Translating Lab: CBC

When it comes to ordering tests, I always start by asking myself two questions: “What am I looking for?” and “How will this change treatment”? The complete blood count (CBC) is a very common test performed for many reasons. I can look at the page of results in a matter of seconds. To those not in the medical field, it’s just a jumbled bunch of medical words, acronyms and numbers. I would like to break down the different components for those who want to know more than the interpretation the doctor provides. When looking at these results, there will usually be a normal range provided on the far-right column. These ranges account for 95% of normal results which means 5% of normal results do not fit in this range. This is where looking at the entire patient is essential for proper interpretation. Side note: newborns are a different subset of patients with their own set of values and physiology so none of the below applies.

The first item is the White Blood Cell (WBC) Count. When the number high (leukocytosis), the distance from normal makes a difference. A mild infection or increased stress causes WBC to be normal or less than 15. When between 15 and 20, this indicates a more significant amount of inflammation. For me, these patients typically prompt a closer follow up of clinical symptoms within 5-7 days, even if lab is not repeated. When greater than 20, my sense of urgency increases that I want to find a reason quickly as in two days or less. The possible causes are vast and require more tests to figure it out.

My exception to high WBCs is if the patient had recently been started on steroids (like prednisone) as this will cause a substantial temporary rise. I won’t geek out explaining demargination except to say in times of stress, steroids are released from our bodies (think fight or flight response). The body mobilizes the WBC to proactively protect itself from this stressful situation. It can’t tell the difference between what we ingest and what we produce ourselves, it just knows steroids are present and releases its fighters. Cool, right?

On the other side of normal are the low WBC (leukopenia). Infections, chronic disease (like rheumatoid arthritis) and medications account for the majority of these low numbers. Low WBCs are concerning in two infrequent situations. The first is when a specific type of WBC called neutrophils is too low (neutropenic). The neutropenic patient is immunocompromised and at risk for serious infections. The other scenario is when there are one or two other cell lines down. The bone marrow produces three types of cell: White Blood Cells, Red Blood Cells, and Platelets. If two or more cell lines are down, this can indicate bone marrow infiltration where something is blocking your bone marrow from making cells.

We often use the differential at this point for high or low WBC – these are typically represented as a percentage (%) or absolute number (#). There are different WBCs associated with certain types of infections and illnesses. Both neutrophils and granulocytes typically fight bacterial infections. Lymphocytes and monocytes are typically involved with viral and fungal infections. Eosinophils mediate allergic and parasitic infections. On a normal differential, these are all present and balanced in the normal ranges. When those ratios shift, that can sometimes aid the clinician in making a diagnosis.

The next “cell line” are the Red Blood Cells (RBCs). While RBC is listed, most of us typically reference the hemoglobin (HGB) and hematocrit (HCT). Why is that? I have no idea, but that’s what we do. The hemoglobin and hematocrit tell us if RBCs are low (anemic), normal, or too high (polycythemic). Number one cause of polycythemia is dehydration. After that, testosterone, sleep apnea and lung disease. High RBC is almost always a reaction to something else rather than a blood problem. Anemia can be caused by a whole heap of reasons.

The next natural step is looking at cell characteristics. The mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) tell the clinician about the size and robustness of the RBC. A low MCV, MCH or MCHC indicates your cells are small or thin typically associated with an iron deficiency or less commonly a genetic disorder called thalessemia. A high MCV, MCH, or MCHC indicates enlarged or thick cells typically associated with a B12 or folate vitamin deficiency although other more serious conditions can cause this. Abnormalities in either direction typically result in more tests. The final cell parameter is the red cell distribution width (RDW). This tells us how uniform your cells look compared to each other. When the bone marrow is stressed and cranking out cells rapidly or when there is a vitamin deficiency, the cells come out in different shapes and sizes instead of being the reliable oxygen transport cells they should be. If no anemia present, these cell parameters are typically considered less important.

Platelets comprise the third cell line produced by our bone marrow. They are responsible for clotting blood as well as mediating inflammation. When normal above 150 and less than 500, it’s great. When abnormal, the diagnosis can be a lot more nuanced. Further work-up is always required when less than 100 (even though bleeding risk does not critical until less than 50) OR when greater than 800 (indicates a serious infection or inflammatory state). As platelet counts can fluctuate greatly based on different disease processes, most of us repeat the test to determine a trend to help us decide the next step. Most causes of platelet abnormalities are benign and related to other health problems rather than a platelet abnormality itself.

So hopefully this helps and is not more confusing than the CBC itself. There is a lot of information we clinicians gain by getting a CBC. Will the above rules account for all possible scenarios? Absolutely not. But it’s a solid place to start. One important point to make when it comes to lab tests is that we use them not only to find a diagnosis, but we also use them to rule out diagnoses as well. When used appropriately, they can answer what I am looking for by ruling something in or out as well as influence how I will manage the patient in front of me by pushing for more tests or not. Don’t forget, normal values on a CBC can take a lot of serious diseases off the table and numbers are just one part of formulating a treatment plan.

A priest, a pastor and a rabbit went to a blood donation clinic. The nurse asked what blood type they were. The rabbit said, "I'm probably a Type O."