How It Works: Complete Blood Count

By Jarrod Beachum, PA-C

Why did Dracula fail art class? He could only draw blood! Speaking of drawing blood, I wanted to talk today about the infamous CBC, or complete blood count. This is a lab test your provider may draw from time to time, but they may not really explain how we use it to make decisions. Keep reading for a brief explanation of the CBC.

A CBC is a blood test that we use to gauge overall health status, screen for, or diagnose, diseases or other conditions that may affect your blood. “Blood” is actually a mixture of many different types of cells suspended in fluid, or plasma. Those cells include red blood cells, white blood cells, and platelets. White blood cells (aka leukocytes) are further divided into neutrophils, lymphocytes, monocytes, basophils, and eosinophils.

Red blood cells are produced in the bone marrow and are vital for carrying oxygen throughout the body. RBC’s are produced continually and circulate for about 120 days before they die. A CBC will test the shape, size, color, and the number of red blood cells to look for conditions such as anemia, vitamin B12 or folate deficiencies, sickle cell, immature blood cells, etc. We also measure hemoglobin and hematocrit. Hemoglobin is the protein in RBCs that carries oxygen. Hematocrit is the percentage of RBCs in the total blood count.

White blood cells are part of the immune system. If your body is a nightclub, the white blood cells are the bouncers. In a normal environment, they exist in stable and predictable numbers. When there is a significant shift in the amount of any of the cells, that gives us an important clue that there may be an infection or inflammatory process going on somewhere in the body. Because blood circulates, we may not know where the problem is at first, but we know to look. Also, the type of cell that is increased is important. High neutrophils may indicate a bacterial infection. Allergies may cause the eosinophil count to rise. Viral infections are associated with increased lymphocytes. CBC may be the earliest lab indication of cancer in some cases.

Platelets, also called thrombocytes, are very small cell fragments that are essential for blood clotting. Without functioning platelets, you could very well bleed out from simple lacerations or procedures. Platelets work by adhering to damaged tissue and clumping, forming a plug. Two conditions that we look for on the CBC are thrombocytosis, which means you have an excess of platelets, and thrombocytopenia, which means you don’t have enough. Both conditions lead to improper clotting in the body and can be serious.

It is important to remember that a CBC is just one piece of evidence. Small variances in the “normal range” are not always a concern and may not need to be treated. It is important as a provider that we treat the patient, not the labs. If you ever have a question about your lab results, you should ask your provider to explain them to How it Works: Complete Blood Count