Complete blood count


A complete blood count (CBC) is a series of tests used to evaluate the composition and concentration of the cellular components of blood. It consists of the following tests: red blood cell (RBC) count, white blood cell (WBC) count, and platelet count; measurement of hemoglobin and mean red cell volume; classification of white blood cells (WBC differential); and calculation of hematocrit and red blood cell indices . The hematocrit is the percentage of blood by volume that is occupied by the red cells (i.e., the packed red cell volume). Red blood cell indices are calculations derived from the red blood cell count, hemoglobin, and hematocrit that aid in the diagnosis and classification of anemia.


The CBC provides valuable information about the blood and to some extent the bone marrow, which is the blood-forming tissue. The CBC is used for the following purposes:


The CBC requires a sample of blood collected from a vein. The nurse or phlebotomist inserting the needle should clean the skin first. The tourniquet should be removed from the arm as soon as the blood flows. If a fingerstick is used to collect the blood, care must be taken to wipe away the first drop, and not to squeeze the finger excessively as this causes the blood to be diluted by tissue fluid. Many drugs affect the results by causing increased or decreased RBC, WBC, and/or platelet production. Medications should be taken into account when interpreting results.


The CBC is commonly performed on an automated hematology analyzer using well mixed whole blood that is added to a chemical called EDTA to prevent clotting. A CBC is a group of tests used to quantify the number of RBCs, WBCs, and platelets, provide information about their size and shape, measure the hemoblobin content of RBCs, determine the percentage and absolute number of the five white blood cell types, and identify early and abnormal blood cells. These tests are performed simultaneously, (usually in less than one minute), using an automated hematology analyzer. When the performance limit of the automated hematology analyzer is exceeded, sample dilution or pretreatment, manual smear review, or manual cell counts may be required. Each laboratory has established rules for determining the need for manual smear review based upon specific CBC parameters. For example, a manual differential is always performed when nucleated immature red blood cells are found on an electronic cell count.

Electronic cell counting

Electronic blood cell counting is based upon the principle of impedance (i.e., resistance to current flow). Some hematology analyzers combine impedance counting with light scattering to measure platelets. A small sample of the blood is aspirated into a chamber (the WBC counting bath) and diluted with a balanced isotonic saline solution that is free of particles. The diluted blood sample is split into two parts, one for counting RBCs and platelets and the other for counting WBCs. The RBC portion is transferred to the RBC/platelet counting bath where it is diluted further. The other portion remains in the WBC bath and a detergent (lysing agent) is added to destroy (hemolyze) the red blood cells. A small portion of the diluted fluid in each bath is allowed to flow past a small aperture. An electrical current is produced in each aperture by two electrodes, one on the inside and the other on the outside of the aperture. The saline solution is responsible for conducting current between the electrodes. The cells move through the aperture one at a time. When a cell enters the aperture, it displaces a volume of electrolyte equal to its size. The cell acts as an electrical resistor, and impedes the flow of current. This produces a voltage pulse, the magnitude of which is proportional to the size of the cell. Instrument electronics are adjusted to discriminate voltage pulses produced by different cells. These adjustments are called thresholds. For example, the threshold for counting a RBC is equivalent to a cell volume of 36 femtoliters or higher. Voltage pulses that are equivalent to volumes of 2–20 femtoliters are counted as platelets. This process is repeated two more times so that the RBC, WBC, and platelet counts are performed in triplicate. Each time frame for counting is several seconds and many thousands of cells are counted. The computer processes the counting data first by determining the agreement between the three counts. If acceptable criteria are met, the counts are accepted and used to calculate the result.

The hemoglobin concentration is measured optically using the solution in the WBC bath. The lysing agent contains potassium cyanide that reacts with the hemoglobin to form cyanmethemoglobin. The optical density of the cyanmethemoglobin is proportional to hemoglobin concentration.

The voltage pulses produced by the white blood cells depend upon the size of the cell and its nuclear density. Therefore, the pulses may be analyzed to differentiate between the types of WBCs found. For example, lymphocytes are the smallest WBCs and comprise the lower end of the size scale. Monocytes, prolymphocytes, and immature granulocytes comprise the central area of the WBC histogram, and mature granulocytes comprise the upper end. In addition to cell sizing, automated instruments may use any of three other methods to distinguish between subpopulations. These are radio frequency conductance, forward and angular light scattering, and fluorescent staining.

Red blood cell count

The red cells, the most numerous of the cellular elements, carry oxygen from the lungs to the body's tissues. They are released from the bone marrow into the blood in an immature form called the reticulocyte that still retains much of the cellular RNA needed for hemoglobin production. Reticulocytes may be counted on some automated analyzers and are an index to recovery from anemia. The average life span of RBCs in the circulation is approximately 120 days.

The red blood cell (RBC) count determines the total number of red cells (erythrocytes) in a sample of blood. Most anemias are associated with a low RBC count, hemoglobin, and hematocrit. Common causes include excessive bleeding; a deficiency of iron, vitamin B 12 , or folic acid; destruction of red cells by antibodies or mechanical trauma; bone marrow malignancy and fibrosis; and structurally abnormal hemoglobin. The RBC count is also decreased due to cancer, kidney diseases, and excessive IV fluids. An elevated RBC count may be caused by dehydration, hypoxia (decreased oxygen), or a disease called polycythemia vera. Hypoxia may result from high altitudes, chronic obstructive lung diseases, and congestive heart failure.

Hematocrit and cell indices

The hematocrit is a test that measures the volume of blood in percent that is comprised of the red blood cells. Automated cell counters calculate the hematocrit by multiplying the RBC count by the mean red cell volume. A decrease in the number or size of red cells also decreases the amount of space they occupy, resulting in a lower hematocrit. Conversely, an increase in the number or size of red cells increases the amount of space they occupy, resulting in a higher hematocrit. Thalassemia minor, a genetic cause of anemia, is an exception in that it usually causes an increase in the number of red blood cells, but because they are small, it results in a decreased hematocrit.

The three main RBC indices are used to determine the average size and hemoglobin content of the RBCs and they help determine the cause of anemia. The three indices are described below:

The mechanisms by which anemia occurs will alter the RBC indices in a predictable manner. Therefore, the RBC indices permit the physician to narrow down the possible causes of an anemia. The MCV is an index of the size of the RBCs. When the MCV is below normal, the RBCs will be smaller than normal and are described as microcytic. When the MCV is elevated, the RBCs will be larger than normal and are termed macrocytic. RBCs of normal size are termed normocytic. Failure to produce hemoglobin results in smaller than normal cells. This occurs in many diseases including iron deficiency anemia, thalassemia (an inherited disease in which globin chain production is deficient), and anemias associated with chronic infection or disease. Macrocytic cells occur when division of RBC precursor cells in the bone marrow is impaired. The most common causes of macrocytic anemia are vitamin B 12 deficiency, folate deficiency, and liver disease. Normocytic anemia may be caused by decreased production (e.g., malignancy and other causes of bone marrow failure), increased destruction (hemolytic anemia), or blood loss. The RBC count is low, but the size and amount of hemoglobin in the cells is normal.

White blood cell count

The majority of CBCs include both a WBC count and an automated differential. A differential determines the percentage of each of the five types of mature white blood cells. An elevated WBC count occurs in infection, allergy, systemic illness, inflammation, tissue injury, and leukemia. A low WBC count may occur in some viral infections, immunodeficiency states, and bone marrow failure. The WBC count provides clues about certain illnesses, and helps physicians monitor a patient's recovery from others. The differential will reveal which WBC types are affected most. For example, an elevated WBC count with an absolute increase in lymphocytes having an atypical appearance is most often caused by infectious mononucleosis. The differential will also identify early WBCs that may be reactive (e.g., a response to acute infection) or the result of a leukemia.

When the electronic WBC count is abnormal or a cell population is flagged, meaning that one or more of the results is atypical, a manual differential is performed. In that case, a wedge smear is prepared. This is done by placing a drop of blood on a glass slide, and using a second slide to pull the blood over the first slide's surface. The smear is air dried, then stained with Wright stain and examined under a microscope using oil immersion (1000x magnification). One hundred white cells are counted and identified as either neutrophils, lymphocytes, monocytes, eosinophils, or basophils based on the shape and appearance of the nucleus, the color of cytoplasm, and the presence and color of granules. The purpose is to determine if these cells are present in a normal distribution, or if one cell type is increased or decreased. Any atypical or immature cells also are counted.

In addition to determining the percentage of each mature white blood cell, the following tests are performed as part of the differential:

WBCs consist of two main subpopulations, the mononuclear cells and the granulocytic cells. Mononuclear cells include lymphocytes and monocytes. Granulocytes include neutropohils (also called polymorphonuclear leukocytes or segmented neutrophils), eosinophils, and basophils. Each cell type is described below:

Platelet count

Platelets are disk-shaped structures formed by the detachment of cytoplasm from megakaryocytes. They aid in the coagulation process by attaching or adhering to the walls of injured blood vessels, where they stick together to form the initial platelet plug. A low platelet count may occur in patients with AIDS, viral infections, lymphoma, and lupus erythematosus, or in patients taking certain drugs, most notably quinine and quinidine. Decreased platelet production is also a cause of thrombocytopenia, and may be due to aplastic anemia, leukemia, lymphoma, or bone marrow fibrosis. A low platelet count can occur due to increased destruction. This can result from antibody production that is often drug-induced (heparin treatment being a prominent cause). Increased destruction also results from autoantibody production as occurs in idiopathic thrombocytopenic purpura (ITP) and thrombotic episodes that consume platelets such as occur in thrombotic thrombocytopenic purpura (TTP), disseminated intravascular coagulation (DIC), and hemolytic-uremic syndrome (HUS). Inherited (congenital) thrombocytopenia can be caused by Glanzmann's thrombasthenia, Fanconi syndrome, and Wiskott-Aldrich syndrome.

Thrombocytosis, an increased platelet count, is most often caused by a reaction to injury or inflammation. In these cases the platelet count increases transiently and is usually within the range of 400,000–800,000 per microliter. Persistent or higher counts are usually associated with myeloproliferative disease (malignant disease involving blood forming cells) such as chronic granulocytic (myelogenous) leukemia, polycythemia vera, or primary (essential) thrombocythemia.

The platelet count is most often measured by impedance counting but is performed manually when the platelet count is very low, platelet clumping is observed, or abnormally large (giant) platelets are present. Often these abnormalities and others such as cryoglobulinemia, cell fragmentation (hemolysis), and microcytic RBCs are signaled by abnormal RBC and platelet indices and abnormal population flags. An abnormal mean platelet volume or platelet histogram indicates that morphological platelet abnormalities are present and the platelets should be observed from a stained blood film to characterize the abnormality. The platelet count can be estimated using the Wright-stained blood smear used for a differential WBC count by multiplying the average number of platelets per oil immersion field by 20,000. Platelet estimates should correlate with actual counts. When they disagree, the platelet count should be repeated and a manual count performed if necessary.


The CBC does not require fasting or any special preparation.


Discomfort or bruising may occur at the puncture site. Applying pressure to the puncture site until the bleeding stops helps to reduce bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be treated by resting awhile.


Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test.

Normal results

CBC values vary by age and sex. Normal values are ultimately determined by the laboratory performing the test. As a guide, the normal values for men and nonpregnant women are as follows:

Normal adult results for red blood cell indices are as follows:

In addition to normal values, critical values (alert, panic values) are established for hemoglobin (and hematocrit), WBC count, and platelet count. Precipitously low hemoglobin is associated with hypoxia that can have life-threatening complications. Extremely low WBCs indicates an inability to fight infection and a high risk of sepsis. A severely reduced platelet count predisposes the patient to spontaneous internal bleeding. Representative critical values are shown below.

Abnormal blood count results are seen in a variety of conditions. One of the most common is anemia, which is characterized by a low RBC count, hemoglobin, and hematocrit. The category into which a person's anemia is placed is in part based upon the red blood cell indices provided. The indices provide a significant clue as to the cause of the anemia, but further testing is needed to confirm a specific diagnosis. The most common causes of macrocytic anemia (high MCV) are vitamin B 12 and folic acid deficiencies. Lack of iron in the diet, thalassemia (a type of hereditary anemia), and chronic illness are the most common causes of microcytic anemia (low MCV). Normocytic anemia (normal MCV) can be caused by kidney and liver disease, bone marrow disorders, leukemia, excessive bleeding, or hemolysis of the red blood cells. Iron deficiency and thalassemia are the most common causes of hypochromic anemia (low MCHC). Normocytic anemias are usually also normochromic and share the same causes. The red cell distribution width (RDW) is increased in anemias caused by deficiencies of iron, vitamin B 12 , or folic acid. Abnormal hemoglobins, such as in sickle cell anemia, can change the shape of red blood cells as well as cause them to hemolyze, or rupture. The abnormal shape and the cell fragments resulting from hemolysis increase the RDW. Conditions that cause more immature cells to be released into the bloodstream, such as severe blood loss, will increase the RDW. The larger size of immature cells creates a distinct size variation.

Infections and leukemias are associated with increased numbers of WBCs. Increases or decreases in the percentage of each white cell can be associated with a number of diseases or conditions, including cancer, leukemia, anemia, multiple sclerosis, allergies, parasitic and viral diseases, infections, and tissue damage.



Chernecky, Cynthia C. and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia, PA: W. B. Saunders, 2001.

Henry, John B. Clinical Diagnosis and Management by Laboratory Methods. Philadelphia: W. B. Saunders, 2001.

Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests. 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Wallach, Jacques. Interpretation of Diagnostic Tests. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2000.

Victoria E. DeMoranville Mark A. Best

Also read article about Complete Blood Count from Wikipedia

User Contributions:

I found this article very informative. As a clinical instructor, I find it useful as a tool for my students.
adela hunt
this article is excellent and easey to to understand taht components of cbc with differtntial.
The information was very understandable and interesting.
mona lisa
It would provide better information if there is a specified value of all the components of CBC for children with detailed ages abd sex.
Finally found the "normal" range of band cells I was looking for. I'm trying to better understand my daughter's CBC, confirms for me that putting her on antibiotics was the right thing to do! Wish the Dr explained it as thoroughly! Thank you!
Please I need an answer for this urine test I did for my daughter. Is this something I should worry? Leucocyte: high and in small groups. RBC :6-8 per new field. What this mean? Please write me in this
Thank You.
I found the Normal ranges.But their are things I cant understand.My friend told me that having sex with someone can help to decrease the CBC.Is that true?pls answer my questions.THANK YOU!..please send your ANSWER in my email add :
Good Information provided on the CBC, i would also like to know if more on the subject, with regards to the patience undergone a Angioplasty, is the increase in Plattes and WBC count things to worry the WBC count is 15.10 and Plattes 457000, is there any further test required in these cases. Kindly let me know as soon as possible. you can write me on
an informative topic which will increase my professional skills
Maria Herrera
Very interesting article, although didn't find the answer to the question about the blood lack of basophils.
Curious about an indicator for leukemia vs bad infection.The
patient had myelodysplasia but was in a nursing home with c diff and mrsa rampant her WBC shot up to 35 but she had diarrhea as well.Her platelets were allowed to go down to 26-30 her hct was
25 or so.
Thank you
amin williams
thank you for this information but i hope you send to my how can i write the comment of cbc (anaemia & mild hypochromic & etc.....................)
mimo aziz
excellent plz how can i get the chemical formula of the hematology analyzer solutions , can u tell me the site or reply at my email , thanks
Thanks for an informative review. I learned this all years ago: now at 75, my husband has essential thrombocythemia, undergoing chemo with complications, and this explains things for him better then I ever could.
I found this article very helpful to me in finding the significance of Cbc result to my patients' condition. Thanks. But it would be better if you included all the components. i didn't find info about segmenters.
I got a blood test done for my son who is 11years old as he has swollen tonsils and results show few immature Atypical cells(8%) and monocytes 9%. TLC is 6700cells/cumm. I am confused as doctor says it is due to infection. Kindly guide me and let me know which tests I should get done. My email is
Easy to understand.. I'd like to ask, what would be the cause of this result... I had my CBC 2 days ago, and my hemoglobin, hematocrit and RBC are low.. as well as my Segmenters, Eosinophils and monocytes, but my leucocytes is 9.8 normal. together with my cbc is my urinalysis, its RBC is 124/uL// Please send your response in this address. Thank you so much..
This is a very nice description about RBC and WBC.IT is very helpful in eradicationg some common questions comming to our mind
Article very helpful. Gave good information about bloodwork and how values relate to each other.
My lab results were RBC 3.70 (low), MCV 104 (high) and MCH 35.4 (high).My Cholestrol is 223 and LDL 164.
I was diagnosed with Fibromylgia 16 years ago and have it pretty much under control with medication. My past labs (past 2 years) have shown the above counts. I feel tired, arm and hand pain and swelling of joints especially knuckles, neck and lower back pain. I am 57 years old, not overwieght and otherwise in good health. What diseases are associated with the information from my lab results? Thanks.
I have had an unusually bad three years, with rashes, allergies (I never had before) and duodenal and gastric problems. My blood tests were supposedly normal, but I had High MCV , MCH, and low Lymp%. Should I be concerned and what kind of doctor should I see?
I want to know what tests are necessary for thalassemia patients before entry to the work of any surgical procedure
it is required to perform the CBC WHILE FASTING?

Very informative. Thank you. I couldn't find such an informative article in the website. Kudos to its author
This is good. maybe a good paper to print and hang onto
I just want to know RDW(rbc distribution width) test.Thank u.
I ask can i calculate rdw by manual count and the blood film
i wanted to ask some questions.
1-dimension of WBC's squares?
2-what is the dilution in TLC and how?
3-how you calculate the area of RBC and WBC sqaures?
this is good info. though they say CBC does not show if a person is HIV positive, but the point is
it gives clues to the doctors if a person is infected with HIV bcos if the results came out normal
that means a person is free to any kind of infection in his/her blood cells. if the results were abnormal in any tests, especially on white blood cells, that means a person is infected and the doctor will call for a further tests like HIV. remember, when u contract HIV, first of all it starts a fight with your white blood cells cos thats your defence in your body. HIV always win the
fight no mater how long it takes bcos its a killer. take care.
My Hametology test is completely normal.Does it mean that i have no HIV?
dr gurpreet
good article .it give me valueable information it is very help for my profional work
I just had my cbc a few days ago, and my hemoglobin result was 9.8, /uLRBC was 4m/uL, the rest were normal, however, i have irregular periods and i just had my period for almost 2 weeks a day before i had my cbc, could that possibly had some effect on my results? I had mild anemia before, but this results just really surprised me..answers would be great! You can email me at
jun lustre
My 12 years old daughter laboratory result lately, her segmenters is low and eosinophils is high what it indicates? thank you. segmenters- 0.505 normal values (0.55- 0.65), eosinophils - 0.093 normal values (0.02- 0.04). At present she undergoes treatment zalpen injection every 21 days for almost 3 years already for the period of 4 years upon doctor's advice because she was diagnosed with rheumatic heart disease.
Well yes and no .White blood cells, or leukocytes, are cells of the mimune system defending the body against both infectious disease and foreign materials.The number of leukocytes in the blood is often an indicator of disease. There are normally between 4d7109 (10 to teh 9th) and 11d7109 white blood cells in a litre of blood, making up approximately 1% of blood in a healthy adult.In conditions such as leukemia, the number of leukocytes is higher than normal, and the red blood cells will be lesser. So its not leukemia.In leukopenia, this number is much lower. The lower limit is generally regarded as 4,000 to 5,000 cells/mmb3. So if your bosses count is this much, then she should be worries. As the principal function of white cells is to combat infection, a decrease in the number of these cells can place patients at increased risk for infection.CausesLow white cell counts are associated with chemotherapy, radiation therapy, leukemia (as malignant cells overwhelm the bone marrow), myelofibrosis and aplastic anemia (failure of white and red cell creation, along with poor platelet production). In addition, many common medications can cause leukopenia (eg. minocyclen, a commonly prescribed antibiotic).Other causes of low white blood cell count include: Influenza, systemic lupus erythematosus, typhus, malaria, HIV, tuberculosis, dengue, Rickettsial infections, enlargement of the spleen, folate deficiencies, psittacosis and sepsis. Many other causes exist, such as a deficiency in certain minerals such as copper and zinc.Pseudoleukopenia can develop upon the onset of infection. The leukocytes (predominately Neutrophils, responding to injury first) are marginalized in the blood vessels so that they can scan for the site of infection. This means that even though there is increased WBC production, it will appear as though it is low from a blood sample, since the blood sample is of core blood and does not include the marginalized leukocytes.There are also reports of Leukopenia caused by Depakote (Divalproex Sodium or Valproic Acid), a drug used for epilepsy (seizures), mania (with bipolar disorder) and migraine
The informtion was important and appreciated. Thanks for you assistance.
I found this article very informative. It is very nice if this article is also collaborated by known hematologists. I appreciated your work on this topic. Thank you and God bless you to continue this work for the laymans understanding.
Today I got the CBC report of my wife and TLC is 2400 cells/ and Platelets Count is 1 lacs/cu. mm Kindly suggest. She is also found Dengue positive.

Ravi Chaddha
Hello, my mother is 61years old, doctor gave her the blood test BT, CT, PLATELETS COUNT andd many
Can any body interpret TLC range found in result 13.20, is that ok??
Luckyman Oryina
It is clear that this article addressed simplicity and details of the tests, procedures and results of this all important medical term.
White count 23 kid func high liver high and platletes only 11000 What does this mean
My cbc was 11.9 about a week ago, which is a tag low if normal us 12. I'm on a pill that defers my cycle for 3 months because I bleed heavy.Right now my arms and thighs get cold is this my body preparing for a period without being able to proceed? Does this impact a cbc reading even though no blood flows?
I had a CBC/W AUTO DIFF recently and the note to patient read "Your CBC or complete blood count shows a normal white blood cell count. You are not anemic but your platelets are clumped so we will follow them. Results RED Blood Cell Count/4.42, Hemoglobin/13.0, Hemoglobin/13.0, Hematocrit/40.8, MCV/92.4, MCHC/32.0, RDW/15.9(high), Platelet Count/TNP, Unable to report due to signifigant platelet clumping. Platelet estimate appears normal. In April I had similar numbers except for the RDW was 13.9 . The April report stated "has lots of clumps in her platelets--we'll recheck this at f/u. If persist may send to hematology." I was told everything was fine. I would like to know what is this eluding to?
Adele Romano
My results from arthrocentesis of my right knee showed a high count of mononuclear cells. 93.3 where as 75.0 is normal.My Dr. did not mention this. Please tell me the meaning of this.Thank you.
Khuraijam Sunanda
My friend's mom is suffering from polycythemia Vera. She is 53 of age and her hematocrit level is 46 instead of 45. Should the excess be removed ?

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