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

percentage

Q. I need help with this question from Robbins:

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.

By the way, 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.

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