A complete blood count (CBC) cannot detect breast cancer. However, doctors may order CBCs before and during cancer treatment to monitor a person’s health and help inform treatment decisions.

A CBC is a blood test that shows levels of the three basic types of blood cells — red blood cells (RBCs), white blood cells (WBCs), and platelets.

This article explains how doctors can use CBC tests during the treatment of breast cancer.

It also discusses how medical professionals diagnose and screen for breast cancer and answers some frequently asked questions about breast cancer and CBCs.

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According to the American Cancer Society (ACS), doctors do not use CBCs or any other type of blood test to diagnose breast cancer.

However, a CBC test can give an indication of a person’s overall health, which can affect decisions involving a person’s treatment.

Doctors may perform a CBC following a diagnosis of breast cancer to help inform decisions about cancer treatment.

According to the ACS, a CBC can offer insights into a person’s overall health. This can help doctors determine whether certain cancer treatments are suitable for a particular individual.

For example, a CBC can help to detect issues such as:

Many cancer drugs can affect blood-forming cells inside the bone marrow, leading to reduced blood cell counts. Doctors must therefore check whether individuals have preexisting issues with their blood cells, as this can impact decisions about the following:

  • the type of cancer treatment
  • when cancer treatment should begin
  • the treatment dosage and duration
  • the need for supportive treatments

According to the ACS, a CBC is the most common lab test a person will have during their cancer treatment. This is because cancer and cancer treatments can reduce the number of red and white blood cells and platelets.

Checking red blood cells

Red blood cells deliver oxygen to the body’s tissues and transport carbon dioxide back to the lungs for exhalation.

A CBC measures each of the following:

  • Hemoglobin (Hgb): The part of each RBC that carries iron.
  • Hematocrit (Hct): The percentage of RBCs in the blood.

According to a 2023 review, doctors may diagnose anemia if:

  • Hgb levels fall below 13.5 grams per deciliter (g/dL) in men or fall below 12.0 g/dL in women
  • Hct levels fall below 41.0% in men or fall below 36.0% in women

Checking white blood cells

White blood cells fight infection. There are many different types of WBCs, but neutrophils are the immune system’s first line of defense against infections.

An absolute neutrophil count (ANC) shows neutrophil levels in the blood. The ACS states that a healthy ANC is between 2,500 and 6,000.

Neutropenia is the medical term for when the ANC drops below 1,000. An ANC below 500 indicates a much higher risk of infection.

Checking platelets

Platelets help to control bleeding. Thrombocytopenia is the medical term for a low platelet count.

Platelet levels below 20,000 increase the risk of bleeding.

Doctors may also order a blood test called a “blood chemistry panel” or “metabolic profile” to check organ function and electrolyte levels while a person is undergoing cancer treatment for breast cancer.

According to the Centers for Disease Control and Prevention, doctors may use the following tests to detect and diagnose breast cancer:

  • Breast ultrasound: A handheld device that uses sound waves to visualize areas inside the breast.
  • Diagnostic mammogram (DM): A mammogram uses X-rays to build up a detailed image of the breast. Doctors may recommend a DM following a screening mammogram (SM) if the SM shows signs of abnormalities.
  • Breast magnetic resonance imaging (MRI): A breast MRI scanner uses strong magnets and radio waves to generate cross-sectional images of areas inside the breast.
  • Biopsy: This procedure involves removing tissue or fluid from the breast and checking it under a microscope. There are different kinds of biopsies, including:
    • fine-needle aspiration
    • core biopsy
    • open biopsy

Breast cancer screening is the most reliable way to detect breast cancer in its early stages.

The ACS provides guidelines for individuals at average risk of breast cancer, as well as those at high risk of the disease.

Those with an average risk of developing breast cancer are those who meet all of the following criteria:

  • no personal history of breast cancer
  • no strong family history of breast cancer
  • no genetic mutation known to increase breast cancer risk, such as a BRCA gene
  • no chest radiation therapy before the age of 30

Screening for those at average risk

The following screening recommendations are for females who are at average risk of developing breast cancer:

  • Age 40–44: Those in this age group have the option to choose yearly screening mammograms.
  • Age 45–54: Those in this age group should receive yearly screening mammograms.
  • Age 55 and older: These people can continue with yearly mammograms or can choose to have a mammogram every other year. Screening should continue as long as a person is in good health and expects to live at least 10 more years.

Screening for those at high risk

Those with a high risk of developing breast cancer or who have a family history of breast cancer should receive yearly a mammogram and breast MRI starting from around the age of 30.

People should speak with a doctor to discuss when they should start to get mammograms.

Below are some answers to frequently asked questions about breast cancer and CBCs.

Does breast cancer affect a person’s blood cell count?

According to the ACS, cancers and cancer treatments can affect blood cell counts, potentially decreasing levels of RBCs, WBCs, and platelets.

Does breast cancer affect a person’s white blood cell count?

Breast cancer and breast cancer treatment can affect a person’s WBC count, potentially causing neutropenia. Very low neutrophil levels significantly increase the risk of infection.

Can CBCs detect other types of cancer?

A CBC can help to detect certain types of blood cancer, such as leukemia and lymphoma.

In leukemia, the bone marrow produces abnormal and rapidly dividing WBCs. These abnormal cells are unable to fight infection and impair the bone marrow’s ability to produce RBCs and platelets. This results in lower RBC and platelet counts.

A CBC can also indicate whether cancer of the lymphatic system, called “lymphoma,” is present in the bone marrow or blood. If this is the case, a CBC may show low levels of RBCs, WBCs, or platelets.

A CBC cannot help doctors detect or diagnose breast cancer. However, doctors may order CBCs before and during cancer treatment to help assess a person’s overall health and inform decisions about their treatment.

Doctors typically use imaging tests or biopsy procedures to diagnose breast cancer. Common imaging tests include breast ultrasound, breast MRI, and mammograms.

Regular breast screenings can help to detect breast cancer in its early stages when cancers are typically more treatable. A person can talk with their doctor about their individual breast cancer risk and arrange screenings accordingly.