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MDS or leukaemia: where the line is drawn | CION Cancer Clinics
The difference is mainly the share of blasts, the very young blood cells, in your bone marrow. In MDS the marrow makes faulty cells but blasts stay below about 20%. At or above that line, or with certain gene changes, doctors call it acute myeloid leukaemia. This page explains how the line is drawn, how the two behave, and what the label cannot tell you. At CION Cancer Clinics, every leukaemia, MDS and MPN case is reviewed by our haematologist and discussed at a tumour board before a plan is agreed.
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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 is the difference between MDS and leukaemia?
- How do MDS and acute leukaemia compare?
- How does the laboratory decide which one it is?
- Which other conditions sit close to MDS?
- What do families often believe about MDS and leukaemia?
- What can this comparison not tell you?
- Common questions about MDS and leukaemia
The short answer
What is the difference between MDS and leukaemia?
The main difference is the share of blasts, the very young blood cells, in your bone marrow. In MDS the marrow makes faulty cells but blasts stay below about 20%. At or above that line, doctors call it acute myeloid leukaemia (AML).
Why the two are so often confused
Both start in the same place, the stem cells in the marrow that make your blood. Both cause low counts, tiredness, infections and bruising. Both can need the same kinds of tests. On a blood report alone they can look alike, which is why a marrow test is needed to tell them apart.
How they behave differently
MDS is usually slower. Many people with lower-risk MDS are watched for years. Acute leukaemia usually moves in weeks, and treatment is planned quickly. Some MDS does change into AML over time, and this is more likely in higher-risk MDS.
Why the name on the report matters to you
The label changes the plan. It decides which medicines are offered, how soon, and whether a transplant needs to be discussed early. It also changes which scheme and insurance codes apply to your care.
The line is drawn by laboratory rules, not by how unwell someone feels. A person with MDS can feel worse than a person with leukaemia.Side by side
How do MDS and acute leukaemia compare?
MDS
Acute myeloid leukaemia
Blasts in the marrow stay below about 20%
Blasts reach about 20% or more, or a defining gene change is found
Often changes slowly, over months or years
Usually changes over days to weeks
Lower-risk types may only need blood tests and support
Usually needs treatment planned without delay
Most often found by chance on a routine blood test
More often found because someone has become unwell quickly
On the marrow report
How does the laboratory decide which one it is?
The blood count and smear
Low counts and oddly shaped cells raise the question. If blasts are already seen in the blood, the marrow test is arranged sooner.
Counting blasts in the marrow
A pathologist counts how many of the marrow cells are blasts. This percentage is the first thing that separates MDS from acute leukaemia.
Looking at chromosomes and genes
Some gene changes mean the disease is called AML even when blasts are fewer. Others point firmly to MDS. These results can take longer to arrive.
Reading it all together
The haematologist joins the counts, blasts, gene results and your symptoms into one diagnosis. When results sit near the line, a repeat test after a short gap may be advised.
Not sure whether this applies to you?
Ask an oncologistNot all leukaemia is the same
Which other conditions sit close to MDS?
"Leukaemia" covers several different diseases. Only some of them are close relatives of MDS.
Acute myeloid leukaemia
The closest relative. It starts in the same myeloid cells as MDS, and MDS that progresses turns into this. When AML follows MDS, doctors often note it on the report because it can change the treatment choice.
CMML
Chronic myelomonocytic leukaemia has features of both MDS and a condition where the marrow makes too many cells. A raised count of a white cell called a monocyte is its main sign.
Chronic myeloid leukaemia
Despite the similar name, this is a different disease. It usually raises the white count rather than lowering it, and it is driven by one specific gene change that tablets can target.
Lymphoid leukaemias
Acute and chronic lymphocytic leukaemia start in a different branch of blood cells. They are not forms of MDS and are treated differently.
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When not to wait for the next appointment
If someone with MDS or leukaemia develops a fever, shivering, bleeding that will not stop, sudden breathlessness or confusion, go to the nearest emergency department the same day, or call 108. Tell them about the blood cancer and the latest counts. Do not wait to see if it settles, and do not start leftover antibiotics at home first.
Commonly believed
What do families often believe about MDS and leukaemia?
"MDS is just early leukaemia, so it will turn into it anyway."
Many people with MDS, especially lower-risk MDS, never develop acute leukaemia. MDS is a condition in its own right, with its own treatments. Your risk group gives a much better idea of what to expect than the name alone.
"MDS is not really cancer, so it is nothing to worry about."
MDS is a blood cancer, even when it behaves gently. It needs a haematologist, regular blood tests and a clear plan. Treating it as a minor blood problem can mean changes are missed.
"Once it is called leukaemia, nothing more can be done."
That is not true. There are treatments for AML that follows MDS, including options that suit older adults. What is offered depends on the gene changes, your fitness and your wishes.
"A second marrow test means the first one was wrong."
Repeat tests are routine. Blasts can shift over time, and a later sample often shows whether the disease is stable or changing. It is how your team keeps the diagnosis accurate.
Being straight with you
What can this comparison not tell you?
This page cannot tell you which one you or your parent has. That comes only from a marrow test and gene results read by a haematologist. A blood report on its own, or a single word in a report, is not enough.
It cannot tell you how fast things will move
Within each name there is a wide spread. Some MDS moves faster than some leukaemia. Gene changes, age and general health shape the picture far more than the label does.
What to ask at the appointment
Ask what percentage of blasts was found, and whether gene results are still pending. Ask whether the diagnosis might change once they arrive. Ask what risk group this places the disease in, and what that means for treatment. At CION the haematology team reviews the full set of reports and presents the case at a tumour board. Where a test or treatment is done elsewhere, the team coordinates it with you.
Bring every earlier blood report you have. A slow change over many months tells a different story from a sudden one.Questions we are asked
Common questions about MDS and leukaemia
Is MDS worse than leukaemia?
Not as a rule. Lower-risk MDS usually behaves more gently than acute leukaemia. Higher-risk MDS can be as serious and is often treated in similar ways. The risk group, gene changes and your general health say far more than the name. Ask your haematologist where your own diagnosis sits.
How will we know if MDS is turning into leukaemia?
Signs include counts falling faster, more transfusions being needed, blasts appearing in the blood, or new infections and bruising. Regular blood tests are how your team watches for this. A repeat marrow test confirms it. Report new symptoms early rather than waiting for the next visit.
The report says "MDS with increased blasts". What does that mean?
It means blasts are higher than in a healthy marrow but have not reached the level used for acute leukaemia. It usually places the disease in a higher-risk group. Your haematologist will explain what that means for treatment and how often you will be checked.
Is the treatment the same for both?
Sometimes it overlaps. Higher-risk MDS and AML in older adults may both be treated with medicines such as azacitidine. Lower-risk MDS is usually treated very differently, with transfusions and medicines that support red cells. A transplant is discussed for fit patients in either condition.
Can a blood test alone tell MDS from leukaemia?
Usually not. A blood test can raise the suspicion and sometimes shows blasts. The diagnosis needs a bone marrow sample, with the blasts counted and chromosome and gene tests done. Without those, any label on a report is only a working guess.
Is either of them passed on to children?
In most families, no. Both usually come from changes that build up in marrow cells during life. A few families carry genes that raise the risk, often when the disease appears young or in several relatives. Your haematologist can advise whether genetic counselling makes sense.
Can it be CMML instead?
Yes. CMML shares features with MDS but also has a persistently raised count of monocytes, a type of white cell. The blood count and marrow findings decide it. The treatment approach overlaps with MDS in several ways, but the risk scores used are different.
Who should we see for a second opinion?
A haematologist, ideally one who can review the actual marrow slides and gene reports, not only the summary. Bring every report and the slides or blocks if you can get them. The CION helpline can tell you what to bring before you travel.
Your Haematologist
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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 — Myelodysplastic syndromes
- National Cancer Institute — Myelodysplastic Syndromes Treatment (PDQ), Patient Version
- NHS — Acute myeloid leukaemia
- Leukemia & Lymphoma Society — Myelodysplastic syndromes
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
MDS Explained: When the Marrow Stops Making Good CellsStart here if MDS is a new word to youWhen MDS Transforms Into AMLThe signs, the tests and what changes in the planCMML: Between MDS and a Myeloproliferative DiseaseThe close relative with a raised monocyte countLeukaemia guideThe different types of leukaemia explained
Talk to us
Not sure which name is on your report?
Tell us what has been found so far. The haematology team will read the reports with you and help you reach the right specialist. One helpline serves every CION centre.