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What is a blast?

“Blast” is one of the most common words in leukemia test reports. Its meaning is simple.

Last updated: ~1 min read
A blast: a young cell with a large nucleus
Figure 1.A blast: a young cell with a large nucleus

01Short answer

A blast is a young, immature blood cell in the bone marrow. In a healthy person there are few blasts (usually under 5% in the marrow) and almost none in the blood, because they quickly mature into working cells. In acute leukemia, blasts multiply without maturing and build up in the marrow and blood. The blast percentage is one of the main measures for assessing diagnosis, remission and relapse.

02Simple explanation

A blast is a “child” cell: it has not yet learned its job.

In leukemia these “children” never grow up and fill the marrow.

03Scientific explanation

Acute leukemia is usually diagnosed when blasts make up 20% or more of the marrow or blood, although some genetic subtypes do not require this threshold.

Flow cytometry shows whether a blast is myeloid or lymphoid.

04What happens in the body?

Normal blasts

New blasts form in the marrow every day and mature step by step. This is part of healthy blood formation.

Leukemia blasts

A genetic change stops the blast maturing. Such blasts multiply and leave no room for healthy cells.

From blast to mature cell
Figure 2.From blast to mature cell

How it shows in tests

The blood film and marrow aspirate report the percentage of blasts. A fall after treatment shows response.

Counting blasts under the microscope
Figure 3.Counting blasts under the microscope

05A blast’s path

Normally and in leukemia.

A young cell — A blast forms in the marrow.1A young cellA blast forms in the marrow.
Normal maturing — The blast becomes a working cell.2Normal maturingThe blast becomes a working cell.
Stuck in leukemia — Maturing stops while dividing goes on.3Stuck in leukemiaMaturing stops while dividing goes on.
Counted in the marrow — The percentage shows diagnosis and response.4Counted in the marrowThe percentage shows diagnosis and response.
Clearer termsReports become easier to read.
Tracking responseThe blast percentage shows treatment effect.

06Key points at a glance

TopicIn brief
Normal blastsNew blasts form in the marrow every day and mature step by step. This is part of healthy blood formation.
Leukemia blastsA genetic change stops the blast maturing. Such blasts multiply and leave no room for healthy cells.
How it shows in testsThe blood film and marrow aspirate report the percentage of blasts. A fall after treatment shows response.

07Key terms

Blast
An immature young blood cell.
Aspirate
A liquid marrow sample taken with a needle.
Maturation
A cell taking on its working form.

08Key points

  • A blast is a young blood cell.
  • Normally there are few.
  • In acute leukemia blasts multiply.
  • The percentage matters for diagnosis and response.

09Frequently asked questions

Are blasts in the blood always leukemia?

No, they sometimes appear after a severe infection or growth factors; the doctor assesses this.

Should the blast percentage be zero?

In remission the marrow can contain a few normal blasts.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Are blasts in the blood always leukemia?

  2. Should the blast percentage be zero?

My questions

12Sources

  1. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: blast
  2. MedlinePlus (U.S. National Library of Medicine). Bone marrow aspiration
  3. National Cancer Institute (NCI). Adult Acute Myeloid Leukemia Treatment (PDQ®) – Patient Version
  4. National Cancer Institute (NCI). Adult Acute Lymphoblastic Leukemia Treatment (PDQ®) – Patient Version
  5. NHS. High temperature (fever) in adults
  6. National Cancer Institute (NCI). Managing Your Care
  7. Anthony Nolan. Understanding stem cell transplants
  8. National Cancer Institute (NCI). Questions to Ask about Cancer Treatment
  9. American Cancer Society. Typical Treatment of Acute Lymphocytic Leukemia
  10. National Cancer Institute (NCI). Side Effects of Cancer Treatment
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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