Complete blood count: what the key parameters mean

A complete blood count (CBC) is one of the most commonly ordered laboratory tests. It provides information about the number and characteristics of red blood cells, white blood cells, and platelets. It can help detect anaemia, infection, inflammation, immune disorders, or abnormalities related to clotting, among others.

It is not, however, a test that can establish a specific diagnosis on its own. The same parameter can change for many different reasons, and a normal result does not always rule out disease. That is why a CBC should be interpreted together with symptoms, medical history, earlier test results, and other laboratory parameters.

How to read a complete blood count

A CBC result is usually divided into three main sections:

1. the red cell line, meaning erythrocytes, haemoglobin, and the indices that describe red blood cells,
2. the white cell line, meaning leukocytes and their individual types,
3. platelets, which are involved among other things in stopping bleeding.

Next to each parameter, the laboratory provides a reference range. Ranges can differ depending on the method used, the equipment, age, sex, pregnancy, and the population on which they were established. That is why a result should be compared primarily with the range printed on the specific laboratory report, not with a random table found online. A reference range usually covers about 95% of a healthy population, which means that a single result slightly outside the range does not automatically indicate disease.

The following also matter in interpretation:

• the size of the deviation from the norm,
• simultaneous changes in other parameters,
• the direction of change across successive tests,
• current symptoms,
• infections, medications, pregnancy, menstruation, smoking, physical activity, and hydration.

A CBC is often one of the basic tests ordered during the work-up of chronic fatigue, alongside ferritin or thyroid parameters.

The red cell line

Red blood cells, or erythrocytes, are responsible above all for transporting oxygen from the lungs to the tissues. They contain haemoglobin, which binds oxygen and enables it to be carried.

RBC, the red blood cell count

RBC describes the number of red blood cells per unit of blood volume.

A low RBC may occur, among other reasons, in:

• anaemia,
• blood loss,
• nutritional deficiencies,
• chronic kidney disease,
• chronic inflammation,
• bone marrow disorders,
• excessive breakdown of erythrocytes.

A high RBC may be related to:

• dehydration and blood concentration,
• smoking,
• staying at high altitude,
• chronic hypoxia,
• certain lung or heart diseases,
• less often myeloproliferative disorders, in which the marrow produces too many blood cells.

The number of erythrocytes alone does not, however, precisely describe the blood's ability to transport oxygen. Haemoglobin concentration is more important in assessing anaemia.

HGB or Hb, haemoglobin

Haemoglobin is a protein found inside erythrocytes. It is one of the most important parameters used to diagnose anaemia.

According to World Health Organization guidelines, in adults aged 15 to 65 anaemia is diagnosed when haemoglobin concentration falls below:

• 120 g/l in non-pregnant women,
• 130 g/l in men.

Different cut-off values apply in pregnancy and depend on the trimester as well. The result also requires interpretation with respect to age, altitude above sea level, and other factors that influence haemoglobin concentration.

Low haemoglobin indicates anaemia but does not yet point to its cause. Possible reasons include:

• iron deficiency,
• vitamin B12 or folate deficiency,
• blood loss, for example due to heavy menstruation or gastrointestinal bleeding,
• chronic diseases and inflammation,
• kidney disease,
• haemolysis, meaning excessive breakdown of erythrocytes,
• bone marrow disorders.

High haemoglobin may result from dehydration, staying at high altitude, smoking, chronic hypoxia, or excessive erythrocyte production.

It is important not to start iron supplementation based on low haemoglobin alone. Anaemia does not always stem from iron deficiency, and its cause should be confirmed with appropriate tests.

HCT, the haematocrit

Haematocrit describes what percentage of blood volume is made up of erythrocytes.

It usually changes in the same direction as haemoglobin. A low haematocrit may accompany anaemia or overhydration, while a high one occurs, among other reasons, in dehydration or an increased number of erythrocytes.

Haematocrit is particularly sensitive to changes in plasma volume. A dehydrated person may have an apparently elevated haematocrit and haemoglobin even though the actual red cell mass has not increased.

Red cell indices

Red cell indices help describe the size of erythrocytes and the amount of haemoglobin they contain. They are especially useful when establishing the possible cause of anaemia.

MCV, the mean corpuscular volume

MCV describes the average size of a red blood cell. On its basis, anaemia can be provisionally classified as microcytic, normocytic, or macrocytic.

Low MCV

A low MCV means that erythrocytes are on average smaller than expected. The most common possible causes are:

• iron deficiency,
• thalassaemias and other disorders of haemoglobin synthesis,
• some anaemias associated with chronic diseases,
• less often disorders of copper metabolism, lead poisoning, or sideroblastic anaemias.

A low MCV is not synonymous with iron deficiency. Further work-up usually assesses, among other things, ferritin, transferrin saturation, CRP, the reticulocyte count, and a blood smear.

Normal MCV

Anaemia can also occur with a normal red cell size. Possible causes include:

• recent bleeding,
• anaemia of chronic disease,
• kidney disease,
• haemolysis,
• bone marrow disorders,
• the coexistence of several deficiencies, for example iron and vitamin B12.

A normal MCV therefore does not rule out iron deficiency or any other cause of anaemia.

High MCV

A high MCV means that erythrocytes are larger than average. It may occur in:

• vitamin B12 deficiency,
• folate deficiency,
• consumption of large amounts of alcohol,
• liver diseases,
• hypothyroidism,
• the use of certain medications,
• increased reticulocyte production after bleeding or haemolysis,
• myelodysplastic syndromes and other bone marrow disorders.

A high MCV does not always mean vitamin B12 deficiency. Conversely, vitamin B12 deficiency can occur without evident macrocytosis, especially early on or when there is a coexisting iron deficiency.

MCH, the mean corpuscular haemoglobin

MCH shows how much haemoglobin is on average present in a single red blood cell.

A low MCH often occurs together with a low MCV and may suggest reduced haemoglobin production, for example in iron deficiency or thalassaemia. A high MCH often accompanies an increased MCV, because a larger cell can contain more haemoglobin.

MCH should be interpreted together with haemoglobin, MCV, MCHC, and RDW. On its own it has limited diagnostic value.

MCHC, the mean corpuscular haemoglobin concentration

MCHC describes the average concentration of haemoglobin within the volume of red blood cells.

A low MCHC means that the cells are less saturated with haemoglobin. It may accompany iron deficiency, among other conditions.

A markedly elevated MCHC is less common. It may appear in certain disorders of erythrocyte structure, but it often also results from factors that interfere with the measurement, such as sample haemolysis, clumping of cells, or a high lipid content in the blood. Such a result requires assessment in combination with the remaining parameters and, if needed, a smear.

RDW, the red cell distribution width

RDW describes how much red blood cells differ from one another in size.

A high RDW means a large variation among erythrocytes. It may appear, among other situations:

• in early iron deficiency,
• in vitamin B12 or folate deficiency,
• after bleeding,
• during blood recovery after starting treatment for anaemia,
• when several different deficiencies coexist.

RDW cannot on its own diagnose a specific disease. It can, however, be helpful in combination with MCV. For example, a low MCV and a high RDW often occur in iron deficiency, whereas a low MCV with a normal RDW may prompt consideration of other causes of microcytosis. This is not, however, a rule sufficient to make a diagnosis.

Reticulocytes

Reticulocytes are young forms of erythrocytes. They are not always part of a basic CBC, but a doctor may order them in the work-up of anaemia.

An elevated reticulocyte count may indicate that the marrow is intensively producing new cells, for example after bleeding, during haemolysis, or in response to effective treatment of a deficiency.

A low or insufficiently high reticulocyte count in the presence of anaemia may suggest limited erythrocyte production, for example due to deficiencies, kidney disease, or bone marrow disorders.

The white cell line

White blood cells, or leukocytes, participate in the immune response. A CBC measures their total number and their distribution among individual cell types.

WBC, the total leukocyte count

An elevated leukocyte count is called leukocytosis. It may appear, among other situations, in:

• infection,
• acute or chronic inflammation,
• intense physical effort,
• strong physical or emotional stress,
• smoking,
• the use of certain medications, including glucocorticoids,
• injury or surgery,
• diseases of the haematopoietic system.

Leukocytosis is not automatic proof of a bacterial infection. The WBC level alone does not distinguish a bacterial infection from a viral one, nor does it identify its location.

A low leukocyte count, or leukopenia, may occur in:

• some viral infections,
• the action of medications,
• autoimmune diseases,
• nutritional deficiencies,
• bone marrow disorders,
• chemotherapy or radiotherapy,
• severe infections.

When the WBC is abnormal, it is crucial to check which type of leukocyte is responsible for the change.

The leukocyte differential

The result may present individual types of leukocytes as percentage and absolute values, usually marked with the abbreviation "abs." or the "#" sign.

Absolute values are usually more useful. For example, a normal neutrophil percentage does not rule out neutropenia if the total leukocyte count is low. Conversely, the lymphocyte percentage may look elevated only because the neutrophil count has decreased.

NEUT, the neutrophils

Neutrophils are an important part of the defence against microorganisms, especially bacteria and fungi.

An elevated neutrophil count may occur in:

• bacterial infections,
• inflammation,
• injury or tissue necrosis,
• strong stress,
• intense physical effort,
• smoking,
• pregnancy,
• the use of glucocorticoids,
• certain bone marrow disorders.

A low neutrophil count is called neutropenia. Its possible causes include viral infections, medications, autoimmune diseases, vitamin B12 or folate deficiency, bone marrow damage, and anticancer treatment.

The lower the absolute neutrophil count, the higher the risk of infection may be. A fever in a person with significant neutropenia requires urgent medical assessment.

LYMPH, the lymphocytes

Lymphocytes are responsible, among other things, for recognising antigens, producing antibodies, and immune memory.

An elevated lymphocyte count may accompany:

• viral infections,
• some bacterial infections, such as whooping cough,
• the recovery period after an infection,
• less often lymphoproliferative disorders.

A low lymphocyte count may occur during acute infections, with the use of glucocorticoids, in autoimmune diseases, immune disorders, malnutrition, and some cancers.

Persistent lymphocytosis, especially with enlarged lymph nodes, night sweats, unintentional weight loss, or changes in other blood lines, requires further work-up.

MONO, the monocytes

Monocytes participate in engulfing microorganisms and damaged cells and in regulating the inflammatory response.

Elevated monocytes may occur, among other situations:

• during or after an infection,
• in chronic inflammation,
• in autoimmune diseases,
• with smoking,
• in certain bone marrow and haematopoietic disorders.

A small, transient deviation is often not very significant. More attention is warranted for monocytosis that persists across successive tests, especially when accompanied by other abnormalities.

EOS, the eosinophils

Eosinophils are involved, among other things, in allergic reactions and the response to certain parasites.

An elevated eosinophil count may appear in:

• allergy,
• asthma,
• atopic dermatitis,
• drug reactions,
• certain parasitic infections,
• autoimmune and inflammatory diseases,
• less often haematological disorders.

Eosinophilia does not automatically mean a parasitic infection. Its interpretation should take into account symptoms, medications taken, travel, diet, and the presence of allergic diseases.

BASO, the basophils

Basophils are involved, among other things, in allergic and inflammatory reactions. They make up a very small fraction of leukocytes, so small percentage changes may result from fluctuations in other types of white blood cells.

An elevated basophil count may occur in allergies, chronic inflammation, some infections, and endocrine diseases. Persistent, marked basophilia may require ruling out myeloproliferative disorders.

Platelets

Platelets are involved in forming a clot and stopping bleeding. Their number does not, however, describe the whole efficiency of the clotting system. A person can have a normal platelet count and at the same time impaired platelet function or another disorder of haemostasis.

PLT, the platelet count

A low platelet count is called thrombocytopenia. It may result from:

• reduced platelet production in the marrow,
• increased destruction or consumption of platelets,
• autoimmune diseases,
• infections,
• the action of medications,
• liver diseases and enlargement of the spleen,
• vitamin B12 or folate deficiency,
• pregnancy,
• bone marrow disorders.

The risk of bleeding depends not only on the platelet count but also on the rate of its decline, the cause of the disorder, medications used, and coexisting conditions. A very low platelet count, spontaneous bleeding, numerous petechiae, or blood in the urine or stool requires urgent consultation.

An elevated platelet count is called thrombocytosis. It is often reactive and may appear in response to:

• infection,
• inflammation,
• iron deficiency,
• bleeding,
• surgery or injury,
• removal of the spleen,
• cancer.

Less often it results from a primary bone marrow disease, such as essential thrombocythaemia. In adults, thrombocytosis is usually considered a value of at least 450 × 10⁹/l, but a single result should be confirmed and assessed in the clinical context.

Does a low PLT always mean thrombocytopenia?

No. It happens that platelets clump together in a blood sample collected into an EDTA tube. The analyser may then count too few of them. This phenomenon is called pseudothrombocytopenia.

In such a situation the laboratory may report platelet aggregates, and a doctor may order a repeat test from a sample collected into a tube with a different anticoagulant, along with a smear assessment. Pseudothrombocytopenia is a laboratory phenomenon, not a real deficiency of platelets in the patient's blood.

MPV, the mean platelet volume

MPV describes the average size of platelets.

Larger platelets are often younger, which is why an elevated MPV may appear when the marrow increases platelet production in response to their loss or destruction. A low MPV may occur with reduced platelet production, but this parameter is sensitive to the measurement method and the time since sample collection.

MPV should not be interpreted on its own or treated as an indicator of a specific disease.

What do the IG, NRBC, and analyser "flags" mean?

Some laboratories report additional parameters.

IG, meaning immature granulocytes, are young forms of cells belonging to the neutrophil line. An elevated number may occur during a strong marrow response, for example in an acute infection, inflammation, or significant stress on the body. The result should be assessed together with the WBC, neutrophils, and symptoms.

NRBC are nucleated red blood cells, which normally remain in the marrow. Their presence in the peripheral blood of an adult may appear with a heavy load on the marrow, significant hypoxia, haemolysis, severe infections, or haematological disorders, and usually requires explanation.

The analyser may also report information such as:

• the presence of immature cells,
• suspected blasts,
• platelet aggregates,
• atypical lymphocytes,
• abnormal cell size or structure.

An analyser flag is not a diagnosis. It is a signal that the sample may require manual microscopic assessment.

What is a blood smear?

A blood smear involves assessing cells under a microscope. It allows one to see their real size, shape, maturity, and other features that cannot always be reliably assessed with an automated analyser.

A smear may be helpful, among other situations, in:

• unclear anaemia,
• significant leukocytosis or leukopenia,
• suspected immature cells,
• abnormalities involving several cell lines,
• suspected haemolysis,
• a very low platelet count,
• the presence of flags on the automated result.

The simultaneous occurrence of abnormalities involving erythrocytes, leukocytes, and platelets may point to a process involving the bone marrow and usually requires more in-depth work-up.

Do you need to fast before a CBC?

A CBC on its own usually does not require fasting. You can eat and drink normally beforehand. If, however, other tests are performed together with the CBC, for example glucose, insulin, or some metabolic parameters, the laboratory may recommend fasting for several hours. It is always worth following the instructions for the whole set of tests.

Before the blood draw it is worth avoiding dehydration and, if possible, very intense training right before the test. For comparability of results, successive tests are best performed under similar conditions.

Summary

A CBC provides a great deal of information, but no single parameter should be interpreted in isolation from the others.

Haemoglobin helps diagnose anaemia, MCV and RDW can narrow the list of its causes, the leukocyte differential shows which immune cells are responsible for a change in WBC, and the platelet count helps assess potential disorders of their production or consumption.

What matters most, however, is looking at the whole set of results, the symptoms, and the changes occurring over time. A single value outside the reference range does not always mean disease. Likewise, a normal CBC does not rule out all deficiencies and health problems. This is one of the reasons why results are best interpreted together with symptoms, as part of a broader preventive approach to health, rather than in isolation.

Frequently asked questions

Does low haemoglobin always mean iron deficiency?

No. Iron deficiency is a common cause of anaemia, but low haemoglobin can also result from vitamin B12 or folate deficiency, kidney disease, chronic inflammation, bleeding, haemolysis, or bone marrow disorders. Ferritin and transferrin saturation, among other tests, are used to assess iron status.

Does a normal CBC rule out iron deficiency?

No. Iron deficiency can develop in stages. At first the iron stores decrease, while haemoglobin and MCV may still remain within the reference range. That is why, in the presence of symptoms or risk factors, a CBC alone may be insufficient.

Do high leukocytes always mean a bacterial infection?

No. Leukocytosis can also appear with a viral infection, inflammation, stress, intense physical effort, smoking, pregnancy, the use of glucocorticoids, and in haematological diseases. It helps to determine which type of leukocyte is elevated.

Is a single abnormal parameter a reason for concern?

Not necessarily. A small deviation may result from natural variability, a past infection, dehydration, physical effort, or factors related to sample collection and analysis. What matters is the size of the deviation, the symptoms, the other parameters, and whether the change persists across successive tests.

Can you start iron supplementation based on a CBC alone?

This is not recommended. Before starting supplementation, the deficiency should be confirmed and its cause established. In the case of iron deficiency it may also be important to clarify why the loss or insufficient absorption of iron occurred.

Do you need to repeat a CBC after an infection?

Many changes associated with an acute infection are transient. If the test was performed during an illness, a doctor may recommend repeating it after the symptoms have resolved. The timing depends on the type and scale of the abnormality and on the patient's condition.

Which matters more in a differential: the percentage or the absolute value?

The absolute value usually matters more. The percentage depends on the total leukocyte count and the proportion of the remaining cells, which is why it may appear elevated or reduced despite a normal actual number of cells.

References

1. World Health Organization. Guideline on haemoglobin cutoffs to define anaemia in individuals and populations. Geneva: World Health Organization; 2024. ISBN 978-92-4-008854-2.
2. El Brihi J, Pathak S. Normal and Abnormal Complete Blood Count With Differential. In: StatPearls. Treasure Island: StatPearls Publishing; updated 8 June 2024.
3. Tefferi A, Hanson CA, Inwards DJ. How to interpret and pursue an abnormal complete blood cell count in adults. Mayo Clin Proc. 2005;80(7):923–936. doi:10.4065/80.7.923.
4. Riley LK, Rupert J. Evaluation of patients with leukocytosis. Am Fam Physician. 2015;92(11):1004–1011.
5. Nagao T, Hirokawa M. Diagnosis and treatment of macrocytic anemias in adults. J Gen Fam Med. 2017;18(5):200–204. doi:10.1002/jgf2.31.
6. National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management. NICE Guideline NG239. 2024.
7. Lardinois B, Favresse J, Chatelain B, Lippi G, Mullier F. Pseudothrombocytopenia: a review on causes, occurrence and clinical implications. J Clin Med. 2021;10(4):594. doi:10.3390/jcm10040594.
8. Rumi E, Cazzola M. How I treat essential thrombocythemia. Blood. 2016;128(20):2403–2414. doi:10.1182/blood-2016-05-643346.
9. Cappellini MD, Russo R, Andolfo I, Iolascon A. Inherited microcytic anemias. Hematology Am Soc Hematol Educ Program. 2020;2020(1):465–470.
10. Palmer L, Briggs C, McFadden S, et al. ICSH recommendations for the standardization of nomenclature and grading of peripheral blood cell morphological features. Int J Lab Hematol. 2015;37(3):287–303.
11. MSD Manual Professional Edition. Laboratory Reference Ranges. Updated: May 2026.
12. Mayo Clinic. Complete blood count: how you prepare. Updated: 27 June 2026.

This article is educational in nature and does not replace an individual medical consultation or the interpretation of results in relation to a specific person's health.