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A complete blood count (CBC) is a screening test used to diagnose and manage many diseases. A CBC measures the status of important features of the blood, including the following:

• number of red blood cells (RBCs)
• number of white blood cells (WBCs)
• the percentage of blood volume composed of cells, called the hematocrit
• total amount of hemoglobin in the blood
• mean corpuscular hemoglobin (MCH)
• mean corpuscular hemoglobin concentration (MCHC)
• mean corpuscular volume (MCV)
• number of platelets

Who is a candidate for the test?

A CBC is a part of routine blood testing often done with physical examinations. It is also used to help diagnose many disorders, including problems with the blood, heart, kidneys, and nutritional status.

How is the test performed?

A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, called a tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them.

A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle and into a syringe or vial. After the needle is withdrawn, the puncture site is covered with a bandage for a short time. This helps stop or prevent bleeding at the site.

What is involved in preparation for the test?

Specific instructions may be given by a healthcare professional. Generally, no preparation is needed.

What do the test results mean?

Normal values are as follows:

• RBC (value changes with altitude): Male, 4.7-6.1 million cells/mcl; female, 4.2-5.4 million cells/mcl
• WBC: 4,500-10,000 cells/mcl
• hematocrit (varies with altitude): Male, 40.7-50.3%; female, 36.1-44.3%
• hemoglobin (varies with altitude): Male, 13.8-17.2 gm/dcl; female, 12.1-15.1 gm/dcl
• MCV: 80-95 femtoliter
• MCH: 27-31 pg/cell
• MCHC: 32-36 gm/dl

Abbreviations:

• cells/mcl = cells per microliter
• gm/dl = grams per deciliter
• pg/cell = picograms per cell

Abnormally high numbers of red blood cells may be a sign of the following:

• congenital heart disease, any of several heart conditions present at birth
• cor pulmonale, a condition in which the right lower part of the heart becomes swollen as a consequence of lung disease
• dehydration, a lack of sufficient fluid in the body that can occur with conditions such as severe diarrhea
• kidney disease with high levels of erythropoietin, a hormone produced in the kidney
• low oxygen tension in the blood
• pulmonary fibrosis, which is a hardening of the lung tissue that can make hard to breathe

High hematocrit may be a sign of:

• burns
• dehydration
• diarrhea
• eclampsia, a serious condition involving high blood pressure, protein in the urine and swelling of your face and hands during pregnancy that can lead to seizures and coma
• erythrocytosis, which is an unhealthy rise in the number of red blood cells
• polycythemia vera, which is an increase in the cell mass or red blood cell levels in your blood
• shock in its early stage, before the body has a chance to mobilize fluid to replace lost blood

Low hematocrit or hemoglobin, or low numbers of red cells (anemia) may indicate that one of these conditions is present:

• blood loss
• bone marrow failure
• hemolysis, which is the separation of hemoglobin, the oxygen-carrying component of red blood cells, from the red blood cells
• erythropoietin deficiency, which occurs when the kidney does not produce enough of the hormone
• leukemia
• malnutrition
• multiple myeloma
• overhydration
• rheumatoid arthritis, a long-term disease in which the connective tissue is destroyed
• any of several specific nutritional deficiencies, especially iron deficiency

Abnormally low numbers of white blood cells may be a sign of:

• bone marrow failure
• collagen-vascular diseases, which are diseases in which the body's immune system attacks white blood cells
• exposure to radiation
• liver or spleen disease
• the presence of substances toxic to cells

High numbers of white blood cells may result from:

• infections (the most common and important cause)
• inflammatory diseases
• leukemia
• tissue damage
• emotional or physical stress