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Blast percentage: the number that shapes the diagnosis | CION Cancer Clinics
Blasts are very young blood cells. In a healthy marrow they make up under 5% of the cells counted. A figure of 20% or more is the usual line for calling it an acute leukaemia, and 5% to 19% often points to a slower marrow condition. This page explains how the number is counted, why reports can disagree, and what it cannot tell you. At CION Cancer Clinics, our haematologist reviews abnormal blood reports with you, orders only the tests that answer your question and explains each result plainly.
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Medically reviewed by Dr. Basudev PokhrelConsultant Haematologist · last reviewed September 2026, next review due September 2027
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On this page
- What does the blast percentage on a marrow report mean?
- Which blast figures matter, and what do they point to?
- How is the blast count actually worked out?
- Why might two reports give different blast numbers?
- What do the other words near the blast figure mean?
- What do families often get wrong about the blast count?
- What can the blast percentage not tell you?
- Common questions about blast percentage
The short answer
What does the blast percentage on a marrow report mean?
The blast percentage tells you what share of the cells counted in the marrow sample were blasts. Blasts are very young blood cells that have not grown up yet. A healthy marrow has only a few. When they fill a large part of the marrow, it usually points to an acute leukaemia.
Why young cells matter
Your marrow is the soft tissue inside the big bones where blood is made. Every red cell, white cell and platelet starts life there as a blast. In a healthy person those blasts mature quickly and leave the marrow as working cells. In an acute leukaemia, blasts keep dividing but stop maturing. They crowd out the normal cells. That is why a person with a high blast count so often has low haemoglobin, low platelets or repeated infections at the same time.
Why one number carries so much weight
Doctors around the world use the blast percentage as a dividing line. It helps separate an acute leukaemia from slower conditions of the marrow, such as myelodysplastic syndrome, often shortened to MDS, which means a marrow that makes faulty cells. The same number is checked again after treatment to see whether the leukaemia has gone into remission, which means no sign of disease on the usual tests.
The blast percentage is a starting point for the diagnosis, not the whole diagnosis. Other tests on the same sample finish the picture.The dividing lines
Which blast figures matter, and what do they point to?
These are the lines most laboratories and haematologists work with. Classification systems are updated from time to time, so your own report may use slightly different wording.
Blasts in the marrow
What it usually points to
Under 5%
Within the range seen in a healthy marrow, or a leukaemia in remission after treatment
5% to 19%
Raised. Often seen in MDS with increased blasts, and watched closely
10% to 19% with certain gene changes
Some newer systems treat this as a borderline zone between MDS and acute myeloid leukaemia
20% or more
The usual line for calling it an acute leukaemia
Any figure, with certain specific gene changes
Some types of acute myeloid leukaemia are diagnosed whatever the blast count
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Ask an oncologistBehind the number
How is the blast count actually worked out?
A pathologist or haematologist looks at a thin smear of the marrow liquid under a microscope. They count several hundred cells one by one, usually 500 cells, and note how many are blasts. The percentage is simply blasts divided by the total counted.
The aspirate and the biopsy do different jobs
The marrow test usually takes two samples from the back of the hip bone. The aspirate is the liquid part, and it is where the count is done. The trephine biopsy, a small core of bone, shows how crowded the marrow is and how the cells are laid out. When the liquid cannot be drawn, the biopsy and special stains are used to judge the blasts instead.
Flow cytometry adds a second view
Flow cytometry is a machine test that reads markers on the surface of thousands of cells. It tells the team what kind of blasts they are, myeloid or lymphoid. Its percentage can differ from the microscope count, because the sample is handled differently. Your haematologist reads both together and does not simply pick the higher one.
When reports disagree
Why might two reports give different blast numbers?
Families are often alarmed when one report says one figure and another says something else. There are ordinary reasons for it.
Blood mixed into the sample
If the needle draws a lot of circulating blood with the marrow, the blasts are diluted. The count comes out lower than the true marrow figure.
Blood report versus marrow report
A blood smear counts blasts in the circulation. A marrow report counts them where blood is made. The two figures answer different questions and are rarely identical.
Microscope versus machine
The microscope count and the flow cytometry figure use different methods. Some cells are hard to classify by eye, and some are lost in the machine preparation.
Time between tests
A marrow repeated a few weeks later may show a real change. Steroids or other medicines given in between can also lower the count before the diagnosis is settled.
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On your report
What do the other words near the blast figure mean?
- Myeloblast
- A blast that would normally grow into certain white cells. Raised myeloblasts point towards a myeloid condition.
- Lymphoblast
- A blast that would normally grow into lymphocytes, the infection fighting cells. Raised lymphoblasts point towards acute lymphoblastic leukaemia.
- Blast equivalent
- A cell counted as a blast even though it looks a little more mature, because it behaves like one. Promonoblasts are one example.
- Differential count
- The full breakdown of every cell type counted, of which the blast percentage is one line.
- Hypercellular
- The marrow is more crowded with cells than expected for your age.
Commonly believed
What do families often get wrong about the blast count?
"A higher blast percentage means less time left."
Not on its own. Once the diagnosis is made, the outlook depends far more on the type of leukaemia, its gene changes, your age and general health, and how it responds to treatment. Two people with the same blast figure can have very different paths.
"Just under the line means it is not serious."
A figure a little below the line still needs a clear plan. Some conditions just below it are treated much like acute leukaemia, and some gene changes settle the diagnosis whatever the count shows.
"If blasts are in the blood, the marrow test is not needed."
Blasts in the blood are a strong warning. The marrow test is still usually needed, because it gives the gene and marker results that decide which treatment fits.
"Blasts under the line after treatment means it has gone for good."
A low count after treatment is very welcome news. It is still one check among several. More sensitive tests look for leukaemia cells too few to see by eye, and follow-up continues for a long time.
Being straight with you
What can the blast percentage not tell you?
The blast percentage cannot tell you which treatment is right, how treatment will go, or what the future holds. It does not name the exact type of leukaemia on its own. Those answers come from the full set of tests read together by your haematologist.
What finishes the picture
Flow cytometry names the kind of blast. Karyotype and FISH look at the chromosomes. Gene panels look for specific changes. Together they place the disease in a risk group, and the risk group guides the plan. Some of these results take longer than the first count, and that wait is normal.
What to ask at the appointment
Ask which sample the figure came from, and whether the flow cytometry and microscope counts agree. Ask which gene and chromosome results are still pending. Ask whether anything already given, such as steroids, could have changed the count. At CION, our haematology team reviews the whole report, presents the case to a tumour board, and coordinates any further testing or care with qualified centres.
Reference ranges and reporting style differ between laboratories. Keep every marrow report, and bring the originals, not only photos.Questions we are asked
Common questions about blast percentage
My father's report says 30% blasts. Is it leukaemia?
A marrow blast count at or above 20% is the usual line for an acute leukaemia, so this figure strongly points that way. The type still has to be confirmed with flow cytometry and chromosome tests. Ask the haematologist how soon treatment needs to start, because acute leukaemia is usually not something to wait on.
What is a normal blast percentage in the marrow?
In a healthy adult marrow, blasts are usually under 5% of the cells counted. Laboratories differ slightly in how they report it, and a single figure is always read alongside the blood counts, symptoms and other results. A figure just above the range needs explaining, but it is not a diagnosis by itself.
Can blasts be raised for a reason other than cancer?
Yes, occasionally. The marrow can briefly show more young cells while recovering from a severe infection, after chemotherapy, or after growth factor injections that push the marrow to work harder. Your haematologist looks at what was happening around the time of the test before drawing a conclusion.
Why is my blood blast count different from the marrow count?
They measure different places. Blasts are made in the marrow and only some spill into the blood. A person can have many blasts in the marrow and few in the blood, or the other way round. The marrow figure is the one used for the diagnosis.
What does 5% to 19% blasts mean?
It usually points to a condition such as MDS with increased blasts, where the marrow makes faulty cells and has more young cells than normal. It is not an acute leukaemia by the usual line, but it is taken seriously. Gene and chromosome tests help decide how closely it is watched and whether treatment is needed.
Will the marrow test be repeated?
Usually, yes. After the first phase of treatment for acute leukaemia, a repeat marrow shows whether blasts have fallen under 5% and whether normal cells are coming back. The team may also send it for more sensitive tests that pick up very small amounts of disease. Your haematologist will tell you when.
Does the blast percentage decide the treatment?
It helps decide the diagnosis, and the diagnosis shapes the treatment. The actual plan depends on the type of leukaemia, its gene and chromosome changes, your age, your fitness and your other health conditions. Two people with the same blast figure may be offered different treatment for good reasons.
Should we get a second opinion on the marrow report?
It is reasonable when the figure sits close to a dividing line, when reports disagree, or when you simply want the slides looked at again. Ask the laboratory for the slides and blocks, not only the printed report. Do not let a second opinion delay treatment that your haematologist says is urgent.
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Dr. Basudev Pokhrel reviews blood counts, transfusion needs and blood disorders, and works with the CION tumour board on blood cancers.
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Sources
- American Cancer Society — Tests for Acute Myeloid Leukemia (AML)
- American Cancer Society — Tests for Myelodysplastic Syndromes
- National Cancer Institute — Adult Acute Myeloid Leukemia Treatment (PDQ)
- Cancer Research UK — Acute myeloid leukaemia (AML)
- Leukemia & Lymphoma Society — Acute Myeloid Leukemia
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Flow Cytometry in Blood Cancer: What It DecidesThe test that names the kind of blastCellularity, M:E Ratio and Dysplasia on a Marrow ReportThe other lines on the same marrow reportKaryotype and FISH in Leukaemia: Why They Change TreatmentThe chromosome results that can override the countLeukaemia guideThe wider picture if leukaemia is confirmed
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