The Tests That Help Diagnose Leukemia

An accurate diagnosis helps find the most effective treatment options.

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Getting an accurate cancer diagnosis is crucial for finding the most effective treatment. Doctors want to find out where the cancer originated, how rapidly it’s progressing, whether it’s spreading, and more—all of which could affect treatment and prognosis.

For leukemia, a cancer that starts in the young blood cells in the bone marrow, many patients’ journeys begin during a routine blood test that shows abnormalities. This could be leukemia, or it could be a number of other conditions (cancerous and noncancerous).

If this is the case, doctors must confirm the diagnosis with other tests for leukemia.

Complete Blood Count

A complete blood count (CBC) measures the number of each type of white blood cells, red blood cells, and platelets, as well as your hemoglobin and hematocrit levels, according to the Leukemia and Lymphoma Society. This checks for abnormalities, which could indicate leukemia (or other types of blood cancer).

“The key components of a CBC are your white blood cells, hemoglobin, and platelets,” says Sangmin Lee, MD, hematologist-oncologist at Weill Cornell Medicine and NewYork-Presbyterian.

For example, your CBC might reveal abnormally high levels of white blood cells (which are actually leukemia cells that don’t function like normal white blood cells).

Bone Marrow Biopsy

Bone marrow tests help doctors assess the health of your marrow tissue and fluid to check for diseases like leukemia. Since leukemia starts in the bone marrow, the cancer cells may appear in the bone marrow before the bloodstream, allowing doctors to catch leukemia earlier.

“Basically, a needle goes into your bone marrow, and a small sample of bone marrow fluid is taken out for testing,” says Dr. Lee. Bone marrow biopsies are typically taken from the hip using local anesthesia.

Lymph Node Biopsy

The lymph nodes are one of the first places cancer spreads for diseases like leukemia. By taking a tiny sample of a lymph node, doctors can detect if leukemia has spread to the lymph nodes—or alternatively, if the cancer is originating in the lymph nodes (a blood cancer known as lymphoma).


If doctors diagnose you with leukemia, they may use immunophenotyping to classify which type of leukemia you have.

“To tell if it’s a certain type of leukemia, such as a myeloid leukemia or lymphoid leukemia, we commonly test for markers that are present on the cells,” says Dr. Lee.

Flow Cytometry

This test to diagnose leukemia is used after a CBC finds an abnormally high white blood cell count. Flow cytometry helps determine if this abnormality is blood cancer—or something else.

The instrument used for this test is called a flow cytometer. It measures the percentage of certain blood cells in a sample, as well as their size, shape, and characteristics.


“Inside every cell, you have DNA. In many types of leukemias, your DNA can be abnormal,” says Dr. Lee. Cytogenetics is one test that can detect these DNA mutations associated with leukemia. It examines the chromosomes under a microscope for abnormalities.

FISH (Fluorescence in Situ Hybridization)

FISH is another type of test that checks for DNA abnormalities associated with leukemia. Like with cytogenetics, it also checks for chromosome changes, but FISH can help detect abnormalities that are not visible under a microscope.

Your doctor will choose from these test options until an accurate diagnosis can be confirmed. “It’s extremely important to know what kind of leukemia you have because treatment of leukemia differs based on what kind of leukemia you have,” says Dr. Lee.