A complete blood count (CBC) test plays an important role in detecting chronic myeloid leukemia (CML).

CML is a slow-growing cancer in which the bone marrow produces too many immature white blood cells, called blasts. Eventually, these blast cells crowd out healthy cells and interfere with the production of healthy white blood cells, red blood cells, and platelets.

Early on, CML may cause only a few vague symptoms — or even none at all.

In many cases, the first sign of this condition is when a CBC test that’s performed as part of a routine physical (or for some other reason) comes back “abnormal.”

This article will go over the role that CBC testing plays in detecting CML and how it’s used to monitor treatment.

A CBC can tell healthcare professionals a lot about your blood. The test measures levels of:

  • red blood cells, which deliver blood to tissues throughout your body
  • white blood cells, which help your body fight infection
  • platelets, which form clots that help stop or prevent bleeding
  • hemoglobin, a protein in red blood cells
  • ratio of red blood cells to plasma (hematocrit)

A CBC often includes a differential test, called a CBC with differential or white blood cell differential. This type of test measures how much of each white blood cell type you have in your body.

There are five main types of white blood cells:

  • neutrophils
  • lymphocytes
  • monocytes
  • eosinophils
  • basophils

The differential test shows whether:

  • They are in an expected proportion to one another.
  • The numbers of each cell type are average, increased, or decreased.
  • Atypical or immature white blood cells are present in the blood.

Diagnosing CML

CBC results that point to CML may include:

  • high white blood cell count
  • too many blasts
  • low red blood cell count
  • low or high platelet count

An “abnormal” CBC does not necessarily mean you have CML.

On the other hand, a “normal” blood count does not always rule out CML, as noted in a case report published in 2015. Other blood and bone marrow tests can help confirm CML or rule it out.

Monitoring CML

CML has three phases:

  • chronic
  • accelerated
  • blast phase

Phases are determined by factors like white blood cell counts and percentage of blasts.

A CBC test can help track the potential progression of the cancer. Periodic CBC testing can also assess whether treatment is working.

You do not need to fast or take any other preparations for a CBC test. However, your doctor may give you different instructions if you’re having other blood work done at the same time.

The CBC test involves a healthcare professional taking a sample of blood from a vein in your arm. It helps to wear short sleeves or sleeves you can roll up. It should take only a few minutes.

The doctor’s office will send the blood sample to a lab for analysis, and your doctor will explain the results.

A CBC is an important tool for detecting CML, but other tests are needed to help a healthcare professional diagnose or monitor the condition.

Peripheral blood smear

After an atypical CBC, a doctor might order a blood smear test. In this test, a healthcare professional will take a sample of blood, smear it on a slide, treat it with a special dye, and examine it under a microscope.

This can show:

  • how the blood cells compare in size, shape, and appearance to typical cells
  • the percentages of different types of white blood cells
  • the number of platelets
  • the proportion of mature to immature cells

Bone marrow aspiration and biopsy

The next step usually involves checking your bone marrow.

In a bone marrow aspiration, a healthcare professional will typically take a sample from your hip bone or sternum. After numbing the area, the doctor will insert a needle and remove a small amount of liquid from the bone marrow.

A medical professional can perform a bone marrow biopsy immediately after the aspiration. Using a wider needle, the doctor will remove a small piece of bone that contains marrow.

Next, a healthcare professional examines the samples under a microscope. A person with CML is likely to have a higher-than-average number of blood-forming cells.

These tests help a healthcare professional make a diagnosis and lets them check how your body is responding to treatment.

Cytogenetic analysis

This involves examining your blood or bone marrow under a microscope to look for changes or abnormalities in chromosomes. Chromosomes are the parts of your cells that give each cell instructions on how to act.

An atypical chromosome called the Philadelphia (Ph) chromosome is a sign of CML.

According to the Leukemia & Lymphoma Society, about 95 percent of people with CML have a Ph chromosome.

Those who don’t have one almost always test positive for the BCR-ABL1 fusion gene on chromosome 22. The BCR-ABL1 fusion gene is not present in healthy blood cells.

Fluorescence in situ hybridization (FISH)

This is a more sensitive type of cytogenetic test. It uses a special dye that makes it easier to test for BCR-ABL1 fusion genes.

A healthcare professional can use FISH to check on how treatment is working.

Quantitative polymerase chain reaction (qPCR)

This is the most sensitive test for detecting BCR-ABL1 fusion genes in blood or bone marrow. It’s also helpful in monitoring treatment.

Blood chemistry tests

Blood chemistry tests do not help diagnose CML.

A healthcare professional will use them to evaluate kidney and liver health. That’s because leukemia and leukemia treatments can affect these organs.

Imaging tests

Tests like CT and MRI scans as well as ultrasounds aren’t needed for diagnosis. But your doctor may order them to determine if your liver or spleen are enlarged due to leukemia.

After diagnosis, you’ll need to make important treatment decisions. To better understand your options, consider asking a healthcare professional these questions:

  • What phase is my CML?
  • What’s my general outlook?
  • What treatment do you recommend and why?
  • What are the potential side effects?
  • How will we know if it’s working?
  • What tests will I need and how often?

If you’re not comfortable with your doctor or treatment recommendations, it’s fine to get a second opinion. It’s best to do this as quickly as possible to avoid a delay in starting treatment.

CML is a slow-growing cancer of the bone marrow and blood. Symptoms can be mild and often go unnoticed early on.

The CBC test is often the first sign that something’s wrong. A high white blood cell count or too many immature white blood cells are signs of CML. Additional tests are needed to confirm the diagnosis.

Once you’re in treatment for CML, CBC testing can keep an eye on how well it’s working.