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Acute Care CMEPhysician Assistant CME

Understanding The Cause of Leukocytosis

John Bielinski, MS PA-C By May 30, 2014January 18th, 2022One Comment


An abnormal elevation of white blood cells is referred to as a leukocytosis, and an abnormally low white blood cell count is referred to as leukopenia. For the purpose of this blog, let’s dig into leukocytosis and its causes.

Infection or no infection

There are two reasons why we would have a leukocytosis – the first step whenever you diagnose leukocytosis is to find out the correct reason, which could either be an infectious problem or a noninfectious problem.
Getting further into infection, there are really three ways to gauge infection from a CBC. For leukocytosis, we are concerned with:

  1. The absolute number of white blood cells,
  2. the percentage of neutrophils, and
  3. the number of immature cells.

The absolute number of white blood cells is another way of saying the total number of white blood cells and what is considered a normal value falls between 5,000 and 10,000. With a leukocytosis patient, we’re immediately going to look at that absolute number.
Is this a white count of 12,000, or 14,000, or 24,000?
That number will really give us the first clue as to whether this leukocytosis is caused by an infection.

Taking a closer look at white blood cells

When it comes to white blood cells, there are really five different kinds: neutrophils, basophils, lymphocytes, monocytes and eosinophils. Neutrophils encompass approximately 75% of all the white blood cells. So, neutrophils are the powerhouse that is in charge of the phagocytosis of bacteria.
Now, when we have a bacterial infection, neutrophils have a tendency to increase in number from that 75%. So, when we identify a leukocytosis patient with a suspected infection, we would note their absolute number of white blood cells – for an example, let’s say it’s 17,000. Then we would also note the number of neutrophils – they are the white blood cell count that is most interesting to us in this diagnosis. So, a patient with a 17,000 white count and 90% neutrophils is strongly suggestive of a bacterial infection.
Now, when I talk about immature cells being noted, I am talking about band cells. Band cells are immature white blood cells, and I like to suggest that they are similar to the United States Marines. That means if you see a bunch of United States Marines in a county, you know there has been a bad fight somewhere, and normally the Marines are the first to go in, the first to fight. Band cells suggest that a pretty intense infection has been going on and has used up all the white blood cells, the neutrophils, that are available to the body and had to call on immature cells to come in and help fight the battle.
Taking a closer look at these three keys can help you successfully determine whether leukocytosis is caused by an infectious or non-infectious problem.
Image courtesy of Habbick

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