When you look at a blood smear, it’s best to have a plan, and it’s best to try to follow it each time. It might sound boring – but you’ll make a much more accurate and complete assessment that way. Otherwise, the temptation is to just put the slide under the microscope, scan around to see if you see anything weird, and then focus on that (while missing some important features).

There are 10 main things you need to be sure to evaluate on a blood smear. I like to start with the red cells, move to the platelets, and save the white cells for last…but you can come up with whatever method suits you.  

1. Red blood cell number
First, make sure you’re in the right part of the smear. You should be a couple medium-power fields in from the “feather edge,” which is the thin edge of the smear where the cells are all spread out and there are huge empty spaces. Just give it a quick glance and make sure the red cells aren’t either piling up all over each other, or spread out too far with lots of holes in between – like the red cells in the image above. Take a look at the RBC on the CBC and make sure it fits with what you’re seeing.

2. Red cell size
Normally, if all the red cells are roughly the same size, your eye won’t be able to tell if they’re microcytic (small) or macrocytic (large). So you have to just look at the MCV for that. What your eye can see, however, is a range of sizes. So take note and see if there are some cells that are smaller, and some that are bigger. If that’s the case, it’s called “anisocytosis” and it should be reflected in the RDW (red cell distribution width) on the CBC. The more anisocytosis (variation in size) there is, the bigger the RDW should be.

3. Red cell shape
Normally, red cells are all nice and round, like the ones in the image above. In some anemias, there are funny-shaped cells, like schistocytes (fragmented red cells), sickle-shaped cells, teardrop-shaped cells, or target cells. Your eye will naturally be drawn to these (which is why you should force yourself to follow a consistent method when looking at a smear – otherwise you just look at what your eye is drawn to!). Take note of whether there are any non-round cells, and if so, describe what kinds of shapes you see.

4. Red cell chromasia
“Chromasia” refers to the amount of hemoglobin in the average red cell. Normally, there is a zone of central pallor (the white dot in the center of the cell) that comprises about 1/3 of the diameter of the cell. Check out the cute zones of central pallor in the red cells above. These cells are called “normochromic.” If there is a huge white dot, and just a thin rim of hemoglobin, then the cells are called “hypochromic.” There really isn’t a “hyperchromic” type of red cell.

5. Reticulocytes
Take a look around and see if you see any polychromatophilic cells (these are slightly bigger than normal red cells, and they have a lilac tinge to them). These are just young red cells whose RNA has not yet been completely extruded (so they stain a bit blue). In normal blood, about 1% of the red cells are reticulocytes (because we’re always making new red cells). That equates to about 1-2 red cells per field. If you see more than that, it means the marrow is kicking out red cells at an increased rate.

6.  Stuff inside red cells
Take a look and see if you see any red cells with stuff inside – like nuclei, Howell-Jolly bodies (little nuclear remnants that didn’t get extruded), Pappenheimer bodies (little iron granules), organisms (like malaria or babesia).

7. Platelet number
There should be between 7 and 21 platelets per high power field, which corresponds to a platelet count between 150 and 450 x 109/L.

8. Platelet morphology
This doesn’t usually yield much – but take a look at the platelets anyway and make sure they’re roughly of normal size, and have some nice granules inside. There are rare platelet disorders in which the platelets are abnormally large, or lack granules, or both.

9. White blood cell count
Do a quick scan of a bunch of high power fields and see how many white cells there are. There should be a few white cells per high power field. Check the WBC and see if it seems to correspond to what you’re seeing. Then, do a differential count: count a few hundred white cells (500 is best) and put them in categories (neutrophils, lymphocytes, monocytes, eosinophils, basophils). Compare this to the automated differential on the CBC, and multiply the percentages by the total WBC to get the absolute counts of each cell type. When you’re trying to determine if a patient has a normal number of a certain cell type, absolute counts are much more reliable than percentages.

10. White blood cell morphology
Finally, check the morphology of the white cells. You’ll probably do this as you’re doing your differential – your eye will be drawn to any abnormalities as you’re classifying the cells. Make sure the neutrophils and lymphocytes look normal, and keep your eye open for any weird-looking cells like blasts or circulating carcinoma cells.

Whatever order you decide to use, if you do it the same way each time, it will start to become automatic – and you’ll be much more likely to do a thorough, accurate job.