Hemoglobin levels measure the amount of hemoglobin in the blood. Hemoglobin is a protein in red blood cells that carries oxygen throughout the body and is usually measured as part of a complete blood count (CBC).
Normal hemoglobin levels vary by age, sex, pregnancy, and the laboratory performing the test. High levels may be linked to dehydration, tobacco use, lung disease, or polycythemia, while low levels may be associated with anemia, blood loss, nutrient deficiencies, or other health conditions.
Changes in hemoglobin levels can cause symptoms such as fatigue, shortness of breath, dizziness, headache, or pale skin. The article also explains common reference ranges, possible causes of abnormal results, and the main types of hemoglobin found in the blood.
Reference ranges
Hemoglobin reference ranges can vary depending on age, sex, pregnancy, and the laboratory performing the test. Results should therefore be compared with the reference range provided by the laboratory.
According to the American Academy of Pediatrics (AAP), normal hemoglobin levels vary by age and sex. Common pediatric reference ranges include:
- Children ages 2 to 6: approximately 11.0 to 13.7 g/dL
- Children ages 6 to 12: approximately 11.2 to 14.6 g/dL
For adults, anemia is generally defined as hemoglobin below 13.6 g/dL in men and below 12.0 g/dL in women.
According to the American College of Obstetricians and Gynecologists (ACOG), anemia during pregnancy is defined as hemoglobin below 11 g/dL in the first or third trimester or below 10.5 g/dL in the second trimester.
Hemoglobin is measured as part of a complete blood count (CBC), which also evaluates red blood cell count and red blood cell indices such as mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). These results should be interpreted together.
High hemoglobin
High hemoglobin levels in the blood can be caused by:
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Tobacco use
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Dehydration
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Lung diseases, such as emphysema or pulmonary fibrosis
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Chronic heart disease
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Erythrocytosis
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Kidney tumors
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Use of anabolic steroids or erythropoietin
High hemoglobin levels may cause headache, dizziness, fatigue, difficulty concentrating, skin redness, and itching. In more severe cases, temporary vision or hearing loss may occur.
These symptoms may be related to increased blood viscosity, which can slow circulation and raise the risk of blood clots and cardiovascular complications. See a doctor if symptoms persist or worsen.
Low hemoglobin
The main causes of low hemoglobin levels are:
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A diet low in iron
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Digestive problems that interfere with nutrient absorption
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Conditions such as hypothyroidism, cirrhosis, lymphoma, leukemia, kidney failure, thalassemia, and porphyria
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Heavy bleeding
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Vitamin B12 and folic acid deficiency
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Use of medications to treat conditions such as HIV/AIDS and cancer
Low hemoglobin levels can cause symptoms such as frequent fatigue, shortness of breath, and pale skin. The underlying cause should be identified so that appropriate treatment can be started under medical guidance.
Types of hemoglobin
Hemoglobin can be classified into different types based on the protein chains it contains:
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HbA: consists of a heme group (Hb) with two alpha chains and two beta chains and is the type found in the highest concentration in the blood
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HbA2: consists of a heme group (Hb) with two alpha chains and two delta chains
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HbF: consists of a heme group (Hb) with two alpha chains and two gamma chains. It is found in higher concentrations in newborns
Other types include Hb Gower I, Hb Gower II, and Hb Portland, which are present during embryonic development. Their concentrations decrease as HbF levels increase closer to birth.
The type of hemoglobin can be identified through hemoglobin electrophoresis, which is also useful for diagnosing conditions that affect hemoglobin production.