Lab results explained
Low hemoglobin: meaning and causes
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Short answer: Low hemoglobin means your blood carries less oxygen than usual, a condition called anaemia. It is generally below 13.5 g/dL (135 g/L) in men and 12 g/dL (120 g/L) in women. Iron deficiency is the most common cause, but blood loss, low B12 or folate, chronic illness and inherited conditions are also possible.
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Typical adult range
Your lab may print a slightly different range. Use the one on your report when there is one.
| Test | Men | Women |
|---|---|---|
| Hemoglobin | 13.5–17.5 g/dL135–175 g/L8.38–10.9 mmol/L | 12–15.5 g/dL120–155 g/L7.45–9.62 mmol/L |
| MCV (red cell size) | 80–100 fL | |
| Ferritin | 24–336 ng/mL | 11–307 ng/mL |
What hemoglobin measures
Hemoglobin (spelled haemoglobin in the UK, India and Pakistan) is the protein inside red blood cells that carries oxygen from your lungs to the rest of your body. When it is low, your organs get less oxygen, which can make you tired, breathless on stairs, pale or dizzy. Mild anaemia often causes no symptoms at all and is found on a routine CBC. Pregnant women, children and older adults have slightly different ranges, so the lab range on your report and your doctor's view matter.
Common causes
The cause matters more than the number, because treatment depends on it. In women of childbearing age, heavy periods and pregnancy are the most frequent reasons. In men and in women after menopause, doctors usually look for slow blood loss from the stomach or bowel, even without obvious symptoms. Diets low in iron or B12, which can happen with vegetarian or vegan eating, are common in South Asia.
- Iron deficiency from heavy periods, pregnancy, diet or slow blood loss
- Low vitamin B12 or folate
- Long-term conditions such as kidney disease or inflammation
- Thalassaemia trait or other inherited hemoglobin conditions
- Recent bleeding, surgery or blood donation
How MCV and ferritin help find the cause
Two other results on this page narrow things down. MCV shows red cell size. A low MCV (under 80 fL) with low ferritin strongly suggests iron deficiency. A low MCV with normal or high ferritin can point to thalassaemia trait, which is common in families from India, Pakistan and Bangladesh and does not need iron. A high MCV (over 100 fL) makes doctors check B12, folate, thyroid and liver health. Ferritin is your iron store; levels under about 24 ng/mL in men and 11 ng/mL in women are low, though infection or inflammation can make ferritin look normal when stores are low.
What usually happens next
Your doctor will usually confirm the result, then order tests for the likely cause: ferritin and iron studies, B12 and folate, kidney function, and sometimes a stool test or a hemoglobin electrophoresis for thalassaemia. If iron deficiency is found, your doctor may suggest iron tablets and will also look for why your iron is low. Hemoglobin usually starts rising within a few weeks of correct treatment, and a repeat CBC checks progress. Do not start iron on your own without a test, because iron does not help other types of anaemia.
When to get help the same day
A hemoglobin under 7 g/dL (70 g/L) is a critical result and needs same-day medical care. Call your local emergency number if you have chest pain, fainting, severe breathlessness at rest, a racing heartbeat, vomiting blood, black tarry stools or heavy bleeding that will not stop.
Questions to ask your doctor
- What type of anaemia does my MCV and ferritin suggest?
- What do you think is causing my low hemoglobin?
- Do I need tests for blood loss, B12, folate or thalassaemia?
- If I need treatment, how long until my levels should improve?
- When should I repeat my blood test?
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Frequently asked questions
What hemoglobin level is considered dangerous?
Below 7 g/dL is usually treated as critical and needs urgent assessment. Between 7 and 10 g/dL is moderate anaemia that needs prompt follow-up. How serious it is also depends on how fast it fell and your symptoms, since a slow drop is often better tolerated than sudden blood loss.
Can diet alone fix low hemoglobin?
Eating iron-rich foods such as lentils, beans, green leafy vegetables, meat and fortified cereals helps keep iron up, and vitamin C helps the body absorb it. Once you are deficient, diet alone is often too slow, so doctors may suggest tablets. Diet does not fix anaemia caused by thalassaemia or chronic illness.
Is low hemoglobin the same as anaemia?
Yes. Anaemia means a hemoglobin level below the normal range for your age and sex. It describes the finding, not the cause. Finding out why it is low, such as iron deficiency, B12 deficiency or blood loss, is the important next step.
Why is my hemoglobin low but my iron is normal?
Iron deficiency is only one cause of anaemia. Low B12 or folate, kidney disease, long-term inflammation and thalassaemia trait can all lower hemoglobin with normal iron. Also, a single serum iron result swings during the day, so ferritin is a better guide to iron stores.
Sources
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