It is important to note that not all smears will have good areas to perform a differential in the vicinity of the bone marrow fragment. When this occurs, you must keep looking on additional smears. This is one of the reasons that several smears are stained and prepared for possible review.
It can take time to recognize from 10x magnification what will be countable on 50x magnification. The best tip is to be patient and do not fail to keep on looking! In fact, sometimes it may be necessary to stain additional smears/slides, if available, to obtain enough readable material.
While you are checking the smears on 10x magnification for readable areas, you should take the time to evaluate and record the following:
- The cellularity of the bone marrow sample
- Presence and number of megakaryocytes
- Presence of tumor cells
- Anything else out of the ordinary, which should be noted on the report (such as evidence of hemophagocytosis, storage disease etc.).
Once you have decided where to count the marrow, you will perform the differential count. Usually a 200-cell bone marrow differential is the minimum acceptable count. However, more cells may be required depending on your laboratory/pathology protocol. Remember, unlike peripheral differentials, all nucleated cells are included in the total count, including all maturation stages of the erythroid cell series.
Cell counts are performed on 40 - 50x magnification with oil depending on the optics of your scope, moving up to 100x magnification with oil as needed for fine detail. Once oil is added to the smear, move systematically through your chosen area until the morphology/cellularity/stain quality is no longer acceptable, then move back to 10x power to find another good area in the vicinity of the fragment to continue your count. You may need to progress from one slide to the next to accumulate enough cells for your differential. In fact, if there is variability in cell distribution from one smear or fragment/spicule to the next, then the count should be split between more than one smear/fragment to avoid a biased final count.
If there are no spicules, then the differential should be performed in any portion of the slide that demonstrates readable morphology. In pull preps and coverslip preps, this will usually be in the thin area near the edge of the smear. If differential-type (wedge) smears are available, then the usual feathered-edge area should be used. On any of these smears, be sure that you are in deep enough from the thin edge so that the numbers of stripped cells are kept at a minimum to avoid skewing the count, as some cell types are more fragile than others.
The pathologist is ultimately responsible for the final sign-out and will change/adjust/return smears for recount if there is any disagreement over numbers and cell types.