Complete Blood Count / Haemogram

Also known as:

  • CBC
  • Hemogram
  • CBC with Differential

Why Get Tested?

To determine your general health status;

to screen for, diagnose, or monitor any one of a variety of diseases and conditions that affect blood cells, such as anemiainfectioninflammationbleeding disorder or cancer

When to Get Tested?

As part of a routine medical exam; when you have signs and symptoms that may be related to a condition that affects blood cells; at regular intervals to monitor treatment or when you are receiving treatment known to affect blood cells

The complete blood count (CBC) is often used as a broad screening test to determine an individual’s general health status. It can be used to:

  • Screen for a wide range of conditions and diseases
  • Help diagnose various conditions, such as anemiainfectioninflammationbleeding disorder or leukemia, to name just a few
  • Monitor the condition and/or effectiveness of treatment after a diagnosis is established
  • Monitor treatment that is known to affect blood cells, such as chemotherapy or radiation therapy

A CBC is a panel of tests that evaluates the three types of cells that circulate in the blood and includes the following:

      • Evaluation of white blood cells, the cells that are part of the body’s defense system against infections and cancer and also play a role in allergies and inflammation:
      • Evaluation of red blood cells, the cells that transport oxygen throughout the body:
      • Red blood cell (RBC) count is a count of the actual number of red blood cells in a person’s sample of blood.
      • Hemoglobin measures the amount of the oxygen-carrying protein in the blood.
      • Hematocrit measures the percentage of a person’s blood that consists of red blood cells.
      • Red blood cell indices are calculations that provide information on the physical characteristics of the RBCs:
        • Mean corpuscular volume (MCV) is a measurement of the average size of RBCs.
        • Mean corpuscular hemoglobin (MCH) is a calculation of the average amount of oxygen-carrying hemoglobin inside a red blood cell.
        • Mean corpuscular hemoglobin concentration (MCHC) is a calculation of the average percentage of hemoglobin inside a red cell.
        • Red cell distribution width (RDW), which may be included in a CBC, is a calculation of the variation in the size of RBCs.
      • Evaluation of platelets, cell fragments that are vital for normal blood clotting:
        • The platelet count is the number of platelets in a person’s sample of blood.
        • Mean platelet volume (MPV) may be reported with a CBC. It is a calculation of the average size of platelets.
        • Platelet distribution width (PDW) may also be reported with a CBC. It is a measurement of the variation of platelet size.

Sample Required?

A blood sample drawn from a vein in the arm.

Test Preparation Needed?


What is being tested?

The Complete Blood Count (CBC) is a test that evaluates the cells that circulate in blood. Blood consists of three types of cells suspended in fluid called plasma: white blood cells (WBCs), red blood cells (RBCs), and platelets (PLTs). They are produced and mature primarily in the bone marrow and, under normal circumstances, are released into the bloodstream as needed.

A CBC is typically performed using an automated instrument that measures various parameters, including counts of the cells that are present in a person’s sample of blood. The results of a CBC can provide information about not only the number of cell types but also can give an indication of the physical characteristics of some of the cells. A standard CBC includes the following:

  • Evaluation of white blood cells: WBC count; may or may not include a WBC differential
  • Evaluation of red blood cells: RBC counthemoglobin (Hb)hematocrit (Hct) and RBC indices, which includes mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), and sometimes red cell distribution width (RDW). The RBC evaluation may or may not include reticulocyte count.
  • Evaluation of platelets: platelet count; may or may not include mean platelet volume (MPV) and/or platelet distribution width (PDW)

The three types of cells evaluated by the CBC include:

White Blood Cells

There are five different types of WBCs, also called leukocytes, that the body uses to maintain a healthy state and to fight infections or other causes of injury. They are neutrophilslymphocytesbasophilseosinophils, and monocytes. They are present in the blood at relatively stable numbers. These numbers may temporarily shift higher or lower depending on what is going on in the body. For instance, an infection can stimulate the body to produce a higher number of neutrophils to fight off bacterial infection. With allergies, there may be an increased number of eosinophils. An increased number of lymphocytes may be produced with a viral infection. In certain disease states, such as leukemia, abnormal (immature or mature) white cells rapidly multiply, increasing the WBC count.

Red Blood Cells

Red blood cells, also called erythrocytes, are produced in the bone marrow and released into the bloodstream as they mature. They contain hemoglobin, a protein that transports oxygen throughout the body. The typical lifespan of an RBC is 120 days; thus the bone marrow must continually produce new RBCs to replace those that age and disintegrate or are lost through bleeding. A number of conditions can affect the production of new RBCs and/or their lifespan, in addition to those conditions that may result in significant bleeding. The CBC determines the number of RBCs and amount of hemoglobin present, the proportion of blood made up of RBCs (hematocrit), and whether the population of RBCs appears to be normal. RBCs normally are uniform with minimal variations in size and shape; however, significant variations can occur with conditions such as vitamin B12 and folate deficiencies, iron deficiency, and with a variety of other conditions. If there are insufficient normal RBCs present, a person is said to have anemia and may have symptoms such as fatigue and weakness. Much less frequently, there may be too many RBCs in the blood (erythrocytosis or polycythemia). In extreme cases, this can interfere with the flow of blood through the small veins and arteries.


Platelets, also called thrombocytes, are special cell fragments that play an important role in normal blood clotting. A person who does not have enough platelets may be at an increased risk of excessive bleeding and bruising. The CBC measures the number and size of platelets present.

Significant abnormalities in one or more of the blood cell populations can indicate the presence of one or more conditions. Typically other tests are performed to help determine the cause of abnormal results. Often, this requires visual confirmation by examining a blood smear under a microscope. A trained laboratorian can evaluate the appearance and physical characteristics of the blood cells such as size, shape and color, noting any abnormalities that may be present. Any additional information is noted and reported to the doctor. This information gives the doctor additional clues as to the cause of abnormal CBC results.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and AnxietyTips on Blood TestingTips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

When is it ordered?

The CBC is a very common test. Many people have a CBC performed when they have a routine health examination. If a person is healthy and has results that are within normal limits, then he or she may not require another CBC until their health status changes or until their doctor feels that it is necessary.

A CBC may be ordered when a person has any number of signs and symptoms that may be related to disorders that affect blood cells. When an individual has fatigue or weakness or has an infectioninflammation, bruising, or bleeding, a doctor may order a CBC to help diagnose the cause and/or determine its severity.

When a person has been diagnosed with a disease known to affect blood cells, a CBC will often be ordered on a regular basis to monitor their condition. Likewise, if someone is receiving treatment for a blood-related disorder, then a CBC may be performed frequently to determine if the treatment is effective.

Some therapies, such as chemotherapy, can affect bone marrow production of cells. Some medications can decreaseWBC counts overall. A CBC may be ordered on a regular basis to monitor these drug treatments.

What does the test result mean?

A doctor typically evaluates and interprets results from the components of the CBC together. Depending on the purpose of the test, a number of additional or follow-up tests may be ordered for further investigation.

The following tables briefly and generally explain what the result for each component of the CBC may mean.

Components of the CBC

Expand TableWBC Evaluation





WBC White Blood Cell Count Known as leukopenia

Known as leukocytosis

Diff White Blood Cell Differential(Not always performed; may be done as part of or in follow up to CBC)
Neu, PMN, polys Absolute neutrophil count, % neutrophils Known as neutropenia

  • Severe, overwhelming infection (sepsis)
  • Autoimmune disorders
  • Reaction to drugs, chemotherapy
  • Immunodeficiency
  • Myelodysplasia
  • Bone marrow damage (e.g., chemotherapy, radiation therapy)
  • Cancer that spreads to the bone marrow
Known as neutrophilia

  • Acute bacterial infections
  • Inflammation
  • Tissue death (necrosis) caused by trauma, heart attack, burns
  • Physiological (stress, rigorous exercise)
  • Certain leukemias (e.g., chronic myeloid leukemia)
Lymph Absolute lymphocytecount, % lymphocytes Known as lymphocytopenia

Known as lymphocytosis

Mono Absolute monocyte count, % monocytes Usually, one low count is not medically significant.Repeated low counts can indicate:

  • Bone marrow damage or failure
  • Hairy cell leukemia
  • Chronic infections (e.g., tuberculosis, fungal infection)
  • Infection within the heart (bacterial endocarditis)
  • Collagen vascular diseases (e.g., lupus, scleroderma, rheumatoid arthritis, vasculitis)
  • Monocytic or myelomonocytic leukemia (acute or chronic)
Eos Absolute eosinophilcount, % eosinophils Numbers are normally low in the blood. One or an occasional low number is usually not medically significant
Baso Absolute basophil count, % basophils As with eosinophils, numbers are normally low in the blood; usually not medically significant
  • Rare allergic reactions (hives, food allergy)
  • Inflammation (rheumatoid arthritis, ulcerative colitis)
  • Some leukemias

RBC Evaluation





RBC Red Blood Cell Count Known as anemia

  • Acute or chronic bleeding
  • RBC destruction (e.g., hemolytic anemia, etc.)
  • Nutritional deficiency (e.g., iron deficiency, vitamin B12 or folate deficiency)
  • Bone marrow disorders or damage
  • Chronic inflammatory disease
  • Kidney failure
Known as polycythemia

  • Dehydration
  • Lung (pulmonary) disease
  • Kidney or other tumor that produces excess erythropoietin
  • Smoking
  • Genetic causes (altered oxygen sensing, abnormality in hemoglobin oxygen release)
  • Polycythemia vera—a rare disease
Hb Hemoglobin Usually mirrors RBC results, provides added information Usually mirrors RBC results
Hct Hematocrit Usually mirrors RBC results Usually mirrors RBC results; most common cause is dehydration
RBC indices
MCV Mean Corpuscular Volume Indicates RBCs are smaller than normal (microcytic); caused by iron deficiency anemia or thalassemias, for example. Indicates RBCs are larger than normal (macrocytic), for example in anemia caused by vitamin B12 or folate deficiency
MCH Mean Corpuscular Hemoglobin Mirrors MCV results; small red cells would have a lower value. Mirrors MCV results; macrocytic RBCs are large so tend to have a higher MCH.
MCHC Mean Corpuscular Hemoglobin Concentration May be low when MCV is low; decreased MCHC values (hypochromia) are seen in conditions such as iron deficiency anemia and thalassemia. Increased MCHC values (hyperchromia) are seen in conditions where the hemoglobin is more concentrated inside the red cells, such as autoimmune hemolytic anemia, in burn patients, and hereditary spherocytosis, a rare congenital disorder.
RDW (Not always reported) RBC Distribution Width Low value indicates uniformity in size of RBCs Indicates mixed population of small and large RBCs; immature RBCs tend to be larger. For example, in iron deficiency anemia or pernicious anemia, there is high variation (anisocytosis) in RBC size (along with variation in shape – poikilocytosis), causing an increase in the RDW.

Platelet Evaluation





Plt Platelet Count Known as thrombocytopenia:

  • Viral infection (mononucleosismeasles, hepatitis)
  • Rocky mountain spotted fever
  • Platelet autoantibody
  • Drugs (acetaminophen, quinidine, sulfa drugs)
  • Cirrhosis
  • Autoimmune disorders
  • Sepsis
  • Leukemia, lymphoma
  • Myelodysplasia
  • Chemo or radiation therapy
Know as thrombocytosis:

  • Cancer (lung, gastrointestinal,breastovarian, lymphoma)
  • Rheumatoid arthritis, inflammatory bowel disease, lupus
  • Iron deficiency anemia
  • Hemolytic anemia
  • Myeloproliferative disorder (e.g., essential thrombocythemia)

Is there anything else I should know?

Many different conditions can result in increases or decreases in the cell populations. Some of these conditions may require treatment, while others may resolve on their own.

Recent blood transfusions affect the results of the CBC.

Normal CBC values for babies and children are different from adults.