For many people living with Postural Orthostatic Tachycardia Syndrome (POTS), symptoms like dizziness, fatigue, or feeling faint while standing can seem mysterious and unpredictable. But behind these symptoms, there’s often an important piece of the puzzle: blood volume.
Blood volume testing helps patients and doctors understand how much blood is circulating in the body — and in people with POTS, this number can provide key insights into treatment and recovery. In this post, we’ll explain what blood volume testing is, why it matters for POTS patients, and how you can get tested safely and effectively.
What Is Blood Volume Testing?
Blood volume testing is a medical procedure used to measure the total amount of blood in your body. This includes both plasma (the liquid part of blood) and red blood cells (which carry oxygen). The test is available only in hospitals with nuclear medicine or autonomic testing facilities.
In simple terms, it tells doctors whether your body has too much, too little, or just the right amount of blood to function properly.
For most healthy adults, the average blood volume is about 70 milliliters per kilogram of body weight. But for people with POTS and other forms of dysautonomia, this number can be much lower than expected — a condition known as hypovolemia (low blood volume).
Why Blood Volume Matters in POTS
Your autonomic nervous system (ANS) controls automatic functions like heart rate, blood pressure, and circulation. In POTS, the ANS doesn’t regulate these properly, leading to problems when standing up or moving suddenly.
Low blood volume can make this worse. When you stand, gravity pulls blood into your legs and lower body. Normally, your ANS adjusts to push blood back to your heart and brain. But if your total blood volume is already low, your heart has to pump faster to keep up — causing the rapid heartbeat and lightheadedness typical of POTS.
How Low Blood Volume Affects POTS Symptoms
- Fatigue: Less blood means less oxygen reaching muscles and tissues.
- Dizziness or Fainting: Blood may not reach the brain quickly enough when you stand.
- Brain Fog: Reduced blood flow can cause cognitive symptoms like poor focus.
- Cold Hands and Feet: Circulation slows as your body tries to preserve blood flow to vital organs.
By identifying low blood volume, doctors can customize treatment — like increasing salt intake, adjusting medications, or recommending IV fluids — to help restore balance and improve symptoms.
Types of Blood Volume Tests
There are a few ways to measure blood volume. Some are more precise than others, and your doctor will choose the right one depending on your medical history and available resources.
1. Nuclear Medicine Blood Volume Test
This is the gold standard for accuracy. A small amount of a safe radioactive tracer is injected into your bloodstream. Then, a specialized machine measures how the tracer moves through your blood, calculating your total blood, plasma, and red cell volume.
It’s painless, precise, and can clearly show if your blood volume is normal, low, or high.
2. Plasma Volume Test (Dye Dilution Method)
A dye (such as Evans Blue or I-131 albumin) is injected into your veins, and blood samples are taken at intervals. The concentration of dye in the blood helps estimate your plasma volume.
This test is less common but still reliable in certain settings.
3. Estimated or Indirect Methods
Doctors sometimes estimate blood volume based on hematocrit (the percentage of red blood cells in blood) and body weight. While convenient, these estimates are less accurate and mainly used when other testing options aren’t available.
What to Expect During a Blood Volume Test
If your doctor recommends a nuclear medicine blood volume test, here’s what the process generally looks like:
Before the Test
- You’ll be asked to avoid caffeine, nicotine, and certain medications before the procedure.
- Wear comfortable clothes and stay hydrated unless told otherwise.
During the Test
- A small amount of tracer or dye is injected into your arm.
- Over the next 30–60 minutes, a technologist draws blood samples or scans your circulation using a specialized camera.
- You’ll be asked to remain seated or lying down during the process.
After the Test
- You can resume normal activities right away.
- Your doctor will analyze the results to determine whether your blood, plasma, or red cell volumes are low.
Interpreting the Results
Your results will show whether your blood volume is:
- Normal (within 5–10% of expected range)
- Mildly Reduced (10–15% below normal)
- Significantly Reduced (more than 15–20% below normal)
Many POTS patients have moderate to severe hypovolemia — sometimes 20–30% below normal levels. This insight helps physicians tailor care plans.
For example:
- If plasma volume is low, saline infusions or oral hydration may help.
- If red blood cell volume is low, treatments that boost blood production (like fludrocortisone or EPO in rare cases) might be considered.
- If both are low, a combination of salt, fluids, and medications may be used.
How to Get a Blood Volume Test
Not every clinic performs this specialized test. Here’s how you can access it safely:
- Ask Your POTS Specialist or Cardiologist:
They can refer you to a nuclear medicine department or an autonomic testing center that offers the procedure. - Find a Dysautonomia-Aware Clinic:
Some specialized centers (like Diekman Dysautonomia or major academic hospitals) understand the nuances of POTS and can order and interpret results accurately. - Check Hospital Listings:
Search for hospitals offering “nuclear medicine blood volume studies” or “plasma volume testing.” - Insurance and Referrals:
Because the test involves nuclear imaging, prior authorization may be needed. Your doctor’s note explaining “orthostatic intolerance” or “hypovolemia evaluation” often helps.
What If You Can’t Access Testing?
While direct measurement is ideal, not every patient can easily get tested. In such cases, doctors may rely on clinical signs and response to therapy instead.
If you show improvement with increased salt intake, fluids, or IV saline, it may indirectly indicate that low blood volume is a contributing factor.
At-Home Supportive Measures
- Increase Fluid Intake: Aim for 2–3 liters of water daily.
- Add Salt (If Approved): Around 3–10 grams per day can help retain fluids.
- Use Compression Garments: They improve blood return from legs to the heart.
- Avoid Long Standing or Heat Exposure: Both can worsen blood pooling.
- Monitor Blood Pressure & Heart Rate: Keep a daily log for your doctor.
Blood Volume Testing and Long COVID
Many patients with Long COVID experience symptoms similar to POTS — including fatigue, tachycardia, and brain fog. Studies suggest that blood volume irregularities might play a role in both conditions.
Testing can help distinguish whether symptoms are caused by autonomic dysfunction, inflammation, or blood pooling issues — allowing for more precise treatment strategies.
Why Early Testing Can Change the Course of Care
For patients with chronic symptoms, blood volume testing isn’t just a diagnostic step — it’s a foundation for better treatment. By understanding your body’s specific imbalances, your medical team can:
- Identify whether low blood volume contributes to your POTS symptoms.
- Avoid unnecessary medications that may not address the root issue.
- Design a more personalized, effective care plan.
When patients and doctors understand the “why” behind the symptoms, managing POTS becomes far less overwhelming — and far more hopeful.
Final Thoughts
Blood volume testing might sound complex, but it provides crucial insight into the body’s inner balance — especially for people living with POTS.
If you often feel lightheaded, exhausted, or unable to tolerate standing, talk to your doctor about whether testing could help. Understanding your blood volume can be the missing piece that transforms your treatment journey — turning confusion into clarity and discomfort into progress.
Remember: You deserve care that listens, tests, and truly understands.
