
A hematocrit test (HCT) measures the percentage of red blood cells (RBCs) in your blood, offering essential insight into your overall blood composition and oxygen-carrying capacity. Red blood cells are critical for transporting oxygen from your lungs to the rest of your body, and this test helps quantify how much of your blood volume is made up of these oxygen-delivering cells.
Elevated hematocrit levels (typically above 44% to 50%) may point to dehydration or chronic health conditions such as heart disease or lung disorders that cause an increase in RBC production. Conversely, low hematocrit levels may indicate anemia or issues that impair red blood cell production (erythropoiesis) or lead to increased destruction (hemolysis).
Read on to explore the causes, symptoms, and treatments associated with both high and low hematocrit test results.
Pro Tip: Hematocrit and other parts of the complete blood count (CBC) reveal essential information about your health. If you wish to learn more about what your blood test means, book a virtual consultation to speak to a healthcare provider.
High HCT Levels Meaning
A hematocrit test result is considered high when the percentage of red blood cells in your blood exceeds the standard range. For adult men, normal hematocrit values fall between 41% and 50%, while women typically range from 36% to 44%. When your results exceed these values, it means your blood has an unusually high concentration of RBCs.
Symptoms of high hematocrit(1) include:
- Fatigue
- Dizziness
- Headaches
- Blurry or impaired vision
- Skin flushing or facial redness
Causes of High HCT
A high hematocrit test result may signal an underlying health condition that increases red blood cell volume or decreases plasma. These causes vary widely, from lifestyle-related dehydration to serious diseases.
Dehydration
One of the most frequent reasons for an elevated HCT level is dehydration(2). When dehydrated, your body loses fluids–often through sweating, vomiting, or diarrhea. This fluid loss reduces the volume of plasma. Since the number of RBCs stays the same, they become more concentrated within the smaller plasma volume. This leads to a higher hematocrit reading.
Lung Disease
Diseases that limit lung efficiency, such as chronic obstructive pulmonary disease (COPD), restrict oxygen absorption. As a compensatory mechanism, your body increases RBC production to improve oxygen delivery, resulting in a higher hematocrit. A hematocrit test, alongside lung function tests, can provide diagnostic clues.
Your body will produce more RBCs to compensate for the oxygen shortage. If you have symptoms of pulmonary disorders, your doctor may order additional laboratory tests, including lung function blood tests.
Heart Disease
People with certain types of heart disease, such as congestive heart failure and congenital heart defects, are often prone to high hematocrit and hemoglobin levels. When the heart cannot pump blood effectively, the body may try to compensate by increasing the production of RBCs (erythrocytes).
As the heart cannot fully function to facilitate blood circulation, your body will counteract the loss with increased erythrocyte production. Thus, an HCT blood test also provides additional information about your risk for cardiovascular diseases, making it a key part of heart health blood tests.
Cancers
Certain cancers, like polycythemia vera(3), cause your bone marrow to overproduce red blood cells. This rare blood cancer increases blood viscosity and leads to complications like clotting and spleen enlargement. Kidney cancer may also raise hematocrit by producing more erythropoietin (EPO), the hormone that stimulates red blood cell creation.
On the other hand, kidney cancer also influences erythrocyte production since it releases the protein (erythropoietin) that prompts the process. Additional tests, such as a red blood cell count, biopsy, MRI, etc can further confirm this.
Did You Know? Tumor biomarkers like PSA (for prostate cancer) or CA-125 (for ovarian cancer) can be detected in the blood before physical symptoms appear. In some cases, elevated hematocrit during routine exams may trigger further cancer screening.
In some cases, these cancers are first spotted through routine exams. Learn more about the types of cancer diagnosed through blood testing.
Treatment for Elevated HCT
Managing elevated hematocrit test results depends entirely on identifying the underlying condition responsible for the increase in red blood cell concentration. Once the cause is determined, treatment can begin.
For example, if the elevated hematocrit is linked to dehydration, your provider may recommend drinking more fluids, consuming oral rehydration solutions (similar to those used by athletes or for treating gastrointestinal fluid loss), or receiving intravenous (IV) fluids in more severe cases. Rehydrating restores blood plasma volume, helping bring the hematocrit level back into a healthy range.
If your hematocrit test reveals a result that is significantly above normal due to overproduction of RBCs, therapeutic phlebotomy may be recommended(4). This outpatient procedure involves drawing blood from a vein, much like a blood donation, to lower the concentration of red blood cells and reduce blood viscosity.
It’s important to recognize that lowering the hematocrit value only addresses the symptom. Identifying and treating the root cause—such as a chronic illness, cancer, or heart or lung condition—is necessary for long-term improvement. Your healthcare provider may recommend medications, dietary changes, or lifestyle adjustments depending on your overall diagnosis.
Low HCT Levels Meaning
If your hematocrit test result is lower than normal (typically less than 41% for men and 36% for women), it indicates that your body may be producing fewer red blood cells or destroying them at a faster rate. This could be potentially due to an underlying illness.
The most common cause of low hematocrit is anemia, characterized by a reduced number of healthy RBCs. You may spot anemia through the following symptoms:
- Fatigue
- Dizziness
- Chest pain
- Headaches
- Pale complexion
- Trouble breathing
- Cold hands and feet
- Irregular heartbeat (arrhythmia)
Causes of Low HCT
The following are some common culprits behind a low HCT, ranging from nutritional deficiencies and blood loss to underlying medical conditions. Understanding these causes is crucial for proper diagnosis and treatment, paving the way to restoring your red blood cell levels and overall health.
Anemias
As mentioned above, low HCT often points to anemia, a condition where your body doesn’t have enough healthy RBCs to carry adequate oxygen to your tissues. There are many different types of anemia, each with varying causes:
- Iron-deficiency anemia: This is the most common type of anemia, often caused by blood loss (like heavy periods or internal bleeding) or insufficient iron intake in your diet. Your body needs iron to produce hemoglobin, the protein in RBCs that carries oxygen.
- Vitamin-deficiency anemia: A lack of vitamin B12 or folate, which are essential for red blood cell production, can also lead to anemia.
- Anemia caused by chronic disease: Long-term health conditions, like kidney disease and rheumatoid arthritis, can disrupt the body’s ability to produce red blood cells, potentially resulting in anemia.
- Genetic conditions: Inherited disorders like sickle cell anemia (where RBCs are abnormally shaped) and thalassemia (where the body produces an abnormal form of hemoglobin) can also cause anemia. Additional blood tests are needed to verify the condition, including the anemia profile blood test.
- Aplastic anemia: This rare condition occurs when your bone marrow doesn’t produce enough new blood cells, including RBCs.
Identifying the specific type of anemia allows your doctor to recommend the most appropriate treatment to address the underlying cause. Essentially, this will help restore your red blood cell count to a healthy level.
H. pylori infection
Those who suffer from bacterial ulcers caused by H. pylori may have decreased HCT in their blood test. H. pylori infection can lead to the formation of sores in the stomach lining, causing irritation and inflammation that may result in bleeding. This chronic blood loss, even if small, can gradually deplete the body’s red blood cell count, leading to lower HCT levels.
Did You Know? It’s possible to experience blood loss without seeing blood in your stool, as the naked eye does not easily detect it. Get the fecal occult blood test to determine if you have chronic digestive bleeding.
Kidney Problems
While their primary role is filtering waste products from the blood, your kidneys also have a crucial hand in producing RBCs. This is done through a hormone called erythropoietin (EPO)(5), which acts like a messenger signal. EPO travels from the kidneys to the bone marrow, where it stimulates the production of RBCs.
When your kidneys are damaged, such as in chronic kidney disease (CKD), they can’t produce enough EPO. This shortage leads to a decrease in RBC production.
Inflammatory Diseases
Chronic inflammatory conditions, such as lupus and rheumatoid arthritis, can significantly impact red blood cell production. This is because inflammation disrupts the intricate process of RBC formation and maturation in several ways:
- Suppressed bone marrow activity: Inflammation can suppress the activity of bone marrow, the primary site of blood cell production. This leads to a decrease in the number of RBCs produced.
- Reduced erythropoietin production: Inflammation often reduces the production of erythropoietin, a hormone that stimulates red blood cell production.
- Iron dysregulation: Inflammatory conditions can interfere with iron metabolism, making it less available for hemoglobin synthesis, an essential component of RBCs.
Similarly, inflammatory bowel disease (IBD)(6) can contribute to anemia and low hematocrit by impairing the absorption of essential nutrients in the digestive tract. These nutrients, including iron, copper, vitamin B12, and folate, are crucial for the production of RBCs.
Blood Cancers
Leukemia and lymphoma are types of blood cancer that disrupt the normal function of bone marrow, the spongy tissue inside your bones where blood cells are produced. This disruption can significantly impact your hematocrit (HCT) levels.
- Leukemia: This cancer causes an overproduction of abnormal white blood cells in the bone marrow. These abnormal cells crowd out the production of RBCs, leading to a low HCT.
- Lymphoma: While primarily affecting the lymphatic system, lymphoma can also spread to the bone marrow. When this happens, it can interfere with blood cell production, potentially resulting in a low HCT.
Treatment for Low HCT
Diagnosing and treating these complex conditions that lower HCT requires a comprehensive approach. Your healthcare provider will carefully evaluate your medical history, conduct a thorough physical examination, and analyze the results of various tests to arrive at an accurate diagnosis.
Once the underlying cause is identified, your doctor will create a tailored treatment plan to address the specific condition and ultimately restore your HCT to a healthy range. This might involve medications, transfusions, or other therapies depending on the nature of the disorder.
If low HCT is due to iron-deficiency anemia, your doctor may recommend supplements and certain lifestyle adjustments. Learn more about iron deficiency anemia and its physical impact, especially among women.
On the other hand, if the condition is linked to more complex disorders like cancer, blood disease, CKD, etc., your healthcare provider will need more information to form an accurate diagnosis, entailing other types of assessment. Only then will your doctor prescribe a treatment that covers the process of bringing HCT back to its normal range.
How Is Hematocrit Measured?
Calculating hematocrit involves measuring the ratio of red blood cells through a reading device. One traditional method involves using a capillary tube, a thin glass tube, to collect a small amount of blood. This tube is then spun in a centrifuge, a machine that spins rapidly, separating the blood components based on their density. Since red blood cells are the densest component of blood, they settle to the bottom of the tube when a blood sample is spun in a centrifuge.
After centrifugation, the tube displays distinct layers: a packed column of RBCs at the bottom, a thin layer of white blood cells and platelets (the “buffy coat(7)“) in the middle, and plasma, the liquid portion of blood, at the top.
To calculate the hematocrit, a special reading device is used to measure the length of the packed red blood cell column and compare it to the total length of the blood sample in the tube. This ratio, expressed as a percentage, represents the hematocrit value.
The timing of this analysis is critical. Ideally, the hematocrit should be measured within 10 minutes of collecting the blood sample. This urgency stems from the fact that over time, the neatly separated layers within the capillary tube can start to mix.
How and When Is It Done?
Your doctor will likely order a hematocrit blood test if you exhibit symptoms of anemia, blood loss, dehydration, and other blood disorders. As part of the CBC test, it also provides critical information on other disorders, such as cancer, lung disease, kidney disease, and cardiovascular diseases.
Unlike the other tests involved in CBC, an HCT blood test will require a venous blood sample. This is drawn from the veins at the back of your elbow and should only be performed by a licensed phlebotomist. HCT measurement needs more blood volume for accurate processing and analysis.
An HCT blood test alone does not require fasting or other special preparations prior to blood collection. But if it is part of a more extensive test or you’re taking another test simultaneously, you may have to abstain from consuming food, drinks, and certain medications.
Pro Tip: Venipuncture, the process of inserting a syringe needle into your vein to collect blood, can be slightly uncomfortable. If you are anxious about your lab test appointment, read our tips on calming yourself before getting blood drawn.
Frequently Asked Questions
What level of HCT is concerning?
Healthy hematocrit levels typically fall between 41% and 50% for men and 36% and 44% for women. HCT test values above or below these ranges signal problems in the blood, ranging from anemia to blood loss for low HCT percentage and dehydration to blood cancer for high lab values. Either way, see your doctor for a proper diagnosis and additional lab tests.
Can high HCT cause fatigue?
Elevated hematocrit (HCT) can lead to persistent fatigue because the increased concentration of red blood cells thickens the blood, making it harder for the heart to pump and slowing the delivery of oxygen throughout the body. This reduced blood flow can leave you feeling tired and sluggish, as your organs and tissues, including the brain, receive less oxygen than they need to function optimally.
Does low HCT mean dehydration?
A low hematocrit (HCT) can indicate conditions like anemia, where there are fewer red blood cells, or certain blood disorders that affect red blood cell production. Conversely, a high HCT often suggests dehydration, as a reduced fluid volume in the body leads to a higher concentration of RBCs. This is because the same number of RBCs occupy a smaller proportion of the total blood volume when plasma levels are low.
Did You Know? Aside from an HCT blood test, other ways to detect dehydration include a urinalysis and the electrolyte blood test panel. Severe dehydration treatment includes IV fluids and oral rehydration solution (ORS).
The Bottom Line
An HCT blood test is a valuable tool for detecting a range of health conditions, particularly those affecting red blood cell production, such as anemia or polycythemia. However, HCT results alone are not a definitive diagnosis and require careful interpretation by a healthcare professional. Misinterpreting these results can lead to unnecessary anxiety or a false sense of security, highlighting the importance of discussing them with your doctor, either during an in-person or virtual consultation.
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