Blood Volume Testing
What is blood volume testing?
A blood volume test measures the amount of blood in your body. It is a type of nuclear medicine test. Nuclear medicine tests use a small amount of a radioactive substance to study how your body is functioning.
What are the components of blood?
Your blood contains:
- Plasma: The liquid component of your blood. Over 90% of plasma is water.
- Platelets: Cells that help with blood clotting.
- Red blood cells: Round-shaped cells that carry oxygen to your body tissues.
- White blood cells: Immune system cells that help protect your body against infections.
Platelets and white blood cells are very important but they make up only a small fraction — less than 1% — of your blood volume. Blood volume tests measure primarily plasma and red blood cells. A blood volume test is also called a plasma volume test or red cell mass test.
How does your body control blood volume?
Your body has complex mechanisms to regulate blood volume. It continuously produces and destroys red blood cells. Your bone marrow (the soft tissue inside your bones) produces red blood cells and your liver destroys them. This process is relatively slow. When you lose blood due to a health problem, accident or injury, it can take days or months to generate new red blood cells.
The regulation of plasma, which is mostly water, is a much faster process compared to the replacement of red blood cells. Fluid enters your body mainly through the food and water you consume. You lose fluids through urine and feces and when you sweat and breathe. When you are healthy, your kidneys maintain a constant balance of fluid.
What factors affect blood volume?
Conditions that can lead to an increase in blood volume include:
- Congestive heart failure.
- Excessive sodium intake.
- Kidney conditions such as kidney failure and nephrotic syndrome.
- Liver failure.
Symptoms of hypervolemia may include:
- Abdominal swelling and discomfort (ascites).
- Congestive heart failure.
- High blood pressure.
- Shortness of breath due to fluid buildup in the lungs.
- Swelling (edema), usually of the feet, ankles, wrists or face.
The main causes of low blood volume are:
When your blood volume drops, hypovolemic shock can occur. This occurs when your body compensates for reduced blood flow by:
- Decreasing blood pressure.
- Increasing heart rate.
- Increasing breathing rate.
Continued blood volume loss can eventually cause irreversible damage to your internal organs and brain.
Does blood volume affect blood pressure?
The pressure inside your blood vessels allows blood to reach all your tissues and cells. When blood volume drops, blood pressure also drops. Your body responds to this by narrowing your blood vessels to help restore blood flow. In severe hypovolemia, your body is unable to compensate fully.
When blood volume increases, it expands your arteries and veins and leads to increased blood pressure. Hypervolemia usually occurs because your body is not able to regulate fluids properly due to impaired kidney or liver function.
When would blood volume testing be needed?
Healthcare providers may use this test during critical care of people with:
- Blood loss.
- Heart failure.
- Kidney or liver failure.
- Serious burns.
Other reasons providers might use blood volume testing include:
- Assessing anemia.
- Determining the cause of fainting (syncope).
- Evaluating high blood pressure or low blood pressure (orthostatic hypotension).
- Presurgical screening.
Where is blood volume testing performed?
You may receive this test at your bedside in the hospital or in the nuclear medicine department of a hospital.
How does blood volume testing work?
To measure the volume of blood in your body, first you receive a specific volume of a radioactive iodine tracer injected into one of your veins. Radioactive tracers allow providers to track certain bodily functions. After the tracer mixes completely with your blood, your provider takes a blood sample to measure the amount of radioactivity in your blood.
Calculation of your blood volume uses the mathematical equation C₁V₁ = C₂V₂, where:
- C₁ is the concentration of radioactivity in the tracer.
- V₁ is the volume of radioactive tracer that was injected.
- C₂ is the concentration of radioactivity in your blood after the tracer is fully distributed in your body.
- V₂ is the volume of blood in your body.
To determine if this blood volume measurement is too high or too low, your provider must compare it to your normal estimated blood volume. Blood volume varies widely from person to person. Your normal estimated blood volume is determined by a calculation based on your height, weight and sex.
How do I prepare for blood volume testing?
Your provider will give you instructions to help you prepare. In general, do not eat or drink anything a few hours before the test.
On the day of the test, you should avoid caffeinated beverages. Caffeine is a diuretic, which increases urine production and draws fluid out of your body. Some medications also contain caffeine. Ask your provider if you need to adjust your medications to prepare for your test.
What happens before a blood volume test?
The technologist will measure your height and weight and establish access to a vein with an intravenous (IV) line. This involves placing a needle in a vein in your arm, wrist or hand.
What happens during a blood volume test?
During the blood volume test:
- You lie still on an exam table.
- A technologist injects a radioactive tracer called iodine-131 human serum albumin (HSA) through the IV.
- You wait about 12 minutes to allow the radioactive tracer to mix with your blood.
- The technologist draws several blood samples about six minutes apart and sends them to the lab for analysis.
- The test usually takes less than 90 minutes.
What should I expect after a blood volume test?
The technologist will remove the IV and you can go home. Some of the radioactive tracer leaves your body through your urine, and the rest breaks down naturally. You should drink extra fluids to help flush the tracer from your body.
What are the risks of blood volume testing?
You should not have this test if you are pregnant or breastfeeding. Your provider may order a pregnancy test before the procedure to make sure you are not pregnant.
There is also a risk of allergic reaction. Let your provider know if you are allergic to iodine, shellfish or eggs. If you do have an allergy, your provider can modify the test to prevent a reaction.
The radioactive iodine tracer can damage your thyroid. To prevent this, your provider will give you an iodine pill prior to the test. Your thyroid absorbs this iodine, which blocks the uptake of the radioactive iodine.
Results and Follow-Up
What type of results do you get and what do the results mean?
The results will show if your blood volume is too low, too high or normal. Your provider can use this information to assess your condition and guide treatment.
A note from Cleveland Clinic
Blood volume testing measures whether the amount of blood in your body is too high or too low. Changes in blood volume are associated with many types of conditions and can lead to severe complications. Blood volume testing occurs most often in people who are in intensive care. It can help guide treatment to bring fluid levels back to normal and promote recovery.
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