Hemoglobin (HB) Test: Meaning, Functions, Causes of Low & High Levels, and Complete Guide
What Is Hemoglobin (HB)?
Hemoglobin is an essential red-colored protein present inside red blood cells (RBCs). Its primary role is to carry oxygen from the lungs to all tissues and organs of the body. After delivering oxygen, it collects carbon dioxide and brings it back to the lungs so it can be removed when you exhale.
The hemoglobin (Hb) test measures the amount of this protein circulating in your blood. Because hemoglobin directly affects how well your body receives oxygen, this test is one of the most commonly ordered blood investigations in hospitals and diagnostic labs. When hemoglobin levels drop, the body struggles to receive enough oxygen, which leads to fatigue, weakness, and other complications. Similarly, excessively high hemoglobin can make the blood thicker, increasing the risk of serious problems like blood clots.
Where Is Hemoglobin Produced in the Body?
Hemoglobin is synthesized inside the bone marrow—the soft, spongy tissue found in bones such as the ribs, pelvis, spine, and long bones of the arms and legs. In the bone marrow, immature blood-forming cells called erythroblasts gradually develop into mature red blood cells. During this development, hemoglobin proteins are produced and packed inside the cells.
Once the RBCs mature, they are released into the bloodstream already filled with hemoglobin. Each red blood cell survives for about 120 days, which means the bone marrow must continuously create new RBCs to maintain healthy hemoglobin levels. Any condition that affects the bone marrow will automatically affect hemoglobin production.
Functions and Importance of Hemoglobin
Hemoglobin plays several vital roles that directly influence overall health. Some of its key functions include:
Oxygen Transportation
Hemoglobin binds with oxygen in the lungs and distributes it to organs, muscles, and tissues. This oxygen supply is crucial for energy, cell repair, and normal body functioning.
Carbon Dioxide Removal
After delivering oxygen, hemoglobin collects carbon dioxide—a waste product—from tissues and brings it back to the lungs to be exhaled. This process helps maintain a healthy internal environment.
Maintaining Blood pH Balance
Hemoglobin contributes to keeping the blood’s pH stable. Even slight imbalances in pH can affect body functions, so hemoglobin plays an indirect but important regulatory role.
Supporting Energy Production
Every cell in the body needs oxygen to produce energy. By transporting oxygen efficiently, hemoglobin helps the body stay active, focused, and energetic throughout the day.
When hemoglobin levels fall too low, tissues do not receive enough oxygen, leading to chronic tiredness and reduced physical performance. When levels rise too high, the blood becomes thick, increasing pressure on the heart and raising the risk of clot formation.
Causes of Low Hemoglobin (Anemia)
Low hemoglobin is commonly referred to as anemia. Several factors can lead to reduced levels, and identifying the cause is important for proper treatment.
Iron Deficiency
This is the most common reason behind low hemoglobin. Iron is needed for hemoglobin production, so a lack of iron—usually due to poor diet, blood loss, or heavy menstrual periods—can decrease levels quickly.
Vitamin Deficiency
Both vitamin B12 and folic acid are required for healthy red blood cell formation. When these vitamins are lacking, RBC production slows down, leading to low hemoglobin.
Chronic Medical Conditions
Long-term illnesses such as kidney disease, tuberculosis, cancer, autoimmune disorders, or chronic infections can reduce hemoglobin levels by affecting RBC production or lifespan.
Bone Marrow Disorders
Conditions such as leukemia or aplastic anemia directly affect the bone marrow’s ability to produce red blood cells, resulting in severely low hemoglobin.
Blood Loss
Any form of bleeding—whether due to accidents, surgery, internal ulcers, or heavy menstrual flow—reduces the number of red blood cells in the body, causing hemoglobin levels to fall.
Genetic Disorders
Inherited conditions like sickle cell anemia and thalassemia affect the structure or production of hemoglobin, resulting in chronic anemia.
Symptoms of Low Hemoglobin (Anemia)
People with low hemoglobin commonly experience:
- Persistent weakness or fatigue
- Pale or yellowish skin and lips
- Shortness of breath during light activities
- Rapid or irregular heartbeat
- Frequent headaches or dizziness
- Cold hands and feet
- Poor concentration and low focus
If anemia is left untreated, it may lead to complications affecting the heart, brain, and other organs due to poor oxygen supply.
Causes of High Hemoglobin (Polycythemia)
High hemoglobin levels are less common but important to evaluate. When hemoglobin is significantly elevated, the condition is known as polycythemia. Several factors may contribute to this:
Living at High Altitudes
People living in mountainous regions breathe air with lower oxygen levels. The body responds by producing more hemoglobin to improve oxygen delivery.
Smoking
Smoking increases carbon monoxide levels in the blood. The body compensates by raising hemoglobin levels to carry more oxygen.
Lung Diseases
Conditions like COPD, asthma, or pulmonary fibrosis reduce the ability of the lungs to oxygenate the blood, leading to higher hemoglobin production.
Heart Disorders
Certain congenital or chronic heart problems cause low oxygen saturation, which triggers the body to create more RBCs and hemoglobin.
Dehydration
When the body lacks fluids, the blood becomes more concentrated, and hemoglobin levels may appear higher than they actually are.
Polycythemia Vera
This is a rare bone marrow disorder where the body makes too many red blood cells, causing excessively high hemoglobin.
Symptoms of High Hemoglobin
People with high hemoglobin may notice:
- Headache, dizziness, or fatigue
- Itching, especially after a hot shower
- Red or flushed skin tone
- Nosebleeds
- Blurred vision
- Elevated blood pressure
- Increased risk of blood clots
Blood clots can lead to heart attacks, strokes, and other serious complications, so high hemoglobin must be taken seriously.
Reference Ranges (may vary slightly by lab)
- Men: 13.8 – 17.2 g/dL
- Women: 12.1 – 15.1 g/dL
- Children: 11.0 – 16.0 g/dL
- Pregnant Women: Around 11.0 g/dL or above
Sample Type
- Sample Required: Whole Blood
- Tube Used: Lavender Top (EDTA Tube)
A simple blood sample is collected from a vein in your arm. The test is quick, generally painless, and results are usually available the same day.
Test Preparation
- You do not need to fast for a hemoglobin test.
- Drink enough water before the test because dehydration can temporarily increase the reading.
- Inform your doctor if you are taking iron supplements, chemotherapy drugs, or blood thinners.
- Doctors often order a hemoglobin test along with a Complete Blood Count (CBC) for a more detailed understanding of your blood health.
When to Consult a Doctor
You should talk to a doctor if:
- You feel tired, weak, or breathless without a clear reason.
- You notice pale skin, dizziness, or frequent headaches.
- Your hemoglobin values are repeatedly below or above the normal range.
- You are pregnant, have kidney or lung disease, or have a family history of anemia or polycythemia.
Regular monitoring helps detect issues early and prevents complications.
Important Word Explanations
- Erythroblasts: Immature cells in the bone marrow that develop into red blood cells.
- Polycythemia: A condition where hemoglobin or red blood cells are higher than normal.
- EDTA Tube: A blood collection tube that prevents clotting.
- CBC (Complete Blood Count): A common test that measures different components of blood.
- Bone Marrow: The soft tissue inside bones where blood cells are produced.
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