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Differentiation syndrome: a reaction to watch for in APL | CION Cancer Clinics
Differentiation syndrome is a reaction to ATRA or arsenic trioxide in APL. Its main signs are breathlessness, fever, fast weight gain and swelling, usually in the first weeks of treatment. It can turn serious quickly, so report these signs at once, day or night, or call 108 if at home. This page explains why it happens, how it is treated and who needs closer watching. 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 differentiation syndrome?
- Which symptoms point to differentiation syndrome?
- What does the treating team do about it?
- What else can look like differentiation syndrome?
- What do families misunderstand about this reaction?
- Who is at higher risk, and what can this page not tell you?
- Common questions about differentiation syndrome
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On ATRA or arsenic? These signs cannot wait
New or worsening breathlessness, fever, fast weight gain, swelling of the legs or face, chest pain, dizziness or fainting, or passing much less urine. In hospital, tell the nurse now. At home, call the ward number or 108, or go to the nearest emergency department. Say "APL on ATRA or arsenic" so the doctor thinks of differentiation syndrome. Do not stop or skip the medicines yourself.
The short answer
What is differentiation syndrome?
Differentiation syndrome is a reaction that can happen when ATRA or arsenic trioxide starts working in APL. The main signs are breathlessness, fever, weight gain and swelling. It can become serious quickly, but it usually settles when it is recognised early and treated.
Why it happens
ATRA and arsenic make the stuck leukaemia cells grow up. As they do, large numbers of maturing cells release chemical signals and can move into the lungs, kidneys and other tissues. Fluid leaks out of small blood vessels. That leak causes the breathlessness, swelling and weight gain. It is a sign the medicine is acting on the cells, which is why it is managed rather than simply feared.
When it usually appears
It most often appears in the first weeks of induction, the first phase of treatment, while the patient is still in hospital. It can happen later, and rarely during later courses. The white count often rises before or during it.
Why the old name still appears
You may see it written as "retinoic acid syndrome" or "RA syndrome" in older notes. The name changed because arsenic can cause it too.
Why families matter here
Nurses check oxygen, weight and blood pressure on a schedule. In between those checks, the relative sitting at the bedside is often the first to see a change. Faster breathing while talking, a patient who suddenly wants to sit up at night, or slippers that no longer fit are all worth mentioning straight away. You will not be wasting anyone's time.
Not sure whether this applies to you?
Ask an oncologistWhat to watch for
Which symptoms point to differentiation syndrome?
No single sign proves it. Doctors look for a combination, so tell the team about each change, even if it seems small on its own.
Breathing
The most important sign. Breathlessness at rest or on small effort, a new cough, or a need to sit up to breathe. The oxygen reading on the finger clip may fall.
Fluid and weight
Weight rising over a day or two without eating more, puffy ankles, legs or face, and tight rings or slippers.
Simple checks that help
- Weighing every morning, same scale
- Noting how much urine is passed
Fever and blood pressure
A fever with no clear infection, low blood pressure, dizziness on standing or a fast heartbeat.
What tests may show
A rising white count, shadows or fluid on a chest X-ray, fluid around the heart or lungs, and worsening kidney or liver tests.
If it is suspected
What does the treating team do about it?
Treat on suspicion
Doctors do not wait to be certain. If the signs fit, treatment usually starts straight away, because it works best early and waiting lets it get worse.
Start a steroid
A steroid medicine, most often dexamethasone, is given into a vein to calm the reaction. Some centres give steroids to prevent it in higher-risk patients.
Support breathing and fluids
Oxygen, careful fluid balance and sometimes a water tablet to clear extra fluid. A few patients need intensive care for a short time.
Decide about ATRA and arsenic
In severe cases the doctor may pause one or both medicines until things settle, then restart. This is a medical decision, never one for the family.
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It could be something else
What else can look like differentiation syndrome?
The look-alike
Why the team still checks
Chest infection or pneumonia
Fever and breathlessness are shared, and infections are common when counts are low
Bleeding into the lungs
APL upsets clotting, so coughing blood or sudden breathlessness needs checking
Fluid overload from drips and transfusions
Swelling and weight gain can come from the fluids given in hospital
Heart strain
Arsenic affects heart rhythm, and fluid can gather around the heart
Commonly believed
What do families misunderstand about this reaction?
"If the medicine causes this, we should stop the medicine."
ATRA and arsenic are what treat the APL. Stopping them without a doctor's decision can let the leukaemia and the bleeding risk return. The team will pause only when it is needed, and restart when it is safe.
"The fever must be an infection, so antibiotics will sort it."
It may be an infection, and antibiotics are often given while tests are done. But fever with breathlessness and swelling on ATRA should also make the doctor think of differentiation syndrome. Both can be treated at the same time.
"Swollen feet are just from lying in bed."
They may be. On ATRA or arsenic, new swelling or weight gain should still be reported the same day, because it can be the earliest sign of fluid leaking out of the blood vessels.
"Having this reaction means the treatment has failed."
It does not. Many people who have differentiation syndrome go on to complete treatment and reach remission. What matters is catching it early.
Being straight with you
Who is at higher risk, and what can this page not tell you?
This page cannot tell you whether a symptom in your family member is differentiation syndrome. That takes an examination, oxygen readings, blood tests and often a chest X-ray. What it can do is help you notice changes early and say the right words to the doctor.
Who needs closer watching
A high white count at diagnosis, or a count that rises fast during treatment, raises the risk. So do kidney problems and being overweight. Some teams use steroids to prevent the reaction in these patients. Ask whether this applies and what the plan is.
What recovery usually looks like
With early treatment most people improve over several days. The breathlessness eases first, and the extra fluid clears more slowly. Keep telling the team about any new change during recovery, because a second episode can happen. Ask the team before discharge which signs mean a call to the ward and which mean going straight to emergency, and write the answers where everyone at home can see them.
Where CION fits
CION's haematology team can review reports and help families understand the plan. This reaction is managed where the patient is admitted. A review never replaces calling the ward or going to emergency.
Questions we are asked
Common questions about differentiation syndrome
How common is differentiation syndrome?
It is one of the recognised complications of APL treatment, common enough that every APL team checks for it daily during the first phase. Many patients never develop it, and in many who do it is mild. Preventive steroids in higher-risk patients have made severe cases less frequent.
Is differentiation syndrome life-threatening?
It can be if it is missed or treated late, mainly because of breathing and kidney problems. That is why doctors treat on suspicion rather than waiting for certainty. With early steroids and support, most people recover. Report breathlessness, fever and weight gain promptly, day or night.
Can it happen after we go home?
It is most common in hospital during induction, but it can occur later, including during day-care courses of arsenic. At home, weigh daily if the team asks, and watch breathing and swelling. Keep the ward phone number and the nearest emergency department noted down before leaving hospital.
Why is the patient being weighed every day?
Sudden weight gain is often the earliest sign that fluid is building up in the body, sometimes before breathlessness starts. A daily weight on the same scale, at the same time, gives the team an early warning. Families can help by making sure it is done and noted.
Do the steroids have side effects of their own?
Yes. Steroids can raise blood sugar, affect sleep and mood, increase appetite and add to infection risk. For a short course these are usually manageable. People with diabetes will have their sugar checked more often. Tell the team about any changes you notice.
Will ATRA be restarted after the reaction?
If it was paused, it is usually restarted once breathing and other signs have improved, sometimes with steroids continuing for cover. The haematologist decides when and how. Do not restart or stop it at home based on how the patient feels.
Can differentiation syndrome come back?
It can, particularly if a medicine is restarted or the white count rises again. The team watches for this, and families should keep reporting new breathlessness, swelling or fever even after the first episode has settled.
Does it happen in other leukaemias?
A similar reaction can occur with some newer targeted medicines used in other types of acute myeloid leukaemia, which also make leukaemia cells grow up. If a family member is on such a medicine, ask the team whether they should watch for the same signs.
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Sources
- American Cancer Society — Treating Acute Promyelocytic (M3) Leukemia
- National Cancer Institute — Adult Acute Myeloid Leukemia Treatment (PDQ) - Patient Version
- Leukemia & Lymphoma Society — Acute Myeloid Leukemia (AML)
- Cancer Research UK — Acute myeloid leukaemia (AML)
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
ATRA and Arsenic: Chemotherapy-Free Treatment for APLThe medicines that can trigger this reactionBleeding Risk in APL: What Families Must KnowThe other early danger in APLWhy APL Is Treated Before the Diagnosis Is ConfirmedWhy the first days move so fastAPL: The Most Curable Leukaemia and the Most UrgentThe basics of APL
Talk to us
Questions about a reaction during APL treatment?
If breathing is getting worse, call the ward or go to emergency first. For questions about reports, CION's haematology team can help. One helpline serves every CION centre.