. Fat vacuoles can still be detected. In a packed marrow, no more fat vacuoles are present and the entire marrow is filled with hematopoietic cells. This picture is typical of the acute leukemias and some myeloproliferative neoplams (e.g. in chronic myelocytic leukemia) and in mature lymphocytic neoplasms.
Bone marrow cellularity can more accurately be assessed with a bone marrow biopsy.