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Your haemoglobin came back low — or flagged. It's the single most important number in your blood test for understanding fatigue, breathlessness, and anaemia. Here's exactly what it means.
Clinical Pathology, Hematology ·
The essentials — before you read the full guide below.
Haemoglobin is the iron-containing protein inside red blood cells that carries oxygen from your lungs to every cell in your body.
Men: 13.5–17.5 g/dL. Women: 12.0–15.5 g/dL. Pregnant women: 11.0–14.0 g/dL. Children vary by age.
A haemoglobin below the reference range is the definition of anaemia. The cause (iron, B12, folate, chronic disease) is what matters next.
Elevated haemoglobin may indicate dehydration, smoking, altitude adaptation, or (rarely) polycythaemia vera.
Reference Ranges
Use the interactive slider below, or read the range cards for a full clinical breakdown.
Drag to see what your Haemoglobin (Hgb / Hb) means
The Science
Haemoglobin is a protein in red blood cells made of four subunits, each containing an iron atom. Each iron atom can bind one molecule of oxygen — so haemoglobin level directly determines how much oxygen your blood can carry per litre. When haemoglobin falls, tissues receive less oxygen, and fatigue follows.
Each haemoglobin tetramer binds 4 O₂ molecules at the lungs and releases them progressively to tissues — a process called cooperative binding.
Iron deficiency is the world's most common nutritional deficiency. Without iron, the body can't produce functional haemoglobin — resulting in microcytic, hypochromic anaemia.
Red cells (and the haemoglobin inside them) circulate for ~120 days. Haemoglobin levels reflect the balance between new RBC production and old RBC destruction.
When to Test
These are the most common reasons a Haemoglobin (Hgb / Hb) test is requested — from symptoms to routine screening.
The most common symptom of anaemia. Cells deprived of oxygen produce less ATP — the energy currency of the body.
Primary symptomWith less oxygen-carrying capacity, the cardiorespiratory system compensates — increasing rate and depth of breathing.
Key symptomPallor of the inner eyelid (conjunctival pallor) is a simple clinical sign of significant anaemia. Check the lower eyelid.
Clinical signThe heart compensates for low haemoglobin by beating faster — to deliver the same amount of oxygen per minute.
Compensation signSickle cell disease, thalassaemia, and hereditary spherocytosis all cause chronic haemoglobin abnormalities — genetic screening is relevant.
Risk factorHaemoglobin is checked in every standard blood panel. Women of childbearing age and pregnant women are screened more frequently.
ScreeningTesting Schedule
Frequency depends on your current health status and your doctor's guidance.
Included in the standard annual blood count. Establishes a personal baseline.
Repeat every 3–6 months during iron, B12, or folate replacement therapy to confirm response.
Checked at booking, ~28 weeks, and again at ~36 weeks — anaemia is common in pregnancy and requires treatment.
CKD, inflammatory bowel disease, and haematological disorders require regular haemoglobin monitoring.
If Your Result Is Abnormal
Haemoglobin is a symptom, not a diagnosis — treatment depends entirely on the cause.
Low Hgb with low MCV strongly suggests iron deficiency. Serum ferritin, iron, and TIBC confirm the diagnosis. Oral iron replacement is first-line.
Ferritin + iron panelHigh MCV with low Hgb suggests megaloblastic anaemia. B12 and folate levels guide replacement therapy — often quickly effective.
B12 + folate panelReticulocytes (young RBCs) show whether the bone marrow is responding. Low reticulocytes suggest a production problem rather than blood loss.
Reticulocyte + blood filmHaemoglobin below 8 g/dL, unexplained anaemia, or abnormal blood film findings warrant specialist review for possible bone marrow or haemolytic cause.
For Hgb <8 g/dLClinician-reviewed articles published in this category — referenced, sourced, and written for patients and practitioners alike.
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