Understanding CBC Blood Counts

June 12, 2024

This page was reviewed under our medical and editorial policy by Leslie Popplewell, M.D., hematologist and medical director of Hematology and Blood and Bone Marrow Transplant, City of Hope^® Cancer Center Atlanta

A complete blood count (CBC) is one of the most common tests that doctors use to understand a patient’s overall health. Along with other tests, CBCs assist in diagnosing, monitoring and evaluating certain cancers and their treatments. These blood tests are particularly useful in helping doctors:

• Diagnose blood cancers, including leukemia and lymphoma
• Determine if cancer has reached the bone marrow
• Determine if cancer treatments are working
• Evaluate the impact of cancer treatments such as chemotherapy on the body

What Does a Complete Blood Count Measure?

A CBC measures a variety of substances in the blood. These may include:

• White blood cells
• White blood cell differential
• Red blood cells
• Hematocrit
• Hemoglobin
• Mean corpuscular volume
• Platelets

Finding out if these blood component counts are high, low or within normal ranges is important to detecting many disorders and diseases and managing a patient’s health.

In terms of cancer care, if certain conditions such as anemia (low red blood cell count), leukopenia (low white blood cell count) or thrombocytopenia (low platelet count) are present, it may point to leukemia or the side effects of chemotherapy in patients who have already been diagnosed with cancer.

Here is a closer look at these blood component counts, including what they are and what each level may indicate to the patient’s medical care team.

Hematocrit

The hematocrit count measures the percentage of red blood cells (RBCs) in the blood. When patients receive their CBC test results, this may be abbreviated as Hct.

Hct Levels

Low hematocrit levels may indicate anemia, which has a number of causes. When the cause of anemia is overproduction of white blood cells, this may be due to:

• Cancers such as myeloma, leukemia or lymphoma
• Cancers that have spread to the bone marrow
• Bone marrow disease

High Hct levels may be explained by:

• Leukemia
• Low blood plasma level due to dehydration or shock
• Polycythemia (a blood disorder marked by overproduction of red blood cells)
• Heart disease or heart failure
• Lung disease

For patients already diagnosed with cancer, normal Hct counts may indicate that treatment is working. A normal reading may also be part of the diagnostic testing used to rule out cancer or cancer metastasis (spread) to the bone marrow.

Hemoglobin

Hemoglobin is a protein in red blood cells that carries oxygen and iron throughout the body and carbon dioxide to the lungs. The hemoglobin count measures the amount of this protein in the blood and is often written as Hgb or Hg on CBC test results.

Hgb Levels

Low levels of Hgb might indicate:

• Various types of anemia
• Blood cancers
• Liver diseases or other diseases
• Thalassemia, a genetic disorder marked by low hemoglobin production
• Iron deficiency

Similar to high Hct results, high Hgb results may be due to polycythemia, leukemia, lung disease or heart disease.

If results are normal, this finding may help a doctor rule out certain diagnoses or confirm that medication for an existing condition that impacts hemoglobin levels is working.

Mean Corpuscular Volume

A third red blood cell measurement is known as mean corpuscular volume, or MCV for short. This measures the size of the patient’s red blood cells.

MCV Levels

People with low MCV counts may have a type of anemia called microcytic anemia that results from thalassemia, lead poisoning or iron deficiency.

High MCV counts, on the other hand, may point to macrocytic anemia. Nutrient deficiencies (such as low vitamin B12 or folate levels) and chemotherapy treatments may trigger this type of anemia.

A normal MCV level doesn’t necessarily mean that the doctor will rule out health issues. This is because normocytic anemia may develop as a result of impaired red blood cell production — even though the size of the RBCs may still be normal. Normocytic anemia means the red blood cells are smaller than normal.

Normocytic anemia might indicate:

• Chronic infections, kidney failure, endocrine diseases, liver diseases or other diseases
• Cancer that has spread to the bone marrow
• Aplastic or early iron-deficiency anemia
• Blood cancers such as leukemia
• Side effects from certain medications

Platelet Count

Platelets help the blood clot to stop bleeding. A platelet count measures the total number of these clotting components in a blood sample. On CBC test results, they may be abbreviated Plt.

Platelet Levels

People whose platelet counts are lower than normal might have:

• Cancer that has spread to the bone
• Cancers that lower platelet levels, such as leukemia and lymphoma
• Spleen cancer (the spleen stores extra platelets)
• The presence of certain antibodies that attack healthy platelets
• Temporary damage to the bone marrow from cancer treatments such as chemotherapy and radiation therapy

The reasons for a platelet count that’s higher than normal may include:

• Genetic mutations (gene changes)
• Hemolytic or iron-deficiency anemia
• Infection
• Inflammatory diseases
• Spleen removal (via surgery)
• Cancers such as lymphoma, ovarian, breast, gastrointestinal and lung

Sometimes the number of platelets is within normal ranges, but these platelets don’t work properly. If clotting issues are occurring and the platelet count is normal, the doctor may order additional tests to check for bleeding disorders such as hemophilia.

Red Blood Cell Count

The red blood cell count, or RBC, is the total number of red blood cells present in a blood sample. Since the scientific name for a red blood cell is erythrocyte, an RBC count is also sometimes called an erythrocyte count.

RBC Levels

Similar to a low platelet count, a low RBC count may owe to cancer treatments like chemotherapy and radiation therapy that impact the bone marrow, the spread of cancer to the bone marrow or the presence of certain blood cancers. Poor nutrition, kidney failure and blood loss from surgery (including cancer surgery) may also lower RBC levels.

A high red blood cell count may indicate:

• Kidney, heart or lung disease
• Lung scars
• Polycythemia
• Dehydration

A normal RBC count might reveal to a doctor that a blood transfusion or medication is working to stabilize levels of these cells in the blood.

It might also mean that a health condition that causes low or high RBC levels is improving. Each case is different, and patients are encouraged to discuss their CBC readings with their doctor.

White Blood Cell Count

The total number of white blood cells, or leukocytes, is measured by the white blood cell count. On the CBC test results, this may be abbreviated as WBC or written as leukocyte count.

WBC Levels

Common reasons for low WBC counts may include:

• Certain medications, such as antibiotics, antihistamines and others
• Blood cancers such as myeloma, lymphoma or leukemia (which drive out healthy white blood cells)
• Chemotherapy or radiation treatments
• HIV or AIDS
• Bone marrow infection or disease (such as cancers that have spread there)
• Certain autoimmune disorders

High white blood cell counts may be attributed to:

• Infections
• Inflammatory diseases
• Allergies
• Leukemia or lymphoma (due to overproduction of abnormal white blood cells)
• Tissue damage owing to surgery or a burn injury

Similar to normal RBC counts, normal WBC counts are used to help doctors determine if certain treatments are working or if a diagnosed cancer such as leukemia or lymphoma is improving.

White Blood Cell Differential

Five types of white blood cells help the immune system fight infections, toxins and disease. A white blood cell differential measures the total numbers of each type of white blood cell, including:

Basophils, which help manage allergy responses

Neutrophils, which fight bacteria, viruses and other invading microorganisms

Monocytes, which strengthen the body’s immune response, help clear away dead cells and fight bacteria, viruses and other invading microorganisms

Eosinophils, which help manage inflammation and allergy responses and also fight parasites and other infections

Lymphocytes, which are primarily T cells (that help fight cancer cells or viral infections) and B cells (that help fend off bacteria, viruses and other germs)

Patients may see this test abbreviated as WBC differential on their CBC results.

White Blood Cell Differential Levels

The blood differential test measures each type of white blood cell as a percentage of all white blood cells: basophils, neutrophils, monocytes, eosinophils and lymphocytes.

High or low counts of these white blood cells indicate various potential health concerns. For instance, an increased level of lymphocytes may prompt a doctor to run more tests to check for lymphoma. Low neutrophil counts (neutropenia) might point to blood cancer, bone marrow injury from cancer treatments or cancer spread.

As with RBC and WBC counts in the normal range, normal WBC differentials help doctors rule out certain diagnoses, monitor cancer progression and evaluate treatment.

CBC Test Results and Cancer

Abnormal CBC counts don’t necessarily mean that cancer or other diseases are present. Lifestyle and factors unrelated to health may affect blood counts, as well.

If the doctor suspects that a patient’s results point to health issues, he or she may use blood counts to help:

• Pinpoint a diagnosis of certain blood cancers like multiple myeloma, leukemia or lymphoma
• Determine if previously diagnosed cancer, such as breast, lung or prostate cancer that has spread is affecting the bone marrow where blood components are created
• Track the effects of treatments such as chemotherapy and radiation therapy on bone marrow, and adjust the treatments accordingly
• Diagnose and treat blood conditions like anemia and thrombocytopenia
• Evaluate the success of cancer treatments

At City of Hope^®, the care team meets with each patient individually to discuss his or her complete blood count test results. The goal is to ensure patients fully understand what their results may mean. Experts are also here to guide patients and their family members through any follow-up testing and, if cancer is diagnosed, to provide comprehensive and supportive cancer care.