16793 Blood Volume Testing

Overview

Blood volume testing is a group of special blood tests that measure how much blood is in your body. It can also measure the two main parts of blood separately: the red blood cells (the cells that carry oxygen) and the plasma (the pale, watery liquid that carries the cells around). A normal blood count tells your doctor how concentrated your blood is, but it cannot tell how much blood there is in total. Blood volume testing fills in that gap. It is usually done when a routine blood test shows a high red blood cell count and your doctor needs to know whether your body is truly making too many red cells, or whether the blood is simply more concentrated because there is less plasma than normal. It is not a test you would have as part of a routine check-up.

No, it is not a common test. Only specialist laboratories offer it, and it is usually arranged by a hospital specialist such as a haematologist (a doctor who specialises in blood conditions) rather than in a routine GP appointment. Most people who have a high red blood cell count never need it.

It can be done at any age, but it is most often used in adults who are being checked for a high red blood cell count. It is occasionally used in children, usually in a specialist children's blood service. The test is not affected by sex or ethnicity, though the normal ranges do differ between men and women.

Symptoms

Common symptoms

• There are no symptoms from the test itself — any symptoms come from the reason the test is being done.
• You may have been offered the test because a blood count showed a high red blood cell count, sometimes with headaches, a flushed red face, dizziness, blurred vision, ringing in the ears, or itchy skin after a hot bath or shower.
• Some people are investigated for the opposite problem — low red cells (anaemia) — with tiredness, breathlessness on exertion, pale skin, or feeling lightheaded.
• Some people have no symptoms at all and are only found through a routine blood test.

Symptoms in children

• Children rarely need this test, and when they do it is usually arranged by a children's blood specialist.
• A parent or carer can normally stay with the child throughout.
• Only very small blood samples are needed, and the team will explain how they keep your child comfortable.
• Cream to numb the skin can be used before a needle is inserted, if your child is anxious about needles.

Symptoms in older adults

• Older adults are more likely to be investigated for a raised red blood cell count, because some related conditions become more common with age.
• Thin or fragile veins may make taking blood samples a little harder, so the team may take extra time or use a warm pack to help.
• Being well hydrated before the appointment helps the samples flow more easily.
• If you take medicines that thin the blood or affect bleeding, tell the team beforehand — this does not usually stop the test but it is helpful to know.

Causes

Main causes

• A high red blood cell count on a routine blood test — the test helps show whether the body is truly making too many red cells (sometimes called polycythaemia vera or a similar condition) or whether the blood is simply more concentrated.
• Blood that is more concentrated because there is less plasma than normal — this can happen with dehydration, smoking, stress, or being at high altitude, and is sometimes called relative or apparent polycythaemia.
• Unexplained anaemia where standard tests do not give a clear answer.
• Suspected hidden or slow blood loss that is not showing up on other tests.
• Checking blood volume before or during certain treatments, or as part of specialist research into how the body controls blood volume.

Risk factors

• Smoking, which reduces the amount of plasma and makes blood more concentrated.
• Living at high altitude, where the body makes more red cells to cope with less oxygen.
• Being dehydrated, including after illness, vomiting or diarrhoea.
• Untreated sleep apnoea (repeated pauses in breathing during sleep).
• Some kidney and liver conditions, and some rare inherited blood conditions that raise red cell production.
• Use of medicines or supplements that increase red cell production, such as certain hormones used in sport — this should always be discussed openly with your doctor.

When to see a doctor

See a doctor urgently if:

• You develop chest pain, severe breathlessness, fainting, or signs of a stroke — call your local emergency number straight away.
• New swelling or pain in one leg, or unusual bleeding or bruising.
• You feel suddenly much more unwell after any procedure, with a high temperature or chills.

Book a routine appointment if:

• You have been told your red blood cell count is high and want to understand what it means.
• You have ongoing tiredness, breathlessness or headaches that are not settling.
• You are waiting for blood volume test results and are unsure what happens next.
• You have questions about preparing for the test, such as whether to eat or take your usual medicines.

Diagnosis

Blood volume testing is not a diagnosis in itself — it is one piece of the picture. Your doctor will put the result together with your symptoms, your other blood tests, your medical history and your lifestyle to work out what is going on. The test itself is done in a specialist laboratory, usually on a single visit of about one to two hours. A small amount of your blood is taken and mixed with a tiny amount of a tracer (a safe, measurable marker). This is put back into your vein, and a few small blood samples are taken at set times over the next hour or so. The laboratory measures how diluted the tracer becomes, which tells them how much blood, red cells and plasma you have.

Tests that may be done

• Full blood count and blood film, to look at the number and shape of your blood cells.
• Reticulocyte count, which shows how quickly your body is making new red cells.
• Kidney and liver function tests.
• A test of the hormone that tells the body to make red cells (erythropoietin).
• Oxygen levels in your blood, measured with a small clip on your finger.
• Genetic tests on your blood, done in some cases, to look for inherited causes.
• An ultrasound scan of your abdomen to check the size of your spleen, in some cases.
• The blood volume test itself, using a small, carefully measured amount of tracer.

What to expect at your appointment

You will usually be asked to eat and drink normally, but do follow any specific instructions the laboratory gives you. Wear loose sleeves so the team can reach your arm easily. A small plastic tube may be placed in a vein in your arm so that samples can be taken without repeated needles. You may feel a brief sting when the tube is inserted, and a cold feeling when the tracer is given. You then rest while the timed samples are taken. You can normally go home straight afterwards and drive yourself if you feel well. The tracer leaves your body naturally over a few days. Your results are usually sent to the doctor who arranged the test and are ready within days to a couple of weeks. Let the team know if you are pregnant or breastfeeding, or might be.

Treatment

The test itself needs no treatment. What happens next depends on what the results show. If your blood volume and red cell mass are normal, you may need no further action beyond a repeat blood count in a few months. If the blood is more concentrated because of dehydration or smoking, simple changes may be enough. If the test shows a true increase in red cells, your specialist will explain the likely cause and discuss options to bring your red cell count down and reduce the risk of blood clots.

Self-care at home

• Drink plenty of water in the days before the test and on the day itself, unless you have been told to limit fluids for another reason.
• Keep a simple note of your symptoms, and when they started, to share with your doctor.
• Be open about smoking, any supplements or hormones you use, and any sleep problems — this genuinely helps the team reach the right answer.
• Ask for a copy of your results and a clear explanation of what they mean for you.
• If you are anxious while waiting, plan something calming for the day of the appointment and ask a friend or family member to come with you.

Medical treatments

There is no medicine given as part of a blood volume test. Treatment is aimed at whatever is found. If the blood is more concentrated than it should be, your doctor will first look at the cause — for example, treating dehydration, helping you stop smoking, or testing for and treating sleep apnoea. If the red cell mass is genuinely too high, a specialist may suggest regularly removing a small amount of blood (sometimes called venesection or bloodletting) to bring the count down, or medicines that reduce red cell production. A specialist may also discuss medicines that lower the risk of blood clots. All of these decisions are individual, and your specialist will explain the benefits and drawbacks for you. Never start, stop or change any medicine without talking to your doctor or pharmacist first.

When is surgery considered?

Surgery is not part of blood volume testing and is rarely needed. In a small number of people with a specific blood condition, a specialist may discuss an operation to remove the spleen, but this is uncommon and would always be a decision made together with your specialist team.

Living with this condition

Most people carry on with their normal life straight after the test. While you wait for results, it helps to keep things in perspective: many raised red cell counts have a simple explanation. If you do have a condition that needs monitoring, you will usually have regular blood tests and check-ups, which most people fit around work and family life without much disruption.

Lifestyle tips

• Stay well hydrated, especially in hot weather or during exercise.
• If you smoke, stopping is one of the most effective things you can do to bring a raised red cell count down — your GP or pharmacist can point you to local stop-smoking support.
• Keep moving — regular moderate activity helps your circulation and general wellbeing.
• Watch your alcohol intake and keep to recommended limits.
• Tell any doctor, dentist or nurse treating you that you have had blood volume testing, and mention any blood condition you are being checked for.
• Avoid long periods of sitting still without moving your legs, particularly on long journeys, if your specialist has told you that your clot risk is raised.

Diet and exercise

No special diet is needed for this test. A balanced diet with plenty of fluids supports healthy blood. Regular activity — anything from walking to swimming — is good for your heart and circulation. If your red cell count is high, your specialist may give you specific advice about exercise and hydration, so follow that guidance rather than pushing through very strenuous activity when you feel unwell.

Mental health and emotional wellbeing

Being investigated for a blood problem can be worrying, and it is very normal to feel anxious while you wait for results or search things online. Try to stick to trusted sources and to write down your questions for your doctor rather than reading late at night. If worry is affecting your sleep, mood or appetite, tell your GP — talking therapies and support groups are available and can really help. If you ever feel overwhelmed or have thoughts of harming yourself, contact your local emergency number or a crisis support service straight away, and reach out to someone you trust.

Prevention

You cannot prevent the need for a blood volume test, and there is nothing you could have done differently — it is simply a way of getting a clearer answer. What you can do is reduce the chance of blood that is more concentrated than normal: stay well hydrated, avoid smoking, get sleep apnoea checked if you snore heavily or feel very sleepy in the day, and tell your doctor about any hormones or supplements you take.

Screening programmes

Blood volume testing is not a screening test and is not offered routinely. If you are having regular blood counts for another reason and a high red cell count is found, your doctor will decide whether further testing such as this is worthwhile.

Complications

If left untreated

• If a truly high red cell mass is not investigated, the blood can become thick, which raises the risk of clots such as a stroke, heart attack or a clot in the leg or lung.
• Some underlying blood conditions can slowly get worse over time if they are not monitored.
• Unexplained anaemia that is not looked into may mean a cause such as slow blood loss is missed.
• Living with an unexplained result can cause ongoing worry — getting an answer usually helps.

Long-term outlook

The outlook is generally good. For many people, the test shows that blood volume is normal or only mildly raised, and the explanation is something simple and manageable such as dehydration or smoking. When a genuine blood condition is found, it can usually be monitored and treated, and most people live full, active lives with regular check-ups. Having the test is a positive step: it replaces uncertainty with information your doctor can act on.

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