Hemoglobin (Hgb)
A protein in red blood cells that carries oxygen from the lungs to the rest of the body and returns carbon dioxide back to the lungs.
Last updated: April 10, 2026
Hemoglobin is the iron-rich protein inside red blood cells responsible for carrying oxygen throughout the body. It is one of the most commonly measured markers in routine blood work and is a key component of the complete blood count (CBC) panel.
Why Is Hemoglobin Tested?
Hemoglobin is tested to evaluate the blood's ability to carry oxygen and to screen for conditions like anemia or polycythemia. Because hemoglobin directly reflects how well the body delivers oxygen to tissues, it is one of the first markers doctors check when investigating fatigue, weakness, or shortness of breath.
Hemoglobin is measured as part of the CBC (complete blood count) panel, which is one of the most frequently ordered blood tests. The CBC is routinely included in annual physicals, pre-surgical evaluations, and monitoring for chronic conditions.
A healthcare provider may also specifically check hemoglobin to monitor ongoing blood loss, track the effectiveness of treatment for anemia, or evaluate nutritional deficiencies involving iron, vitamin B12, or folate.
Normal Reference Ranges
Ranges may vary by lab, age, sex, and testing method. Always refer to the ranges printed on your specific lab report.
| Category | Range | Unit |
|---|---|---|
| Adult Male | 13.5 – 17.5 | g/dL |
| Adult Female | 12.0 – 16.0 | g/dL |
| Children (ages 6–12) | 11.5 – 15.5 | g/dL |
| Pregnancy | 11.0 – 14.0 | g/dL |
| Status | Men (g/dL) | Women (g/dL) |
|---|---|---|
| Low | Below 13.5 | Below 12.0 |
| Normal | 13.5 – 17.5 | 12.0 – 16.0 |
| High | Above 17.5 | Above 16.0 |
What Does a High Hemoglobin Level Mean?
A high hemoglobin level means the blood is carrying more oxygen-transporting protein than usual, which may indicate the body is compensating for low oxygen or producing too many red blood cells.
Common causes:
- Living at high altitude (the body produces more red blood cells to compensate for thinner air)
- Chronic smoking (carbon monoxide reduces oxygen-carrying capacity)
- Dehydration (concentrates the blood, making hemoglobin appear higher)
- Chronic lung disease such as COPD or emphysema
Less common causes:
- Polycythemia vera (a bone marrow disorder that overproduces red blood cells)
- Congenital heart disease
- Kidney tumors that produce excess erythropoietin (EPO)
Temporary or benign causes:
- Intense physical exercise or athletic training
- Mild dehydration from not drinking enough fluids before the blood draw
Mildly elevated hemoglobin is often related to lifestyle factors and may not require treatment. Significantly elevated levels (above 18.5 g/dL in men or 16.5 g/dL in women) may warrant further investigation.
What Does a Low Hemoglobin Level Mean?
A low hemoglobin level means the blood has a reduced capacity to carry oxygen, which is the hallmark of anemia. Anemia is one of the most common blood conditions worldwide.
Common causes:
- Iron deficiency (the most frequent cause of low hemoglobin)
- Heavy menstrual periods or chronic blood loss
- Vitamin B12 or folate deficiency
- Chronic kidney disease (reduced production of erythropoietin)
Less common causes:
- Bone marrow disorders (aplastic anemia, leukemia, myelodysplastic syndromes)
- Hemolytic anemia (red blood cells break down faster than they are produced)
- Sickle cell disease or thalassemia (inherited hemoglobin disorders)
Temporary or benign causes:
- Pregnancy (blood volume expands, diluting hemoglobin concentration)
- Recent blood donation
- Overhydration before the blood draw
Mild anemia (hemoglobin of 10–12 g/dL in women, 10–13.5 g/dL in men) is common and often treatable. Hemoglobin below 7–8 g/dL is considered severe and typically requires urgent medical attention.
Factors That Can Affect Your Results
Several factors beyond disease can influence hemoglobin readings. Understanding these helps put results into proper context.
- Age: Hemoglobin naturally decreases slightly with age, especially after 65
- Sex: Men typically have higher hemoglobin than women due to testosterone's effect on red blood cell production
- Pregnancy: Blood volume increases by up to 50%, diluting hemoglobin concentration
- Altitude: Living above 5,000 feet can raise hemoglobin by 1–2 g/dL
- Hydration: Dehydration falsely raises hemoglobin; overhydration can lower it
- Exercise: Endurance athletes may develop "sports anemia" from expanded blood volume
- Medications: Certain drugs, including some chemotherapy agents and antiretrovirals, can lower hemoglobin
- Smoking: Carbon monoxide exposure triggers the body to produce more hemoglobin
- Time of day: Hemoglobin levels can vary slightly throughout the day, tending to be higher in the morning
Should I Be Concerned About My Hemoglobin Level?
A slightly abnormal hemoglobin result is usually not a reason to worry. Many factors, including hydration status, the time of your blood draw, and even your altitude, can shift your hemoglobin reading by a full gram per deciliter or more.
If your hemoglobin is just below or above the normal range, your doctor may simply recommend retesting in a few weeks, especially if you have no symptoms. Common symptoms of significantly low hemoglobin include fatigue, pale skin, shortness of breath, dizziness, and cold hands or feet.
If your hemoglobin is significantly outside the normal range — for example, below 10 g/dL or above 18.5 g/dL — your healthcare provider will likely recommend additional tests and evaluation to determine the cause. This does not automatically mean something is seriously wrong, but it does warrant further investigation.
Common Next Steps
Follow-up depends on whether hemoglobin is high or low and by how much. A healthcare provider may recommend:
- Checking ferritin and iron levels to evaluate iron stores
- Measuring hematocrit and RDW for a more complete picture of red blood cell health
- Testing vitamin B12 and folate levels
- Ordering a reticulocyte count to see how quickly new red blood cells are being produced
- A peripheral blood smear to examine red blood cell shape and size under a microscope
- Dietary changes such as increasing iron-rich foods (red meat, leafy greens, legumes)
- Iron supplementation if iron deficiency is confirmed
Frequently Asked Questions
What is a normal hemoglobin level?
Normal hemoglobin is 13.5–17.5 g/dL for adult men and 12.0–16.0 g/dL for adult women. Children and pregnant individuals have different ranges. Always compare your results to the reference range on your specific lab report, as methods and instruments vary between laboratories.
Should I worry about slightly low hemoglobin?
A slightly low hemoglobin (within 0.5–1 g/dL of the lower limit) is common and may be caused by hydration levels, recent exercise, or normal individual variation. Your doctor may suggest a recheck in a few weeks rather than immediate action.
Can dehydration affect my hemoglobin result?
Yes. Dehydration concentrates the blood, which can make hemoglobin levels appear artificially higher than they truly are. Overhydration has the opposite effect. Staying normally hydrated before your blood draw helps ensure the most accurate result.
How often should hemoglobin be retested?
For borderline results, retesting in 4–8 weeks is common. If hemoglobin is significantly abnormal, your doctor may retest sooner or order additional workup immediately. For people on treatment for anemia, follow-up every 1–3 months is typical until levels stabilize.
Do I need to fast before a hemoglobin test?
No fasting is required for a hemoglobin test or a standard CBC. However, if your blood draw includes other tests like glucose or a lipid panel, your doctor may ask you to fast for those specific tests.
What is the difference between hemoglobin and hematocrit?
Hemoglobin measures the amount of oxygen-carrying protein in the blood (in grams per deciliter), while hematocrit measures the percentage of blood volume made up of red blood cells. Both are part of the CBC and tend to rise and fall together. Hematocrit is roughly three times the hemoglobin value.
What is borderline hemoglobin?
Borderline hemoglobin typically means a result that falls just below the lower limit of normal — for example, 11.5–12.0 g/dL in women or 12.5–13.5 g/dL in men. This zone is not clearly normal but also not definitively anemic. Your doctor may recommend follow-up testing with iron studies.
Can exercise lower hemoglobin?
Regular intense endurance exercise can lead to a condition sometimes called "sports anemia." This happens because blood volume expands with training, diluting the hemoglobin concentration. It is generally considered a normal adaptation rather than true anemia and typically does not require treatment.
What tests are done alongside hemoglobin?
Hemoglobin is always measured as part of the CBC panel, which also includes hematocrit, white blood cells, platelets, and RDW. If anemia is suspected, ferritin and iron studies are typically added.
Sources
- Mayo Clinic. Hemoglobin test — Overview. mayoclinic.org
- MedlinePlus. Hemoglobin Test. U.S. National Library of Medicine. medlineplus.gov
- Cleveland Clinic. Hemoglobin: What It Is, Function & Normal Levels. clevelandclinic.org
- World Health Organization. Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. WHO/NMH/NHD/MNM/11.1
Related Tests and Biomarkers
Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges vary by laboratory, testing method, and individual factors. Always discuss your results with a qualified healthcare provider.