By Katie E. Golden, MD

A complete blood count (CBC) is a common laboratory test to include as part of a basic medical evaluation. If you have recently been to the emergency department (ED), there is a good chance your blood counts were tested. And if you looked at your results, it probably looked like a confusing report of abbreviations, numbers, and percentages.

Blood is a lot more complicated than meets the eye. There are many types of blood cells, as well as ways to measure and analyze those cells. And what’s more, very few people have a ‘normal’ CBC. Differences in these cells and their characteristics are as common as the differences in the color of our eyes or shapes of our noses. So let’s go through what kinds of cells are found in blood, translate all those letters and numbers on the CBC, and help you understand what your results mean for your health.

What is blood?

This may seem like a simple question, but blood is actually a complicated concoction of cells and fluid – all with different jobs. To start, there are four main components to blood:

  • Red Blood Cells (RBCs), also called ‘erythrocytes’
  • White Blood Cells (WBCs), also called ‘leukocytes’
  • Platelets also called ‘thrombocytes’
  • Plasma

RBCs are the most abundant cell in our blood. Their job is simple – to deliver oxygen and remove carbon dioxide from the body. Similar to your neighborhood postal worker, they travel along their route, pick up oxygen from the lungs, drop it off to the rest of the body, pick up any carbon dioxide in the process, and then bring it back to the lungs so we can then breathe it out.

WBCs make up a much smaller percentage of our blood, but they are a lot more complicated. Their main job is to protect us from infection. There are many different types of WBCs, and all of them play slightly different roles in our immune system. Some of them are like the infantry, quickly deployed to kill any unwanted invaders like bacteria or viruses. Others are more like the special forces, tasked with learning about the enemy and developing strategies to neutralize them. In other words, some of our WBCs are trained to win the battle, and some are trained to win the war.

Platelets are not actually cells, but parts of cells whose job it is to help our blood clot. They travel through our blood vessels, looking for leaks, and plugging in the holes when necessary.

And finally, there’s plasma. This is the actual liquid component of blood that carries the cells through the blood vessels. It has lots of other good stuff mixed in to keep us healthy, like nutrients, electrolytes, and proteins. It also picks up the waste that we no longer need, so it can be excreted from the body.

So there you have it. Your RBCs are like the postal service. Your WBCs are like the army. Your platelets are like plumbers. And your plasma is like the chocolate river in WIlly Wonka. Or sewer water, depending on the way you look at it.

What is a CBC?

The CBC is the lab test that analyzes a sample of our blood and gives us a report card of what each cell type is doing. This can give us a lot of important information, for example:

  • Do we have enough RBCs to keep our body well oxygenated?
  • If not, is the body losing blood somewhere? Or not making enough new red blood cells?
  • Do the WBCs look like they are preparing to fight a battle?
  • Is the immune system prepared if the body should be attacked?
  • Do we have enough platelets to make sure our blood will clot as it should?

The CBC also measures different characteristics of each cell, like size and color, to make sure the cells can fully perform their function. For example, if we don’t have enough iron in our body, our red blood cells look smaller, and their color looks more pale. The CBC gives us many important clues into what is going on inside our bodies.

Breaking down the CBC 

Here is an example of a CBC report. (These results are de-identified and provided with patient consent.)

When you look at the results of your CBC, all the abbreviations on the left represent different cells or characteristics of these cells, which I will translate below. To the right of those abbreviations will be your results, and to the right of that, the normal range for that result. So for example, this patient’s white blood cell count is 5.1, which is within the normal range of 3.4 to 10.8.

Here is what all those abbreviations in the left-hand column mean:

  • WBC – Overall white blood cell count.
  • RBC – Red blood cell count. Below this, there will be several abbreviations that all describe different characteristics of the RBCs:
    • HGB – Hemoglobin. This is the molecule in red blood cells that actually carries the oxygen molecules, and this number measures how much of that molecule is in your blood.
    • HCT – Hematocrit. The percentage of your blood that is red blood cells.
    • MCV – Mean corpuscular volume. The average size of your red blood cells.
    • MCH – Mean corpuscular hemoglobin. The average amount of hemoglobin in the blood.
    • MCHC – Mean corpuscular hemoglobin concentration. The average concentration of hemoglobin in the blood.
    • RDW – Red cell distribution width. This measures the variation in the size of your RBCs.
  • PLT – Platelet count
    • MPV – Mean platelet volume. The average size of your platelets.
    • PDW – Platelets distribution width. The variation in the size of your platelets.
  • The ‘Differential’
    • Sometimes, a CBC is also reported with ‘diff’, which stands for ‘differential’. These are all the remaining rows you see labeled as neutrophils, monocytes, lymphs, etc.
    • This is a further breakdown of all the different types of white blood cells, like we talked about earlier.

A lot of times, the small changes in the WBC differential are not clinically significant. But sometimes they matter for people with immune systems that may be functioning differently for a variety of reasons (young children or older adults, people on chemotherapy, people recovering from illness, etc.) The differential can also show if the immune system is preparing for battle, which can signal if a bacteria or virus has entered the body before you even have any symptoms.

What does it mean that my CBC is abnormal?

If you look up your CBC results and see a lot of abnormal numbers, typically shown in RED — don’t panic. Abnormalities are common, and often don’t mean that you are sick or something is wrong. For example, it is common for young women to have lower hemoglobin levels simply because they menstruate. And even brief episodes of stress can even increase your WBC count (and who isn’t stressed when they are in an ED?).

Rest assured that if there is a lab value that is concerningly abnormal, your provider will discuss it with you and the best way to address it. Of course, you should never hesitate to ask them about a value that is concerning to you. Keep in mind that modern medicine, with its advanced technologies, is a blessing and a curse. It means we can pick up on subtle changes with exquisite sensitivity, but remember that our bodies are dynamic. These fluctuations are just one piece of the puzzle that make up our complicated and unique physiology. And plus – who wants to be normal anyway?