What's the big deal about the white blood cell count?

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.

If you’re interested about reading more about related topics such as the basics of making diagnoses, using laboratory testing, or teaching the topic of clinical decision making, please check out my book: A Guide to Clinical Decision Making.

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