Haemoglobin test explained (HGB)

Haemoglobin is the main protein in red blood cells. The iron in its molecule binds oxygen in the lungs and releases it to the tissues, carrying some carbon dioxide back on the return trip. The amount of haemoglobin in the blood (g/l) is one of the most important measurements in a full blood count: when there is too little of it, tissues receive less oxygen — a condition called anaemia. Anaemia has many causes, the most common being iron deficiency, so haemoglobin is usually assessed together with ferritin, red-cell indices (MCV, MCH) and sometimes vitamin B12 and folate. A raised haemoglobin is less common and can be linked to a lack of oxygen (lung disease, sleep apnoea, smoking, living at high altitude) or to the body producing too many red blood cells. The WHO updated its thresholds by age, sex and pregnancy status in 2024 — these are also used in Lithuanian laboratories, though the exact reference range printed on a given lab report can differ slightly.

• As part of a routine full blood count — preventive check-ups and before surgery
• When there is weakness, dizziness, breathlessness, cold hands and feet, or pale skin
• When there is a family history of inherited blood disorders
• With chronic infection or illness, or after significant blood loss from injury or surgery
• During pregnancy — to detect and monitor anaemia
• To monitor treatment of a diagnosed anaemia

Under the WHO's 2024 guideline, this value is classed as mild anaemia for women aged 15–65, since it falls below the 120 g/l threshold; for men it falls within the broader 110-129 g/l mild-anaemia band, since the male threshold is 130 g/l. For pregnant women in the first or third trimester, 110 g/l or above is still considered normal. A single result in this range does not diagnose anything on its own — laboratory methods and reference intervals vary slightly, and haemoglobin can shift temporarily with fluid balance or a recent infection. A doctor typically checks whether it is a one-off reading or a trend, repeats the test after some time, and checks iron markers (ferritin) and red-cell size (MCV) to help separate the most common cause — iron deficiency — from other causes, alongside factors such as heavy periods, diet and chronic illness.

Moderate anaemia: 80-109 g/l

The WHO 2024 guideline treats 80-109 g/l as a single moderate-anaemia band for men and non-pregnant women aged 15–65 (and for children from 5 years) — both 105 g/l and 85 g/l fall into this same category, even though they represent quite different degrees of drop from normal. The closer a result is to the lower 80 g/l boundary, beyond which severe anaemia begins, the greater the deviation from normal, and the more often tiredness, breathlessness on exertion or dizziness appear — so such cases are usually assessed more urgently than ones nearer 109 g/l. A doctor usually checks ferritin and other iron markers, since iron deficiency is the most common cause, together with red-cell size (MCV) to help distinguish iron-deficiency anaemia from anaemia caused by low vitamin B12 or folate, and looks for a possible source of blood loss (heavy periods, gastrointestinal bleeding) or a chronic illness suppressing red-cell production. In pregnancy the thresholds differ by trimester: in the second trimester the normal cut-off is 105 g/l, so part of this band (95-104 g/l) counts as mild rather than moderate anaemia for a pregnant woman, and 105-109 g/l may still be normal. The WHO cites research linking haemoglobin below 95 g/l in pregnancy with a higher risk of an adverse outcome for the newborn, a risk that doubles below 80 g/l; children aged 6-23 months and second-trimester pregnant women also have a lower severe-anaemia cut-off of 70 g/l rather than 80 g/l.

Under the WHO 2024 guideline, below 80 g/l is classed as severe anaemia for adults (except second-trimester pregnant women and children aged 6-23 months, whose threshold is 70 g/l) — the most severe of the WHO's four categories. This calls for urgent medical assessment and is not a case to put off, since oxygen delivery around the body can be significantly impaired. Symptoms are often more pronounced — marked tiredness, breathlessness even with light exertion, a fast heartbeat, dizziness and, at times, impaired consciousness — though severity also depends on how quickly the haemoglobin dropped, since a slow decline is sometimes tolerated better than a sudden one. Causes in this range vary widely, from acute or chronic blood loss to severe iron, vitamin B12 or folate deficiency, chronic illness, or a blood-forming disorder; because low haemoglobin can directly threaten oxygen supply to the organs, the doctor decides on next steps based on overall condition, not the lab number alone.

Raised haemoglobin

A raised haemoglobin is less common than a low one. The usual causes are conditions in which the body gets less oxygen and compensates by producing more red blood cells — lung disease, heart disease, sleep apnoea, smoking. A rarer cause is polycythaemia vera, where the bone marrow produces too many red blood cells. Dehydration can also raise haemoglobin temporarily. Next steps typically considered: whether dehydration affected the sample and the test is worth repeating; possible causes of low oxygen, such as breathing pauses during sleep, lung or heart disease, and smoking; other full-blood-count values — red-cell count, haematocrit, white cells, platelets; and when to repeat the test.

A low haemoglobin means anaemia. The most common cause is iron deficiency, but anaemia is also caused by vitamin B12 or folate deficiency, blood loss, liver disease, chronic illness and cancer. Ferritin, red-cell size (MCV) and reticulocytes help identify the cause. Symptoms often include weakness and fatigue, dizziness, breathlessness, cold hands and feet, and pale skin. Next steps typically considered: ferritin and other iron markers, to check whether iron deficiency is the cause; red-cell size (MCV) — a low value points to iron deficiency, a high value to B12 or folate deficiency; a possible source of blood loss; vitamin B12 and folate, if MCV is raised; and when to repeat the test to track the change. The tests used to look into anaemia are described separately.

Why the threshold differs by sex, age and pregnancy

Under the WHO 2024 guideline, the anaemia threshold is 120 g/l for non-pregnant women aged 15–65, 130 g/l for men of the same age, and lower, trimester-based thresholds for pregnant women — 110 g/l in the first and third trimester, 105 g/l in the second. For children, the threshold rises with age: 105 g/l at 6-23 months, 110 g/l at 24-59 months, 115 g/l at 5-11 years, and 120 g/l for girls aged 12-14, approaching the adult female threshold. The WHO's guideline explains that the thresholds for the 6-23 and 24-59 month groups differ because of notable differences in growth and nutritional needs between these two age bands — so the single 6-59 month threshold used previously was split into two, rising gradually from 105 to 115 g/l. When interpreting a child's result, it matters to know their exact age in months or years, since the threshold changes fairly quickly. Always compare a specific result against the range printed on the lab report, which can differ slightly from the WHO figure, and note whether it was taken during pregnancy, when different trimester-based thresholds apply.

Haemoglobin is measured as part of a full blood count. At the Medicina practica laboratory in Lithuania, that test costs from €10 (September 2026 pricing).