Hematologist
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Haematocrit targets in polycythaemia vera | CION Cancer Clinics
In polycythaemia vera, most haematologists aim to keep the haematocrit below 45%, sometimes printed as 0.45. Above that level the blood is thicker and clots are more likely, so venesection is timed to hold the count under target. Your own target may differ. This page explains what the number measures, why the target is set there, what pushes results up or down, and what one result 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 haematocrit should I aim for with polycythaemia vera?
- What does my result mean against the target?
- Why is the target set below 45%?
- What can push your haematocrit up or down?
- How is the target reached and then kept?
- What do people often get wrong about the target?
- What can this page not tell you?
- Common questions about haematocrit targets
The short answer
What haematocrit should I aim for with polycythaemia vera?
Most haematologists aim to keep the haematocrit below 45% in polycythaemia vera. Above that level the blood is thicker and clots are more likely, so treatment such as venesection is adjusted to hold it under the target.
What haematocrit actually measures
Haematocrit is the share of your blood made up of red cells. A result of 45% means just under half of the blood volume is red cells. Some laboratories print it as a fraction, so 0.45 means the same thing. Your report may also call it HCT or PCV, which stands for packed cell volume.
Why it is the number treatment follows
Haemoglobin and haematocrit usually move together, but haematocrit reflects how thick the blood is more directly. That is why venesection sessions are booked, skipped or brought forward based on this number rather than on the haemoglobin alone.
Your target may be different
The below 45% target is a general rule, not a fixed law. Some haematologists set a lower target for particular people, for example when symptoms continue or after a clot. Others accept a slightly different number for someone with heart or lung disease. Always follow the target your own team gives you.
Reference ranges and printing styles differ between laboratories. A single result is always read alongside symptoms and repeat tests.Reading your result
What does my result mean against the target?
Where your result sits
What usually happens next
Clearly above your target
Venesection is usually brought forward or repeated until it comes down
Just above target on one test
Your team may repeat the count or plan a session, depending on your trend
Below target and steady
The schedule stays the same and checks may be spaced out
Well below target, with tiredness
Sessions may be paused and iron levels checked
Not sure whether this applies to you?
Ask an oncologistThe evidence
Why is the target set below 45%?
The target comes from a large study in people with polycythaemia vera, which compared a strict target below 45% with a looser, higher one. People kept below 45% had noticeably fewer serious clots and heart problems.
What that means for you
Holding the haematocrit below target is one of the main ways treatment lowers your risk of stroke, heart attack and clots in the legs or lungs. It matters whether or not you feel unwell. Many people with a high haematocrit feel perfectly fine until a clot happens.
What the study did not show
It does not mean that a result a little above target on one day is dangerous in itself, or that going lower and lower is always better. Pushing the count far below target can leave you tired and short of iron. The aim is steady control, not the lowest possible number.
Where the target does not fit neatly
The study looked at polycythaemia vera. It does not apply in the same way to secondary polycythaemia, where extra red cells may be helping someone with lung disease carry oxygen. It also does not set a target for essential thrombocythaemia, where platelets rather than red cells are the main concern. If your diagnosis is not yet confirmed, ask before assuming this number applies to you.
Between tests
What can push your haematocrit up or down?
A change from one test to the next is not always the marrow. These are worth mentioning to your team.
How much you drank
Being short of water concentrates the blood and makes the haematocrit read higher. Drink normally before every test and every session.
Iron levels
Repeated venesection lowers iron, which slows red cell production. Taking iron tablets or tonics without advice can send the haematocrit back up quickly.
Smoking and oxygen
Smoking, sleep apnoea and lung disease all push red cells higher on top of the marrow condition.
The test itself
Different laboratories and machines give slightly different results.
To keep results comparable
- Use the same laboratory where you can
- Test at a similar time of day
- Keep every report in date order
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The pathway
How is the target reached and then kept?
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Starting point
At diagnosis your haematocrit and other counts are measured, and your clot risk is assessed from your age, past clots and other conditions.
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Bringing it down
Venesection sessions are booked close together until the result is below target. Smaller volumes may be used if you are older or have heart problems.
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Holding it steady
Blood counts are repeated regularly. A session is added whenever the result drifts above target.
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Reviewing the plan
If sessions become very frequent, are poorly tolerated, or other counts rise, your haematologist may add a medicine that calms the marrow.
Commonly believed
What do people often get wrong about the target?
"I feel fine, so the number does not matter."
Clot risk rises with a high haematocrit whether you feel well or not. Many clots happen without any earlier warning, which is why the number guides treatment rather than how you feel.
"Lower is always better."
Going far below target does not add much protection and can leave you tired and short of iron. Steady control around your own target is the goal.
"Drinking lots of water before the test will fix a high result."
Good hydration stops a falsely high reading, which is useful. It does not lower the true red cell mass, so a genuinely high result still needs the planned treatment.
"The target is the same as the normal range on my report."
The normal range printed by the laboratory is for healthy people. Your treatment target is set by your haematologist for polycythaemia vera and may differ from it.
Being straight with you
What can this page not tell you?
This page cannot tell you your own target, whether one result needs action, or what your outlook is. Those decisions depend on your trend over time, your other counts, your symptoms and your clot risk.
The target is only one part of treatment
Aspirin, where your team advises it, lowers clot risk in a different way. Stopping smoking and controlling blood pressure, sugar and cholesterol matter just as much. A haematocrit on target does not make those less important.
What to ask at your next visit
Ask what your personal target is, how often counts will be checked, what to do if a result comes back high between visits, and when a medicine might be considered. At CION, the haematology team tracks your trend, adjusts the plan and explains each change.
Do not change your venesection schedule, aspirin or any medicine on your own because of one result. Call the team first.Questions we are asked
Common questions about haematocrit targets
My report says 0.47. Is that above target?
Yes. A haematocrit of 0.47 is the same as 47%, which is above the usual below 45% target. Some laboratories print a fraction, others a percentage. Share the result with your haematology team, who will decide whether a session is needed based on your trend and your own target.
Is the target the same for men and women?
The below 45% target is generally used for both. Normal ranges differ between men and women, and some haematologists set a lower target for particular people, depending on symptoms or clot history. Your own team will tell you the number that applies to you.
Does haemoglobin have a target too?
Haemoglobin is watched, but treatment decisions in polycythaemia vera usually follow the haematocrit. Once iron runs low, the two can move a little apart, with haemoglobin falling faster. That is one reason your team looks at the whole count rather than one number.
How often should my haematocrit be checked?
Often at the start, while venesection brings it down, and less often once it is steady. The exact gap depends on how fast your count rises and what treatment you are on. Follow the schedule your team sets, and ask for an earlier check if symptoms return.
Can I check it myself at home?
Home devices for haematocrit are not reliable enough to guide venesection. Use a laboratory blood count, ideally at the same laboratory each time, so results can be compared. Do not act on a reading from a pharmacy or gym machine.
What if my haematocrit stays high despite venesection?
Tell your team. They will check whether you are drinking enough, whether iron is being taken, and how often sessions are happening. If the count still rises quickly, a medicine that calms the marrow may be added. That decision belongs to your haematologist.
Is a result just below target safe?
A result below target means treatment is doing its job for now. It does not remove all clot risk, which also depends on platelets, age, past clots and your heart health. Keep up the rest of the plan, including aspirin where advised and not smoking.
Does pregnancy change the target?
Pregnancy changes blood volume and counts, so haematologists plan it carefully and may adjust the approach. If you are pregnant or planning a pregnancy, tell your haematologist early, so they can work with your obstetrician on a plan suited to you.
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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
- Leukemia & Lymphoma Society — Polycythemia vera
- NHS — Polycythaemia
- National Heart, Lung, and Blood Institute — Polycythemia vera
- National Cancer Institute — Chronic Myeloproliferative Neoplasms Treatment (PDQ), Patient Version
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
Venesection: Why Blood Is Removed as TreatmentThe treatment used to reach the targetMPN Monitoring: Which Tests and How OftenHow often counts are usually checkedThrombosis Risk in MPN and How It Is ScoredThe other factors that set your clot riskPolycythaemia Vera ExplainedThe condition behind the number
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Share your reports with us. CION's haematology team will read your trend and explain what it means for your plan. One helpline serves every CION centre.