Hemoglobin: normal values, reasons for decrease and increase

Hemoglobin is an iron-containing protein found in red blood cells that carries oxygen from the lungs to the tissues. It is responsible for the majority of oxygen delivery to cells, and its concentration in the blood remains one of the key indicators of a complete blood count. If hemoglobin levels are low, tissues receive less oxygen, while if there is too much, the blood may become more viscous, increasing the risk of vascular complications. [1]

Structurally, hemoglobin is a tetramer consisting of four globin chains, each subunit containing a heme with ferrous iron. This allows one hemoglobin molecule to bind up to four oxygen molecules. This molecular architecture makes hemoglobin not just a "blood dye," but a central instrument of gas exchange. [2]

In a healthy adult, the predominant form is hemoglobin A. In the fetus and newborns, hemoglobin F predominates, which has a higher affinity for oxygen and facilitates gas exchange between the blood of the mother and fetus; then, in the first 1-2 years of life, it is mainly replaced by hemoglobin A. In addition to normal types, there are abnormal variants such as hemoglobin S, hemoglobin C, and hemoglobin E, which underlie a number of hemoglobinopathies. [3]

It's important to understand that even a "normal" hemoglobin level does not always indicate normal oxygen delivery. Carboxyhemoglobin, which forms from exposure to carbon monoxide and smoking, and methemoglobin, where iron is in an oxidized form, impair hemoglobin's ability to transport and release oxygen. Therefore, hemoglobin assessment should always take into account the clinical context, not just the number on the form. [4]

The hemoglobin test itself is not a separate, rare diagnostic test, but a standard part of a complete blood count. It helps detect anemia, erythrocytosis, the consequences of blood loss, chronic hypoxia, hereditary blood disorders, and a number of systemic diseases. However, this indicator alone does not explain the cause of the abnormality and requires further interpretation, along with the red blood cell count, hematocrit, red blood cell indices, and clinical data. [5]

Table 1. The main forms of hemoglobin and their significance

Form of hemoglobin	Where it is found	Clinical significance
Hemoglobin A	The main form in healthy adults	Normal oxygen transport
Hemoglobin F	Predominant in the fetus and newborns	Higher affinity for oxygen
Hemoglobin S	Pathological variant	Associated with sickle cell disease
Hemoglobin C	Pathological variant	May cause mild to moderate hemolytic anemia
Hemoglobin E	Pathological variant	It is more common in Southeast Asia and often has a mild course.

The data in the table summarize the clinically significant hemoglobin types described in current reference materials on hemoglobin electrophoresis and hemoglobinopathies. [6]

Hemoglobin levels: why we can't talk about a single, universal number today

The most common mistake is to assume that the "normal hemoglobin level" is the same for everyone. In practice, there are two different levels of assessment. The first is the standard laboratory reference interval, which depends on the method and the specific laboratory. The second is the diagnostic threshold for anemia, which the World Health Organization refined in 2024 to take into account age, gender, pregnancy, altitude, and smoking. [7]

In adults, typical laboratory reference intervals are typically around 138–172 g/L for men and 121–151 g/L for women, but these are laboratory benchmarks rather than universal diagnostic limits. For the diagnosis of anemia, the World Health Organization uses lower thresholds: less than 130 g/L in men aged 15–65 years and less than 120 g/L in non-pregnant women aged 15–65 years. Therefore, a person may be “below the laboratory reference” but not yet meet the international threshold for anemia, and vice versa. [8]

During pregnancy, interpretation should be separate. In modern practice, trimester-specific thresholds are used: less than 110 g/L in the first trimester, less than 105 g/L in the second trimester, and less than 110 g/L in the third trimester. This is due to the physiological increase in plasma volume, which outpaces the increase in red blood cell mass and creates hemodilution. [9]

For children, thresholds also vary by age. According to the updated 2024 World Health Organization guidelines, anemia is defined as hemoglobin levels below 105 g/L in children 6-23 months, below 110 g/L in children 24-59 months, below 115 g/L in children 5-11 years old, and below 120 g/L in children 12-14 years old. This is why children's test results cannot be evaluated using adult charts. [10]

Another important modern amendment concerns altitude and smoking. In 2024, the World Health Organization recommended adjusting hemoglobin levels for anemia diagnosis, taking into account altitude and smoking, as both factors can artificially elevate measured hemoglobin levels. Without such adjustments, anemia may be underestimated or, conversely, its absence misinterpreted. [11]

Table 2. Modern diagnostic thresholds for anemia according to the World Health Organization

Group	Anemia threshold
Children 6-23 months	Less than 105 g per l
Children 24-59 months	Less than 110 g per l
Children 5-11 years old	Less than 115 g per l
Children 12-14 years old	Less than 120 g per l
Non-pregnant women aged 15-65 years	Less than 120 g per l
Men 15-65 years old	Less than 130 g per l
Pregnancy, first trimester	Less than 110 g per l
Pregnancy, second trimester	Less than 105 g per l
Pregnancy, third trimester	Less than 110 g per l

These thresholds reflect the diagnostic limits of anemia rather than the typical laboratory reference interval.[12]

Why does hemoglobin decrease?

A decrease in hemoglobin indicates anemia, but anemia is not a diagnosis, but a general syndrome. In practice, the causes come down to three major mechanisms: blood loss, insufficient red blood cell production, and hemolysis, or accelerated red blood cell destruction. The key to the initial diagnosis lies not only in the hemoglobin itself, but also in the reticulocyte count: elevated values most often indicate blood loss or hemolysis, while low or normal values in anemia indicate insufficient production. [13]

The most common cause of anemia worldwide is iron deficiency. It is characterized by a gradual decrease in hemoglobin, often microcytosis, an increased red blood cell distribution width, and low ferritin. According to the Merck Manual, a ferritin level of less than 30 ng/ml is specific for iron deficiency, although in the presence of inflammation, even levels as low as 100 ng/ml can still be consistent with iron deficiency. [14]

Equally important are anemia of chronic inflammation and anemia in chronic kidney disease. In chronic inflammation, iron utilization is impaired and erythropoiesis is suppressed, while in chronic kidney disease, erythropoietin production is reduced. Normocytic or moderately microcytic anemia is typical for these patients, and without clinical context, it can easily be confused with other forms. [15]

Macrocytic anemia most often leads to investigations for vitamin B12 deficiency, folate deficiency, alcoholism, liver disease, myelodysplasia, and some drug effects. A characteristic feature of vitamin B12 deficiency is that neurological impairment can appear even before overt anemia and even without pronounced macrocytosis. A reference level for vitamin B12 is less than 200 pg/ml, and in case of questionable values, methylmalonic acid and homocysteine are helpful. [16]

If reticulocytes are elevated, the physician should consider hemolysis or blood loss. In the case of hemolysis, a peripheral blood smear, indirect bilirubin, lactate dehydrogenase, and haptoglobin are important; in the case of blood loss, a search for an obvious or hidden source of bleeding is essential. Hemoglobinopathies, including sickle cell disease and thalassemia, are also included in the differential diagnosis, especially if there is a family history, ethnic predisposition, or characteristic changes in the smear and indices. [17]

Table 3. The main mechanisms of hemoglobin reduction

Mechanism	What's happening	Common causes
Blood loss	The body loses red blood cells faster than it can replenish them.	Menstrual loss, gastrointestinal bleeding, surgery, trauma
Insufficient production	Bone marrow produces few red blood cells	Iron deficiency, vitamin B12 deficiency, folate deficiency, kidney disease, inflammation, myelodysplasia
Accelerated destruction	Red blood cells live shorter than normal.	Autoimmune hemolysis, hereditary membranopathy, hemoglobinopathies, microangiopathy

The table reflects the modern clinical principle: first, one must understand the mechanism of anemia, and only then look for the exact nosology. [18]

Table 4. What does the size of red blood cells indicate with low hemoglobin?

Type of anemia by red blood cell size	What makes you think about first?	Which tests are particularly useful?
Microcytic	Iron deficiency, thalassemia, anemia of chronic inflammation	Ferritin, iron, transferrin saturation, hemoglobin electrophoresis
Normocytic	Acute blood loss, hemolysis, chronic kidney disease, inflammation	Reticulocytes, creatinine, hemolysis markers, inflammation markers
Macrocytic	Vitamin B12 deficiency, folate deficiency, liver disease, alcohol, myelodysplasia	Vitamin B12, folate, blood smear, liver function tests

This morphological scheme does not replace the diagnosis, but helps to quickly select the correct next set of tests. [19]

Why does hemoglobin increase?

High hemoglobin levels don't always indicate bone marrow disease. First, we need to distinguish between relative and true increases. A relative increase occurs when plasma volume decreases, especially with dehydration, when hemoglobin concentration increases without an actual increase in red blood cell mass. [20]

A true increase in hemoglobin, or erythrocytosis, is often secondary. The most common causes are chronic hypoxia due to smoking, living at high altitudes, chronic lung diseases, certain congenital and acquired heart diseases, and obstructive sleep apnea. In these cases, the body increases the number of red blood cells in an attempt to compensate for the lack of oxygen. [21]

There are other secondary causes: androgenic steroid use, illicit erythropoietin use, some sodium-glucose cotransporter 2 inhibitors, and tumors and cysts that increase erythropoietin production. Kidney tumors and other erythropoietin-producing neoplasms are particularly important if high hemoglobin is associated with an unusual clinical picture. [22]

Polycythemia vera, also known as polycythemia vera, is a special concern. This is a myeloproliferative neoplasm in which the bone marrow produces excessive amounts of red blood cells, and often white blood cells and platelets as well. The Merck Manual states that suspicion often arises at hemoglobin levels above 165 g/L in men and above 160 g/L in women, although hemoglobin and hematocrit can be misleading due to plasma shifts and concomitant iron deficiency. [23]

In practice, this means the following: a single high hemoglobin level does not yet equate to a diagnosis. A repeat test is needed, along with an assessment of hydration status, smoking history, altitude, sleep apnea symptoms, and testosterone and other medication use. In the case of persistent erythrocytosis, a targeted examination for secondary causes and myeloproliferative processes is necessary. [24]

Table 5. Common causes of increased hemoglobin

Group of reasons	Examples
Relative increase	Dehydration, severe hemoconcentration
Chronic hypoxia	Smoking, high altitude, lung disease, sleep apnea, heart disease
Medicinal and hormonal	Testosterone, androgens, erythropoietin
Renal and tumor causes	Renal tumors and cysts, other erythropoietin-producing tumors
Primary bone marrow process	Polycythemia vera

The table reflects the modern diagnostic approach to high hemoglobin: first, relative and secondary causes are excluded, then primary erythrocytosis is assessed. [25]

How to correctly interpret a hemoglobin test

A proper interpretation always begins not with a single hemoglobin number, but with a complete blood count. This requires a red blood cell count, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin content, red blood cell distribution width, reticulocytes, and a peripheral blood smear. This set of parameters allows us to determine whether we are dealing with deficiency anemia, hemolysis, blood loss, renal failure, hemoglobinopathy, or bone marrow failure. [26]

If hemoglobin is low, the next key step is usually assessing iron stores. In microcytosis, the first laboratory test in most algorithms is ferritin. Low ferritin supports iron deficiency, but normal or elevated ferritin does not rule it out if the patient has inflammation, infection, tumor, or chronic liver disease. [27]

If iron deficiency is not confirmed or the blood smear is macrocytic, the physician typically proceeds to vitamin B12, folate, and a blood smear. If hemolysis is suspected, reticulocytes, lactate dehydrogenase, bilirubin, and haptoglobin are needed. If there is a family history, ethnic predisposition, microcytosis without iron deficiency, or characteristic changes in the smear, hemoglobin electrophoresis and other tests for hemoglobinopathies are added. [28]

For high hemoglobin, the algorithm is different. Here, it is necessary to determine whether the result is relative due to dehydration, then rule out chronic hypoxia, smoking, sleep apnea, and testosterone and erythropoietin use. If persistent deviations occur, look for signs of a myeloproliferative process. In some cases, mutation testing, primarily for JAK2, and an in-depth hematological evaluation are required. [29]

Finally, it's not the hemoglobin level alone that needs to be treated, but its cause. Iron deficiency requires identifying the source of iron loss and replenishing it; vitamin B12 deficiency requires correction of the deficiency and the cause of malabsorption; chronic kidney disease requires a separate nephrological strategy; and polycythemia vera requires hematological treatment. Even the issue of blood transfusion isn't automatically decided based on the number alone: according to current international guidelines, a restrictive strategy with a threshold of approximately 70 g/L is typically considered for most hemodynamically stable hospitalized adults, but the final decision depends on symptoms, bleeding, concomitant cardiovascular diseases, and the clinical situation. [30]

Table 6. What additional tests are usually prescribed after hemoglobin abnormalities?

Situation	The most useful tests are as follows:
Low hemoglobin and microcytosis	Ferritin, iron, transferrin saturation, blood loss screening
Low hemoglobin and macrocytosis	Vitamin B12, folate, blood smear, liver function tests
Low hemoglobin and high reticulocytosis	Lactate dehydrogenase, bilirubin, haptoglobin, blood smear
Low hemoglobin and normocytosis	Creatinine, inflammatory markers, reticulocytes, sometimes bone marrow
High hemoglobin	Repeat testing, hydration assessment, oxygen saturation, smoking, sleep apnea, erythropoietin, JAK2 as indicated
Suspected hemoglobinopathy	Hemoglobin electrophoresis and other special tests

The table summarizes clinical logic and does not replace the individual physician's decision.[31]

When hemoglobin abnormalities require urgent evaluation

Low hemoglobin is dangerous not only in itself, but also because of the rate at which it declines. Symptoms of anemia are generally nonspecific and include weakness, fatigue, drowsiness, dizziness, fainting, and shortness of breath during exertion. However, if hemoglobin drops rapidly or the patient already has heart or lung disease, symptoms can be much more severe even at less extreme levels. [32]

Severe anemia may present with tachycardia, chest pain, severe shortness of breath, hypotension, heart failure, or shock. Particularly concerning are black stools, hematemesis, heavy menstrual loss, trauma, jaundice, dark urine, and sudden weakness—these signs require immediate investigation for blood loss or hemolysis rather than routine monitoring. [33]

High hemoglobin levels can also be harmful. With severe erythrocytosis, headaches, dizziness, itching, sweating, visual disturbances, thrombosis, and other manifestations of blood hyperviscosity are possible, especially in polycythemia vera. For such patients, not only the test results are important, but also a history of vascular events. [34]

Pregnant women with anemia, children with significantly reduced hemoglobin, and patients with suspected hemolysis, acute bleeding, severe hypoxia, blood tumors, or polycythemia vera require special urgent attention. For these groups, the cost of delay is higher, and "trying again later" can be a dangerous tactic. [35]

The main practical conclusion is simple: a hemoglobin assessment should always answer three questions: How severe is the deviation? How quickly did it develop? Does the patient have signs of tissue hypoxia, blood loss, hemolysis, or blood hyperviscosity? These three questions determine whether the patient can be observed calmly or whether urgent assistance is needed. [36]

Table 7. Red flags for abnormal hemoglobin levels

Situation	Why is this dangerous?
Rapidly increasing weakness, shortness of breath, chest pain with low hemoglobin	Severe tissue hypoxia is possible.
Fainting, hypotension, signs of shock	Massive blood loss or severe anemia may occur.
Jaundice, dark urine, high reticulocytosis	Suspicion of hemolysis
Black stool, blood in vomit, visible bleeding	Continued acute blood loss is possible
Persistently high hemoglobin with itching, headache, thrombosis	Possible erythrocytosis or polycythemia vera
Pregnancy with anemia or a child with significantly low hemoglobin	An expedited assessment of the cause and risk of complications is needed.

The table is needed for practical guidance and helps to understand when one cannot limit oneself to just a repeated analysis. [37]

FAQ

What is the normal hemoglobin level for an adult?
Typical laboratory reference intervals in adults are often around 138-172 g/L for men and 121-151 g/L for women, but the diagnosis of anemia is made using separate diagnostic thresholds. The World Health Organization in 2024 uses a threshold of less than 130 g/L for men 15-65 years old and less than 120 g/L for non-pregnant women 15-65 years old. [38]

Does a hemoglobin level of 118 g/L in a woman always indicate anemia?
In a non-pregnant adult woman, 118 g/L is already below the anemia threshold according to the World Health Organization criteria. However, the clinical significance depends on age, symptoms, pregnancy, inflammation, altitude, and other blood test parameters. [39]

Why do pregnant women often have lower hemoglobin levels?
During pregnancy, plasma volume increases faster than red blood cell mass, leading to physiological hemodilution. This is why the diagnostic threshold is lower in the second trimester than in the first and third, but iron deficiency during pregnancy remains a very common pathological cause of anemia. [40]

Is high hemoglobin a good thing?
Not necessarily. Sometimes it's simply a consequence of dehydration, but other times it's a marker of chronic hypoxia, smoking, sleep apnea, tumor production of erythropoietin, or polycythemia vera. Excessively high hemoglobin can increase blood viscosity and the risk of thrombosis. [41]

What is the most common deficiency associated with low hemoglobin?
The most common cause of anemia worldwide is iron deficiency. Ferritin is the best laboratory test for iron deficiency, but a normal or elevated ferritin does not rule out deficiency due to inflammation. [42]

If hemoglobin is low, is simply taking iron sufficient?
No. First, you need to understand the cause. In adults, iron deficiency is often associated with chronic blood loss, including from the gastrointestinal tract. In some patients, low hemoglobin has nothing to do with iron and is caused by vitamin B12 deficiency, kidney disease, hemolysis, inflammation, or hemoglobinopathy. [43]

Which is more important: hemoglobin or ferritin?
They are different indicators. Hemoglobin indicates the presence or absence of anemia, while ferritin indicates iron stores. Iron deficiency can exist even before hemoglobin drops, and ferritin can falsely normalize or increase with inflammation. [44]

When is a vitamin B12 test needed?
It is especially important in cases of macrocytosis, neurological symptoms, long-term veganism, malabsorption, after stomach or small intestinal surgery, and if megaloblastic anemia is suspected. In severe cases, neurological impairment due to vitamin B12 deficiency can occur even without significant anemia. [45]

When is hemoglobin electrophoresis performed?
This test is necessary if a hemoglobinopathy is suspected: family history, microcytosis without iron deficiency, unclear hemolytic anemia, ethnic background, or an atypical complete blood count. Electrophoresis helps evaluate hemoglobin types and identify abnormal variants. [46]

When does hemoglobin require urgent medical attention?
A rapid assessment is necessary in the presence of shortness of breath, chest pain, fainting, severe tachycardia, ongoing bleeding, black stools, jaundice, dark urine, severe weakness, as well as persistently high hemoglobin with symptoms of hyperviscosity or thrombosis. The danger is determined not only by the number but also by the patient's overall condition. [47]

When is red blood cell transfusion considered?
For most hemodynamically stable hospitalized adults, current international guidelines support a restrictive strategy, with transfusion typically considered at hemoglobin levels less than 70 g/L. However, this is not a universal rule for all situations: in cases of active bleeding, myocardial ischemia, after certain surgeries, and in cases of severe symptoms, the decision may be different. [48]