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Venesection: why blood is removed as treatment | CION Cancer Clinics
Blood is removed in polycythaemia vera because there are too many red cells, which makes the blood thick and clots more likely. Venesection takes some out, much like a blood donation, and thins the blood within days. Sessions are steered by your haematocrit, often aiming below 45%. This page explains what happens at a session, how often it is needed, who it does not suit and what to avoid. 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
- Why is blood removed to treat polycythaemia?
- What happens at a venesection session?
- How often will I need it?
- How can you make a session easier?
- Who does venesection not suit, or need adjusting for?
- What do families worry about with venesection?
- What can this page not tell you?
- Common questions about venesection
The short answer
Why is blood removed to treat polycythaemia?
Blood is removed because there are too many red cells in it, and that makes the blood thick. Taking some out, a treatment called venesection, quickly thins the blood and lowers the chance of a clot in the brain, heart, lungs or legs.
Why it sounds strange, and why it works
Most families are used to hearing that blood loss is dangerous and that a high haemoglobin is healthy. In polycythaemia vera the opposite is true. The marrow keeps making red cells the body does not need. Each session takes out some of those extra cells, and the fluid your body replaces afterwards dilutes what is left.
The number it is aiming at
Venesection is steered by the haematocrit, the share of your blood made up of red cells. Most haematologists aim to keep it below 45%, because clots become more likely above that level. Your own target may be different, and your team will tell you what it is.
Why it is often the first treatment
It works within days rather than months, needs no daily tablets, and is one of the oldest and most understood treatments in haematology. For many people it is used alongside aspirin, and sometimes with a medicine that calms the marrow.
Venesection controls the count. It does not change the marrow itself, so it is usually repeated over the long term.On the day
What happens at a venesection session?
A blood count first
A small sample is checked so the team knows your haematocrit that day. If it is already at target, the session may be skipped.
Blood pressure and pulse
A nurse checks these before starting and asks how you have been, including any dizziness after the last session.
Blood is taken
A needle goes into a vein in the arm, much like a blood donation, and about half a litre flows into a bag. You sit or lie back for around half an hour.
Rest and a drink
You stay seated for a short while, drink fluids and have a snack before getting up and going home.
Not sure whether this applies to you?
Ask an oncologistOver time
How often will I need it?
It changes over time. The schedule follows your haematocrit, not the calendar.
At the start
Sessions are usually close together, sometimes a few days apart, until the haematocrit reaches target. Smaller amounts may be taken more often if you are older or have heart problems.
Keeping it steady
Once at target, the gap between sessions widens. Many people need one every few weeks or every few months, guided by regular blood counts.
As iron runs low
Each session removes iron. Over time, lower iron slows red cell production, and many people need fewer sessions. Mild tiredness can come with this.
When plans change
If sessions become too frequent or poorly tolerated, a medicine may be added.
Options your team may discuss
- Hydroxyurea
- Interferon
- Ruxolitinib
Before you go
How can you make a session easier?
- Drink plenty of water the day before and on the morning of the session
- Eat a normal meal beforehand, do not come fasting
- Wear a loose sleeve that rolls up easily
- Bring someone to ride home with you for the first few sessions
- Avoid heavy lifting and hard exercise for the rest of the day
- Tell the nurse if you felt faint or dizzy last time
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Honest limits
Who does venesection not suit, or need adjusting for?
Venesection suits most people with polycythaemia vera, but not everyone in the same way. Some people need smaller amounts, extra care or a different approach altogether.
People with heart or lung problems
A sudden drop in blood volume can strain the heart or cause dizziness. Your team may take smaller volumes and replace them with fluid through a drip.
People with difficult veins
Frequent needles can scar veins over time. If access becomes hard, the plan may lean more on medicines.
People with secondary polycythaemia
When red cells are high because of lung disease or low oxygen, the extra cells may be helping carry oxygen. Removing blood is used far more cautiously and only when a specialist advises it.
People whose counts rise quickly
If the marrow is very active, or platelets are also very high, venesection alone may not be enough to lower clot risk.
Commonly believed
What do families worry about with venesection?
"Taking blood out will make him weak."
The blood removed is extra blood the body did not need. Some people feel tired or light-headed for a day, which usually passes. Feeling better, with fewer headaches, is common once the count is down.
"She should take iron tablets to recover."
Low iron is partly how venesection keeps the count steady. Taking iron without advice can make the marrow produce red cells faster and undo the treatment.
"Donating at a blood bank is the same thing."
Blood banks usually do not accept blood from people with a blood cancer, and donation does not follow your target. Venesection is timed against your own counts, under medical supervision.
"Once the count is normal, the sessions can stop for good."
A normal count means the treatment is working. The marrow keeps overproducing, so stopping usually lets the haematocrit climb again without warning.
Being straight with you
What can this page not tell you?
This page cannot tell you your own target, how often you will need sessions, or whether venesection alone is right for you. Those depend on your counts, age, clot history, heart health and how you respond.
It cannot predict your outlook
Venesection lowers clot risk. How polycythaemia vera behaves over many years depends on factors your haematologist will explain for your own situation. These include your age, whether you have had a clot, other conditions such as high blood pressure or diabetes, and how the counts behave over time.
It cannot replace the rest of the plan
Venesection works best alongside the things you control. Stopping smoking, staying active, drinking enough water and keeping blood pressure, sugar and cholesterol in check all lower clot risk further. Aspirin, where your team advises it, adds a different kind of protection.
What to ask your team
Ask what your haematocrit target is, how often counts will be checked, what to do if you feel faint afterwards, and when a medicine might be added. At CION, the haematology team plans venesection around your counts and reviews the plan as things change.
Do not skip sessions or change the schedule on your own. If you cannot attend, tell the team so they can rearrange it.Questions we are asked
Common questions about venesection
Does venesection hurt?
You feel a sharp scratch when the needle goes in, much like a blood test. The rest is usually comfortable. The needle is slightly larger than for a routine sample, so some bruising at the site is common. Tell the nurse if the arm feels sore or numb during the session.
Can I go to work afterwards?
Many people return to desk work the same day. Avoid heavy lifting, hard exercise and hot baths for the rest of that day. If your job involves driving long distances, operating machinery or working at height, take the rest of the day off after the first few sessions until you know how you react.
What if I feel faint after a session?
Lie down with your legs raised and drink water. Mild light-headedness usually passes quickly. Tell the team at your next visit, as they may take less blood or give fluids. If you faint, have chest pain or breathlessness, call 108 or go to the nearest emergency department.
Why do I feel tired even though my count is better?
Tiredness can come from low iron, which builds up over repeated sessions, or from the condition itself. Tell your haematologist rather than taking iron on your own. They can check your iron levels and decide whether anything should change.
Can I eat normally on venesection days?
Yes, and you should. Coming on an empty stomach makes fainting more likely. Eat a normal meal and drink well beforehand. Afterwards, have a drink and a snack before leaving. There is no special diet for venesection.
Is the removed blood used for anything?
It is usually discarded safely. Rules on using blood from people with a blood cancer mean it is generally not given to other patients. This is a safety policy, not a sign that your blood is harmful to you or your family.
Will I need venesection for the rest of my life?
Many people need it on and off for years, though often less frequently once iron levels fall. Some move to a medicine instead. Your haematologist reviews the plan regularly and explains any change and the reasons for it.
Is venesection covered by Aarogyasri or insurance?
It depends on the scheme and policy, and on whether it is billed as day care or outpatient care. Aarogyasri, CGHS, ECHS, EHS, PM-JAY and cashless insurers have their own rules, which change. Call the helpline with your card details so the team can check your current cover.
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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
- NHS — Polycythaemia
- Leukemia & Lymphoma Society — Polycythemia vera
- National Heart, Lung, and Blood Institute — Polycythemia vera
- National Health Mission — National Health Mission
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.
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