Hemoglobin
Hgb· also: Hb, Hemoglobin
Clinical Overview
Hemoglobin (Hgb) is the iron-containing protein in red blood cells responsible for carrying oxygen from the lungs to body tissues and returning carbon dioxide to the lungs. It is the primary indicator of anemia and is measured in every complete blood count (CBC).
Why This Test Matters
Hemoglobin is the cornerstone test for diagnosing anemia and polycythemia. It guides decisions about blood transfusions, iron supplementation, and treatment of bone marrow disorders. Monitoring trends over time is as important as a single value — a rapidly falling hemoglobin requires urgent evaluation even if the absolute number is not yet critically low.
Reference RangesWHO/IFCC standards
| Age Group | Sex | Reference Range | Unit | Notes |
|---|---|---|---|---|
| Adults (18–64) | Male | 13.5 – 17.5 | g/dL | — |
| Adults (18–64) | Female | 12 – 15.5 | g/dL | — |
| All ages | Female | 11 – 14 | g/dL | Pregnancy |
| Children / Pediatric | All | 11 – 16 | g/dL | Age 6 months–12 years (approximate) |
Also reported in: g/L, mmol/L.
Critical (Panic) Values
Critical Low: < 7 g/dL. Critical High: > 20 g/dL. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High Hgb Causes
- Polycythemia vera (bone marrow overproduction)
- Chronic mountain sickness / high-altitude living
- Dehydration / hemoconcentration
- Chronic obstructive pulmonary disease (COPD)
- Heavy smoking (carbon monoxide effect)
- Anabolic steroid or erythropoietin use
- Congenital heart disease with right-to-left shunt
Low Hgb Causes
- Iron deficiency anemia (most common worldwide)
- Vitamin B12 or folate deficiency (megaloblastic anemia)
- Chronic kidney disease (reduced erythropoietin)
- Thalassemia and hemoglobinopathies
- Bone marrow failure (aplastic anemia)
- Hemolytic anemia (immune, hereditary spherocytosis, G6PD deficiency)
- Acute or chronic blood loss (GI bleed, heavy menstruation)
- Anemia of chronic disease / inflammation
Signs & Symptoms to Watch For
How to Prepare for This Test
No special preparation is required for a hemoglobin test. It is typically drawn as part of a complete blood count (CBC) from a routine blood draw. Inform your doctor of any recent blood transfusions, as these will temporarily raise the result.
Factors That Can Affect Results
- Recent blood transfusion (falsely raises result)
- High-altitude residence (naturally higher baseline)
- Severe dehydration (falsely elevated due to hemoconcentration)
- Pregnancy (physiologically lower due to plasma expansion)
- Smoking (raises carboxyhemoglobin, slightly raises Hgb)
- Lipemia or icterus in blood sample (interfere with spectrophotometry)
Related Topics
Frequently Asked Questions
What hemoglobin level requires a blood transfusion?
Most guidelines recommend transfusion when hemoglobin falls below 7–8 g/dL in stable patients. In patients with active cardiac disease or ongoing blood loss, transfusion may be considered at higher levels (8–10 g/dL). The decision always depends on symptoms and clinical context, not just the number.
What is the difference between hemoglobin and hematocrit?
Hemoglobin measures the actual amount of hemoglobin protein in grams per deciliter of blood. Hematocrit measures the percentage of blood volume occupied by red blood cells. Both reflect the same underlying condition (anemia or polycythemia) and typically move together — hematocrit is approximately three times the hemoglobin value.
Can low hemoglobin be caused by something other than iron deficiency?
Yes. While iron deficiency is the most common cause globally, low hemoglobin can result from vitamin B12 or folate deficiency, chronic kidney disease, thalassemia, bone marrow disorders, hemolytic anemias, chronic inflammation, and blood loss. The MCV and other CBC parameters help distinguish between these causes.
How quickly does hemoglobin change after starting iron supplements?
Reticulocytes (young red blood cells) begin rising within 3–5 days of starting iron therapy. Hemoglobin typically rises by 1–2 g/dL within 4 weeks of adequate iron treatment. Full correction of iron stores (reflected by ferritin) takes 3–6 months even after hemoglobin normalizes.