Hemoglobin
Also known as: Hgb, Hb
What Does Hemoglobin Measure?
Hemoglobin (Hgb or Hb) is a protein found inside red blood cells that is responsible for carrying oxygen from the lungs to every tissue and organ in the body, and transporting carbon dioxide back to the lungs for exhalation. The test measures the concentration of this protein in a given volume of blood, typically expressed in grams per deciliter (g/dL). Each hemoglobin molecule contains iron-rich compounds called heme groups, which bind to oxygen molecules — this is why iron is so critical for healthy hemoglobin levels.
Why Does Hemoglobin Matter?
Hemoglobin is one of the most clinically important biomarkers because it directly reflects the blood's ability to deliver oxygen to the body's tissues. Low hemoglobin levels indicate anemia, which can cause fatigue, shortness of breath, dizziness, and impaired cognitive function. Chronically low levels may signal nutritional deficiencies, chronic disease, blood loss, or bone marrow disorders. Conversely, abnormally high hemoglobin can increase blood viscosity and raise the risk of clots, stroke, or heart attack, and may indicate conditions like polycythemia vera or chronic hypoxia from lung disease or high-altitude living. Hemoglobin is routinely measured as part of a Complete Blood Count (CBC) and is essential for monitoring anemia treatment, surgical readiness, and overall health.
Normal Ranges
Males
13.5–17.5 g/dL
Females
12.0–15.5 g/dL
Children
Varies by age: newborns 14–24 g/dL; infants 9.5–13 g/dL; ages 6–12: 11.5–15.5 g/dL
Causes of High Levels
- Polycythemia vera (a bone marrow disorder causing overproduction of red blood cells)
- Chronic obstructive pulmonary disease (COPD) or other lung diseases causing low blood oxygen
- Living at high altitude, where the body compensates for lower oxygen levels
- Dehydration or reduced plasma volume, which concentrates the blood
- Heavy smoking, which raises carboxyhemoglobin and stimulates red blood cell production
- Erythropoietin (EPO) misuse or testosterone therapy in athletes
Causes of Low Levels
- Iron deficiency anemia, the most common cause worldwide, often due to poor diet or blood loss
- Vitamin B12 or folate deficiency, impairing red blood cell production (megaloblastic anemia)
- Chronic diseases such as kidney disease, cancer, rheumatoid arthritis, or inflammatory conditions
How to Improve Your Hemoglobin
Diet
- Eat iron-rich foods daily, including red meat, poultry, fish, lentils, tofu, and dark leafy greens like spinach
- Consume vitamin C-rich foods (citrus fruits, bell peppers, strawberries) alongside iron-rich meals to enhance iron absorption by up to 3-fold
- Include folate-rich foods such as beans, lentils, asparagus, and fortified cereals to support red blood cell production
- Add vitamin B12 sources like eggs, dairy, beef liver, and salmon, or fortified plant-based milks if vegetarian or vegan
- Avoid drinking tea or coffee with meals, as tannins and polyphenols can inhibit iron absorption by up to 60%
Supplements
- Iron supplements (ferrous sulfate 325 mg or ferrous gluconate 300 mg daily) — only if deficiency is confirmed by a doctor, as excess iron is harmful
- Vitamin B12 supplements: 500–1000 mcg/day of methylcobalamin for those with deficiency or following a plant-based diet
- Folic acid 400–800 mcg/day to support red blood cell maturation, especially important during pregnancy
- Vitamin C 250–500 mg taken alongside iron supplements to significantly improve absorption
Related Biomarkers
Frequently Asked Questions
What is the difference between hemoglobin and hematocrit?
Hemoglobin measures the actual concentration of the oxygen-carrying protein in your blood (in g/dL), while hematocrit measures the percentage of your total blood volume that is made up of red blood cells. They both reflect similar information about red blood cell health, and in most healthy individuals, hematocrit (%) is approximately three times the hemoglobin value (g/dL). Both tests are performed as part of a Complete Blood Count (CBC).
How quickly can hemoglobin levels improve after starting iron supplements?
With appropriate iron supplementation and dietary changes, most people begin to see an increase in hemoglobin levels within 2–4 weeks. However, it typically takes 1–3 months of consistent treatment to fully normalize hemoglobin levels. Your doctor may recommend continuing supplements for an additional 3–6 months after your levels normalize to replenish iron stores. Regular blood tests are important to monitor progress.
Can dehydration affect hemoglobin test results?
Yes. Dehydration causes a reduction in plasma (the liquid part of blood), which concentrates the red blood cells and can make hemoglobin appear higher than it truly is — a phenomenon called pseudo-polycythemia or relative erythrocytosis. Conversely, overhydration or receiving intravenous fluids can dilute the blood and make hemoglobin appear lower. For the most accurate results, stay normally hydrated before a blood test and avoid strenuous exercise the day before.
What hemoglobin level is considered dangerously low?
A hemoglobin level below 7.0 g/dL is generally considered severely low and often requires medical intervention, which may include blood transfusion, especially if the person is symptomatic (feeling faint, having chest pain, or experiencing severe shortness of breath). Levels between 7.0 and 10.0 g/dL represent moderate anemia requiring prompt medical evaluation and treatment. Levels below 8.0 g/dL are typically a threshold for transfusion consideration in hospitalized patients, though clinical context always guides the decision.