Pretty much every position I’ve worked in has had some
teaching and the importance (or lack thereof) of the WBC often comes up in
discussion with students and residents. Below is a summary of what I try to
tell my trainees. It is not a complete description of how to use the WBC in
clinical medicine, but it gives trainees a good start when understanding this
test. I hope that students, residents, and other clinicians in teaching roles
find this useful.
The WBC is a common test we acquire in clinical medicine. Some
clinicians lean on it heavily for their clinical decision making. Other
clinicians have the opinion that the WBC has little usefulness. What I teach my
trainees is that the WBC is a valuable tool that has limitations. We must understand its limitations to use
that tool correctly.
Let’s take a step back and consider testing in general. Then
let’s see how the WBC fits in this paradigm specifically. From a certain
perspective, we have 2 kinds of tests, tests that:
1.
Help to define
a diagnosis
2.
Measure severity
of disease
The WBC generally falls into the second category, it’s a measure of severity. Outside of
hematology, it’s typically not a test that gives us a diagnosis. Theoretically,
any physical stress can elevate the WBC. A WBC value cannot tell you whether or
not there is pyelonephritis, or pneumonia.
However, it can help us judge the severity of a
pyelonephritis or pneumonia.
For example, consider
- a patient with pyelonephritis and a WBC of 16 and
- a patient with pyelonephritis with a WBC of 8
If all other aspects of the
patient are equal, most clinicians would consider the first patient sicker.
The WBC is also useful to track progress of treatment.
For example, in that same
pyelonephritis patient with a WBC of 16, there
- is improvement if the previous day’s WBC was 20
- is worsening if the previous day’s WBC was 12.
A WBC is not a very good screening test either.
- A normal WBC does not completely rule out an inflammatory process. For example, a patient with appendicitis sometimes has a normal WBC.
- An elevated WBC does not always mean that an infection is severe. For example, strep pharyngitis can elevate WBC markedly.
The WBC is an
imperfect test, neither perfectly sensitive or specific. To use the WBC
wisely as the clinician, you have to see the big picture, the context that is
the patient’s situation. Without
the context, the WBC means very little. We need to use other tools to
determine the diagnosis: the history, the physical exam, imaging, etc. We also
have to use other data to help measure severity: heart rate, blood pressure,
electrolytes, lactic acid, sedimentation rate, etc. For measuring severity of
disease, the WBC is just one tool among several others that we often use in
combination to give us a better understanding of the greater context.
