The short answer
A blast is an immature blood cell. A blood smear, where blood is looked at under a microscope, checks for blast cells. In some leukemias, abnormal blasts build up in the blood and bone marrow and crowd out healthy cells. A bone marrow test is usually needed to find out more. Your care team explains what your result means.
A blast is an immature blood cell.
A blood smear looks at blood under a microscope and checks for blast cells.
In some leukemias, such as AML, abnormal blasts build up and crowd out healthy blood cells.
A bone marrow test is often used to look further.
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The short version
A blast is an immature blood cell. NCI describes the abnormal ones in acute myeloid leukemia (AML) as myeloblasts. Blasts may be mentioned on a blood smear report or a bone marrow report. Seeing this word can be frightening. It usually means more tests are needed, not that anything is certain yet.
What a blood smear checks
MedlinePlus explains that a blood smear spreads blood on a glass slide. The blood is stained and looked at under a microscope. It checks the size, shape and number of red cells, white cells and platelets. NCI notes that it also checks for blast cells.
Why blasts matter in some leukemias
NCI explains that in acute myeloid leukemia (AML), immature white blood cells called myeloblasts increase. These abnormal cells do not become healthy white blood cells. As they build up in the blood and bone marrow, there is less room for healthy cells.
For some blood cancers, the share of blasts in the bone marrow helps describe the disease. For example, NCI says a new AML diagnosis requires at least 20% blasts in the bone marrow, or certain gene changes.
What usually comes next
If a blood smear is not normal, more tests are usually ordered. NCI describes a bone marrow aspiration and biopsy, where a hollow needle removes marrow, blood and a small piece of bone to check under a microscope.
What to ask
Ask what your smear showed and what test comes next. Call your care team right away for a fever, unusual bleeding or bruising, or feeling very unwell while you wait. Call 911 for severe bleeding or confusion. Your hematology team will explain your own result once the tests are back.
Sources
Common questions
Does blasts on my report mean I have leukemia?
Not by itself. A report that mentions blasts usually leads to more tests. MedlinePlus notes that abnormal blood smear results usually lead to more tests to find the cause. Your care team will explain your result.
What is a blood smear?
MedlinePlus explains that a drop of blood is spread on a glass slide, stained and looked at under a microscope. It checks the size, shape and number of red cells, white cells and platelets. NCI notes it also checks for blast cells.
Why does the percentage of blasts matter?
For some blood cancers, the share of blasts in the bone marrow helps describe the disease. For example, NCI says a new AML diagnosis requires at least 20% blasts in the bone marrow, or certain gene changes.
What test usually comes next?
NCI describes bone marrow aspiration and biopsy, where a hollow needle removes bone marrow, blood and a small piece of bone and checked under a microscope. Your care team will explain what is needed for you.
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Sources last checked: 2026-10-02 what this meansLast updated: 2026-10-02Next planned review: 2027-09-27
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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