Acomplete blood count is one of the most common and useful tests in medicine. If you have ever had routine blood work, you have probably had this test. Most adults will have many CBCs over a lifetime. A recent study from researchers at Massachusetts General Hospital in Boston found that the normal range for CBC numbers may not be one-size-fits-all. The normal range represents what could be normal for millions of people, but inside that range, there is a smaller range of numbers that may be normal for you. The researchers call these numbers your individual setpoints.
The study is published in the journal Nature. The research team analyzed close to 5,000 CBCs in healthy adults between 2002 and 2021. They included only CBCs that were repeated at least five times and separated by at least 90 days. The key finding of the study was that for each type of CBC result, individual setpoints were stable for more than 20 years, and that slight changes were often early warning signs for future problems and opportunities for early intervention.
Inside the numbers
A CBC evaluates your overall health and can give you important information about a wide variety of important disorders. It is commonly used as a screening test to find a possible problem early, before symptoms start. It can also be used to help find the cause of symptoms you already have. Finally, it can be used after a diagnosis or treatment to find out if your condition is getting better or worse.
It is not important to know the actual numbers because normal values vary depending on the test used as well as your sex and age. The best way to read your CBC is to see if anything is out of the normal range, and if you can, to compare it to past CBCs to see if there are changes within the normal range. Both may be important and are worth discussing with your healthcare provider.
The CBC measures three important types of blood cells: red cells, white cells, and platelets.
- Red blood cells (RBCs) carry oxygen from your lungs to the rest of your body. The CBC measures the number, size, concentration, and oxygen-carrying capacity of your RBCs.
- White blood cells (WBCs) are the cells of your body’s defense system (immune system). They help defend against infections and play a role in inflammation.
- Platelets are small cell fragments that you need to help your blood clot and to stop bleeding when you are injured.
RBC numbers
The most important RBC results are your total RBC count, hemoglobin, and hematocrit. The RBC count is the total number of red blood cells in your blood. Hemoglobin (Hb) is the protein in your RBCs that binds to oxygen. Hematocrit (Hct) measures how much of your blood volume is made up from RBCs.
A significant drop in your RBC count, Hb, or both is called anemia. There are many causes of anemia, including bleeding and blood disorders that reduce red blood cell production, which occurs in the soft center of your bones, called bone marrow.
Your RBC numbers can also be too high. An increase in your RBC count, Hb, or Hct could be due to dehydration, heart disease, or a condition that causes excessive red blood cell production, called polycythemia. An increase in your Hb can also be caused by high altitude or by a lung disease.
Symptoms that might lead your provider to order a CBC for diagnostic purposes can include weakness, fatigue, trouble breathing, or dizziness. Anemia caused by bleeding may be due to an injury, but may also be caused by internal bleeding that is not obvious. Anemia can be caused by less RBC production in bone marrow, called bone marrow failure. Bone marrow failure occurs with certain types of cancers. Decreased production of RBCs can also be caused by iron or vitamin B12 deficiency. Destruction of RBCs in the bloodstream, called hemolysis, can be caused by toxins, immune system disorders, and infections like malaria or HIV.
WBC numbers
WBCs are your body’s first line of defense against germs and other harmful invaders, so abnormal WBC counts often signal an immune system reaction or a problem with white blood cell production in the bone marrow. The most important WBC number is your total WBC count. A high WBC count is called leukocytosis. It is often caused by an infection, inflammation, or, less commonly, by a white blood cell cancer that causes increased production of abnormal WBCs. A drop in WBC count is called leukopenia, and can be caused by bone marrow disorders, immune system diseases, or the effect of certain medications. Leukopenia is dangerous because it leaves you less able to fight off infections.
Most CBCs today will also include numbers for specific types of WBCs, called the WBC differential. There are five types of WBCs, but the most important types are neutrophils, lymphocytes, and eosinophils:
Neutrophils are the most common and important WBCs. They may be the first warning that your immune system is reacting to a foreign invader. Neutrophils increase quickly to fight infections. Many causes of inflammation or stress are also associated with a high neutrophil count. Examples include infections, trauma, inflammatory diseases like gout and arthritis, and other immune system disorders. A drop in neutrophils could be caused by bone marrow failure, cancer, some viral infections, and some drugs that are toxic to neutrophils.
Lymphocytes are also immune system cells. They are associated with a longer-lasting immune system response. Lymphocytes may go up in response to viral infections, and go down in response to white blood cell cancers or immune diseases.
Eosinophils are white blood cells that increase if you have an allergic reaction or a parasitic infection.
Platelets
A low platelet count, called thrombocytopenia, increases your risk of bleeding. It may be caused by bone marrow failure, white blood cell cancer (leukemia), certain medications (like chemotherapy), viral infections, or autoimmune diseases. Symptoms might include bleeding and excessive bruising.
A high platelet count, called thrombocytosis, may occur because of long-term inflammation, certain cancers, or bone marrow disorders. Thrombocytosis can increase the risk of abnormal blood clotting. An abnormal clot, called a thrombus, can form inside a blood vessel and block blood flow or break lose and travel to the brain or heart causing, a stroke or heart attack.
Talking to your healthcare provider
As you can see, there are many causes that can make CBC numbers go up or down. Your CBC is a snapshot of your current overall physical condition, but a CBC itself can’t diagnose a specific condition. It provides important clues that can help your provider determine whether further testing or treatment is needed. Knowing how to read your CBC is important and can alert you to changes in your health, but that is not a substitute for an evaluation and discussion of the results with your healthcare provider.
