Why you need to look at absolute numbers (not just percentages) of white cells

percentage

Q. I need help with this question from Robbins regarding percentages vs. absolute values of white blood cells.

15 year old boy with fever of 10 days. Petechial hemorrhages on trunk and extremities. CBC shows Hg 13.2, Hct 38.9%, MCV, 93, plt ct 175,000 and WBC ct 1860 with 1% segs, 98% lymphs, and 1% monocytes. BM biopsy shows no abnormal cells. What’s the diagnosis?

The answer was overwhelming bacterial infection! It said you are supposed to multiply the percentages in the differential by the total WBC to get absolute values. Any tips for reading WBC counts and diffs? They are so daunting! Would it possibly be included in The Complete But Not Obsessive Hematopathology Guide? I’ve definitely gotten some questions right just from reading PathologyStudent though! Such as the difference between a CML and leukomoid reaction! 🙂

A. This is a great question! Yes – there is quite a bit about the diff in The Complete Hematopathology Guide. You might just start by downloading the Top 10 Anemias if you haven’t already (it’s free) – I have a bit in there about normal CBC values. I think once you understand the diff and the CBC, it becomes a lot less overwhelming. You do need to know how to multiply the percentages to get the absolutes…but you definitely don’t have to do that on every case. And for most cases, it’s fine to just do a ballpark figure.

Why do you need to look at absolute counts?

The reason you need to look at absolute counts is because if you just look at percentages, you could mess up. Let’s say you have 60% neutrophils in a particular patient. You might take a quick look at that and go, oh, yeah, that looks fine. But if you don’t take the time to find out the absolute neutrophil count, you could be missing something. If the WBC count in that particular patient is very low, say 1.5 (normal being 4-11), then the patient would have a low number of neutrophils even though the percentage of neutrophils is normal.

Conversely, if the same patient had a very high WBC (say 120), then 60% of the total WBC would be a very high number! Higher than normal for sure. So if you just looked at the percentage of neutrophils in that case, you’d go okay, that looks normal – when in fact the number of neutrophils would be very elevated.So that’s the theory behind looking at the absolute numbers. In reality, most of the time you can just take a look at the WBC, and if it’s really high or low, then go ahead and figure out the absolute numbers of the individual white cells (by multiplying the percentage of that particular cell times the actual WBC). If the WBC is normal, or pretty close to normal, you probably don’t need to go to that amount of work.

Neutropenia and infection

The Robbins question is trying to get you to a) recognize that the patient is neutropenic (in addition to being leukopenic overall), and b) figure out that the reason the neutrophil count is so low is because the patient has a massive infection, and the neutrophils are leaving the bloodstream to go the tissues where they are needed (hence the number of neutrophils in the blood is actually low). Usually in an infection (a bacterial one anyway), the WBC is high, and the percentage (or at least the absolute number) of neutrophils is way increased. This leaving-the-bloodstream phenomenon that happened in this patient is uncommon – but it does occur.

Bottom line

If you just looked at the % of lymphocytes in this question, and didn’t think too much about the WBC, you might (incorrectly) conclude that the patient has a lymphocytosis (98% lymphocytes! That sounds like a lot.) In fact, the patient is actually leaning towards being lymphocytopenic (since the total number of white cells is really low). The normal absolute lymphocyte count is somewhere between 1 and 4; in this patient, it’s about 1800 (98% of 1860).

Hope that helps! I hope you can get to the point where it doesn’t feel totally overwhelming – because it’s actually pretty straightforward and doable, once you understand some general principles.

Myelodysplasia: not quite leukemia



Myelodysplasia (MDS) is often thought of as “pre-leukemia.” Which is kind of misleading, because only some cases of MDS go on to become leukemia; others stay the same and never progress. (more…)

Aplastic anemia



Aplastic anemia falls into the category of “anemias-in-which-the-cells-don’t-look-weird” category. (more…)

Leukoerythroblastotic reaction


Here’s a long term: leukoerythroblastotic reaction. Despite its length, it’s a pretty good term, because it describes a reactive condition in which you see young red cells (erythroblasts) and young white cells (leuko-) out in the peripheral blood.

(more…)

How to read a blood smear

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.

That 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.

Where do you see Auer rods?


Q.In which type of AML do you see the most Auer rods?

A. Auer rods are elongated structures seen in malignant cells of the neutrophil lineage. (more…)

How to identify normal leukocytes in a blood smear


Q. I’m not sure if I can identify leukocytes correctly. Could you give me some tips? Thanks very much.

A. Sure! When you are just starting out in hematopathology, it can be a bit overwhelming. It’s really not as difficult as it seems. There are just 5 kinds of cells that you see in normal peripheral blood, and with a few guidelines, you can tell them apart pretty easily.

Neutrophils
These are the most numerous white cells in normal blood. There are two in the above image (from WebPath), one at about 3 o’clock and one at about 10 o’clock. They are part of a category of white cells called “granulocytes,” which refers to the cytoplasmic granules you see in these cells. In neutrophils, the cytoplasmic granules are mostly small, pale peachy-pink granules. These granules (called “specific granules”) are what give the neutrophil cytoplasm its pinkish color. There are also scattered larger, dark purple (or “azurophilic”) granules. These are called “primary granules” because they are the granules that appear first as the neutrophil matures. If you forget the thing about neutrophil maturation, you can remember which is which by remembering that the p words go together (primary=purple).

The nucleus of a normal neutrophil is also unique-looking. It’s segmented – pinched off into different sections, like sausage links – rather than round, like most other cells. Neutrophils are sometimes called polymorphonuclear leukocytes because there are several (poly) bodies (morpho) in the nucleus (nuclear). Rarely, you might see a “band” cell (which is the neutrophil at 10 o’clock), which is the stage of neutrophil right before the nucleus becomes segmented. Neutrophil chromatin in general is clumpy, and you can’t see any nucleoli.

Lymphocytes
These are the second most numerous type of white cell in normal blood. There’s one lymphocyte at 8 o’clock in the above image. Lymphocytes are generally a bit smaller than neutrophils, and the thing that sets them apart is their chromatin, which is both clumpy and smudgy at the same time. It looks like someone took a finger and rubbed the nucleus before the ink fully dried. Although there are clumps in lymphocyte chromatin, there aren’t discrete white spaces between the clumps, like you see in neutrophil chromatin. Check out the above image and you’ll see what I mean. Sometimes you’ll see lymphocytes that are a bit larger, with more cytoplasm and maybe a few coarse granules. T cells often have this appearance (though you really can’t tell for sure without doing some special studies).

Monocytes
Monocytes are big cells (there’s one at about 8 o’clock above) with lots of cytoplasm. The cytoplasm often has a “dishwater” appearance, meaning it is sort of cloudy and grayish. Sometimes, as in the cell above, it’s more of a pale purple color. You can see some fine purple granules scattered about as well. The nucleus is big and it’s usually indented, or horseshoe shaped. The chromatin is pretty fine (finer than neutrophil or lymphocyte chromatin), and it has a weird “raked” appearance on high power (it looks like someone messed up the chromatin by dragging a rake across it).

Eosinophils
These cells, along with basophils, are probably the easiest to spot (there’s an eosinophil at 2 o’clock above). Both eosinophils and basophils are granulocytes. The granules in eosinophils are beautiful – they are large, luminous, and reddish-orange. The word eosin comes from the Greek word eos, which means “flush of the dawn sky.” Very cool name for these gorgeous, sunrise-colored granules. The nucleus is nothing to write home about, really – it’s segmented into a few different parts, and it looks kind of like a neutrophil nucleus.

Basophils
You can tell a basophil from a mile away: it’s the cell with the big, super-dark-purple-blue granules (there’s one at 4 o’clock above). The granules are so numerous and dark that they often obscure the nucleus (which is a rather boring nucleus, usually divided into two segments). Basophils are the least numerous of all white blood cells – you may have to look several fields to find one.

And that’s it! When you start looking at different diseases (like infection, or leukemia), it gets a bit more complicated, because you often see immature cells out in the blood. But for now, you can just focus on the normal, mature white cells. Once you get familiar with these, they start looking like little friends that you happily recognize from across the street.