Hematocrit (HCT) and hemoglobin (Hb) are two blood tests used to detect abnormalities in your red blood cells (RBCs). Hematocrit is the percentage of RBCs in your blood, while hemoglobin is a protein in RBCs that carries oxygen to cells. The HCT and Hb values are reported as part of a complete blood count (CBC).
If hematocrit and hemoglobin levels are below the normal range, a person is said to have anemia. Causes for low hematocrit and hemoglobin include malnutrition, bleeding, chronic kidney disease, liver disease, bone marrow disorders, and blood cancers like leukemia or lymphoma.
Learn more about hematocrit and hemoglobin and what high or low values say about your health.
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What Is Hemoglobin?
Hemoglobin is an iron-containing protein in RBCs that carries oxygen and carbon dioxide. It also gives blood its red color.
Hemoglobin is essential for transporting oxygen from your lungs to your tissues and cells. As oxygen reacts with glucose (blood sugar) inside of cells, it produces carbon dioxide as a waste product that hemoglobin carries back to the lungs to be expelled.
Without hemoglobin to enable cellular respiration, tissues and cells of the body could not survive.
Hemoglobin is formed in the bone marrow as red blood cells are produced. Iron is needed to create hemoglobin, so low iron levels in the body can lead to low Hb counts.
How Hemoglobin (Hgb) Blood Tests Work
What Is Hematocrit?
Hematocrit is the proportion of blood composed of RBCs, reported as a percentage (%). In addition to RBCs, blood is comprised of plasma (the liquid part), white blood cells (WBCs), and platelets.
While plasma accounts for around 55% of the total volume of blood, RBCs account for the largest share of blood cells.
Because the purpose of RBCs is to transfer oxygen from the lungs to tissues, the hematocrit level is essential for determining whether there are enough RBCs to meet the body's demand for oxygen.
Understanding the Hematocrit Test
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How Hemoglobin and Hematocrit Are Measured
Hemoglobin and hematocrit are among of battery of tests included in a complete blood count (CBC). Your healthcare provider might order a CBC if you have signs of an infection, anemia, blood loss, diseases of the immune system, or blood cancer.
The CBC requires a simple blood draw from a vein. Results can usually be delivered within a day of taking the test.
Hb Count
The Hb count is measured with a technology known as spectrophotometry. This is a highly sensitive tool that measures how much of a chemical substance—in this case, hemoglobin—is in a solution as a beam of light passes through it.
Because hemoglobin absorbs light differently than other components of blood, spectrophotometry can accurately measure the proportion of Hb in a small blood sample. The value can then be mathematically translated into grams of Hb per liter of blood (g/L).
HCT
A hematocrit test is also known as a packed cell volume (PCV) test. The test starts by spinning a test tube of whole blood at high speed, which causes the plasma and blood cells to sediment in layers in the tube.
The HCT is then calculated by dividing the total volume of packed red blood cells by the total volume of all blood cells and plasma.
Relationship of Hemoglobin and Hematocrit
Your hemoglobin and hematocrit are typically both normal or increase and decrease in tandem with the other. Sometimes hemoglobin may be low due to vitamin B12 deficiency or iron deficiency before the hematocrit becomes low.
Normal Ranges of Hb and HCT
There are established values for normal hemoglobin and hematocrit. The values differ by age and sex.
| Normal Values | Hemoglobin | Hematocrit |
|---|---|---|
| Males | 138 to 172 g/L | 40.7% to 50.3% |
| Females | 121 to 151 g/L | 36.1% to 44.3% |
| Infants | 95 to 130 g/L | 32% to 42% |
| Newborns | 140 to 240 g/L | 45% to 61% |
Low Hemoglobin and Hematocrit
Broadly speaking, there are three key reasons why your hemoglobin and hematocrit levels may be low:
Blood Loss
Blood loss from injury or illness can affect both your Hb and HCT levels. Examples include:
- Heavy menstrual bleeding
- Internal bleeding from a traumatic injury
- Internal bleeding from an ulcer or diverticulitis
- Gastrointestinal cancers, including colon cancer
Bone Marrow Dysfunction
You can also have a low Hb or HCT if your bone marrow isn’t producing enough red blood cells. This can occur with:
- Anaplastic anemia
- Iron deficiency anemia
- Vitamin deficiency anemia
- Chronic kidney disease (CKD)
- Cirrhosis
- Hypothyroidism
- Inflammatory bowel disease (IBD)
- Rheumatoid arthritis
- Lead poisoning
- Leukemia
- Lymphoma
- Multiple myeloma
- Myelodysplastic syndromes
- Osteonecrosis
Hemolysis
Hemolysis occurs when red blood cells are being broken faster than they can be replaced. This can lead to hemolytic anemia.
Causes include:
- Porphyria (a group of disorders caused by an overaccumulation of porphyrin which helps hemoglobin)
- Sickle cell anemia (an inherited disorder that causes red blood cells to become misshapen and break down)
- Splenomegaly (enlarged spleen)
- Thalassemia (an inherited blood disorderin which your body makes less hemoglobin)
HighHemoglobin and Hematocrit
There are two broad reasons why your hemoglobin and hematocrit levels may be rising, either chronically (over the long term) or acutely (over the short term):
Increased Need for Oxygen
When you are deprived of oxygen, your body will make more red blood cells to carry what oxygen it receives in the lungs to cells and tissues.
Causes of this include:
- Smoking
- Obstructive sleep apnea
- Exposure to high altitudes where oxygen levels are low
- Heart failure which decreases the output of blood from the heart
- Obstructive lung diseases like COPD that reduce the intake of oxygen into the lungs
- Degenerative lung diseases like cystic fibrosis that causes lung scarring
- Carbon monoxide poisoning
Bone Marrow Dysfunction
Some rare diseases and conditions cause the bone marrow to make too many RBCs, including:
- Polycythemia vera, a type of blood cancer
- Erythropoietin (EPO), a drug used to treat anemia in people with cancer
Dehydration can also cause your hematocrit levels to rise, although it doesn't actually increase the number of red blood cells. Instead, your hematocrit increases only because RBCs make up a larger proportion of the total blood volume.
High Hemoglobin Count
Summary
Hemoglobin and hematocrit are among a battery of tests included in a complete blood count (CBC). Hemoglobin is the iron-rich protein that carries oxygen to cells, while hematocrit is the percentage of red blood cells in relation to the total volume of blood.
Low hemoglobin and hematocrit may be due to bleeding, bone marrow diseases (including blood cancers), and hemolysis (the destruction of red blood cells). High hemoglobin and hematocrit levels occur with high altitude, heart or lung disease, or diseases like polycythemia vera that cause the overproduction of red blood cells.
10 Sources
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Karakochuk CD, Hess SY, Moorthy D, et al.Measurement and interpretation of hemoglobin concentration in clinical and field settings: a narrative review.Ann N Y Acad Sci. 2019;1450(1):126-146. doi:10.1111/nyas.14003
Institute for Quality and Efficiency in Health Care. What does blood do?
MedlinePlus.Hematocrit test.
MedlinePlus. Complete blood count (CBC).
Karakochuk CD, Hess SY, Moorthy D, et al. Measurement and interpretation of hemoglobin concentration in clinical and field settings: a narrative review. Ann N Y Acad Sci. 2019 Aug;1450(1):126-146. doi: 10.1111/nyas.14003
MedlinePlus. Hemoglobin.
American Society of Hematology. Anemia.
MedlinePlus. Hematocrit.
Schop A, Stout3en K, Riedl JA, et al. A new diagnostic work-up for defining anemia etiologies: a cohort study in patients ≥ 50 years in general practices. BMC Fam Pract. 2020;21:167. doi:10.1186/s12875-020-01241-7
National Heart, Lung and Blood Institute.Hemolytic anemia.
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By Heidi Moawad, MD
Dr. Moawad is a neurologist and expert in brain health. She regularly writes and edits health content for medical books and publications.
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