Lab Series: What does a CBC (Complete Blood Count) tell us?
Quick Summary: A complete blood count (CBC) is a typical lab run in urgent and routine medical evaluations. This lab gives us a window into your ability to deliver oxygen, your infection/immune status, and a general screen for bleeding disorders. More functionally, we can look at markers in this lab to tease out nutritional deficiencies. Learn more below.
The big picture.
A complete blood count (CBC) looks at your three primary blood cells: red blood cells, white blood cells, and platelets.
Red Blood Cells (RBC): These cells deliver oxygen through the carrier protein, hemoglobin (Hb). A CBC will also look at "RBC indices" (i.e., shape/size). We look at RBC size and shape to evaluate nutritional or disease status.
White Blood Cells (WBC): These immune cells help fight off bacteria, viruses, and fungi and mediate inflammatory/allergic responses.
Platelets: Platelets monitor your blood vessels to form plugs at injury sites in your blood vessels. These give us a small peek into your clotting/bleeding disorder status.
Disclaimer: I am not a hematologist (e.g., blood and blood cell specialist). I am a primary care doctor specializing in concussion/PCS. We'll take a very abbreviated look at the CBC through a functional primary care lens. That said, do not take this post to act as a comprehensive guide. Always talk with your licensed healthcare provider before making any interpretations or changes to your healthcare plan.
A deeper look at RBCs.
Are you ready to have your mind blown? Assuming normal physiology, you produce roughly two million red blood cells every second. If that were nickels, I'd have paid off my student loans by the time you read this paragraph. Let's look at what the RBC and RBC Indices tell us. We'll keep this "grade school level" and consider high vs. low. (Generally, "low" in this category indicates some anemia.)
RBCs: When your RBCs are low, we first think of anemia. When elevated, it can be as simple as dehydration or someone who lives in altitude or smokes. Variations in your RBC number can also point toward conditions like kidney disease, Polycythemia Vera, etc...
Hemoglobin & Hematocrit:
Hemoglobin (Hb) is the protein that binds, carries, and delivers oxygen to your cells. When this is low, we consider it anemia and dig further into "why."
If we look at your whole blood (i.e., serum + WBC + RBC + Platelets + Albumin +, etc...), your hematocrit (Htc) is the % of whole blood that is RBCs. When your Htc is low, we also think of anemia and need to figure out why. Hb and Htc can be high for benign reasons (e.g., dehydration or living in altitude) or pathological reasons.
RBC Indices: These include your MCV, MCH, and MCHC. The MCV tells us about cell volume (i.e., "V"). The MCH and MCHC tell us about hemoglobin (i.e., "H"). The RDW tells us how consistent the size is between red blood cells (e.g., are they primarily average-sized, or are some big and some small?).
Bigger Cells: When your cell size/volume increases (e.g., elevated MCV), we begin thinking about B-vitamin deficiency anemia, hypothyroidism, liver disease, excessive alcohol intake, etc...
Smaller Cells: When your cells start getting smaller (e.g., low MCV, low MCH), we start thinking about iron deficiency anemia, anemia of chronic disease, etc...
A deeper look at WBCs (and "differential").
Your white blood cells are far more complicated and diverse than your RBCs. While your RBCs are just that - red blood cells, your WBCs have different types. There are neutrophils, lymphocytes, monocytes, eosinophils, and basophils.
Neutrophils are generally going to be associated with bacterial infections when elevated.
Lymphocytes are generally going to be related to viral infections when elevated.
Monocytes can be involved in multiple types of infections. Their job is to either "eat" the invaders or alert other immune cells to the problem.
Eosinophils can be associated with parasite infections and allergic reactions.
Basophils are associated with histamine and allergic reactions.
Important Note: Let's put "highs" and "lows" into context. Your CBC, particularly the WBCs, can give a window into serious diagnoses (e.g., cancer). That does not mean that every bolded lab value is pathology. In clinical practice, when a lab value is notably high, we will repeat the test to rule out a lab error. And the lab will immediately contact the physician if the value is alarmingly high or low. So, rest easy if you see that your lymphocyte count was 0.2 over the lab standard. You may have just been dealing with a common viral infection, stress, etc., at the time of the blood draw.
A "not-so-deep look" at platelets.
We are going to keep it super brief:
Too low? You're at risk of excessive bleeding.
Too high? You're at risk of excessive clotting.
Generally, the reasons for having clinically low (or high enough) levels of platelets move into drugs, serious infections, autoimmunity, liver disease, cancer, etc.
Wrap-Up
So, whether your CBC was run for an annual exam or because you have low-grade headaches and fatigue, you know a bit more about the numbers behind the lab. And remember, just like an MRI or X-ray, this is just a “snapshot” of your blood at the particular time of the blood draw. You can repeat a CBC every 4-6 months to monitor RBC changes, infection resolution, etc…
In my practice, I offer in-person care here in Arizona and virtual concussion/PCS programs for folks outside the state and abroad. Over the course of three visits, we dive deeply into autonomic, metabolic, visual/vestibular, cervical, and psychological assessment and screening. By completing this comprehensive assessment in a matter of 2-3 weeks, patients are provided with a comprehensive rehab plan that can shave months off their recovery times.
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