Red/White blood cell info
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Why It Is Done
How To Prepare
How It Is Done
How It Feels
What Affects the Test
What To Think About
Complete Blood Count (CBC)
A complete blood count (CBC) provides important information about the kinds and numbers of cells in the blood: red blood cells, white blood cells, and platelets. A CBC can help a health professional evaluate symptoms (such as weakness, fatigue, or bruising) and diagnose conditions (such as anemia, infection, and many other disorders).
A CBC test usually includes:
White blood cell (WBC) count. White blood cells protect the body against infection. If an infection develops, white blood cells attack and destroy the bacteria, virus, or other organism causing it. White blood cells are bigger than red blood cells and normally fewer in number. When a person has a bacterial infection, the number of white cells can increase dramatically. The white blood cell count shows the number of white blood cells in a sample of blood. The number of white blood cells is sometimes used to identify an infection or to monitor the body's response to cancer treatment.
White blood cell types (WBC differential). There are five major kinds of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Immature neutrophils, called band neutrophils, are also included and counted as part of this test. Each type of cell plays a different role in protecting the body. The numbers of each one of these types of white blood cells give important information about the immune system. An increase or decrease in the numbers of the different types of white blood cells can help identify infection, an allergic or toxic reaction to certain medications or chemicals, and many conditions (such as leukemia).
Red blood cell (RBC) count. Red blood cells carry oxygen from the lungs to the rest of the body. They also help carry carbon dioxide back to the lungs so it can be exhaled. The red blood cell count shows the number of red blood cells in a sample of blood. If the RBC count is low, the body may not be getting the oxygen it needs. If the count is too high (a condition called polycythemia), there is a risk that the red blood cells will clump together and block tiny blood vessels (capillaries).
Hematocrit (HCT, packed cell volume, PCV). This test measures the amount of space (volume) red blood cells occupy in the blood. The value is given as a percentage of red blood cells in a volume of blood. For example, a hematocrit of 38 means that 38% of the blood's volume is composed of red cells.
Hemoglobin (Hgb). Hemoglobin is the major substance in a red blood cells. It carries oxygen and gives the blood cell it's red color. The hemoglobin test measures the amount of hemoglobin in blood and is a good indication of the blood's ability to carry oxygen throughout the body.
Red blood cell indices. There are three red blood cell indices: mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). They are measured by a machine and their values are determined from other measurements noted during a CBC. The MCV shows the size of the red blood cells. The MCH value is the amount of hemoglobin in an average red blood cell. The MCHC measures the concentration of hemoglobin in an average red blood cell. These numbers help in the diagnosis of different types of anemia.
Red cell distribution width (RDW). The RDW reports whether all the red cells are about the same width, size, and shape. This helps further classify the types of anemia.
Platelet (thrombocyte) count. Platelets (thrombocytes) are the smallest type of blood cell. They play a major role in blood clotting. When bleeding occurs, the platelets swell, clump together, and form a sticky plug that helps stop the bleeding. If there are too few platelets, uncontrolled bleeding may be a problem. If there are too many platelets, there is a risk of a blood clot forming in a blood vessel. Also, platelets may be involved in hardening of the arteries, or atherosclerosis.
Blood smear. In this test, a drop of blood is spread (smeared) on a slide and stained with a special dye. The slide is then examined under a microscope. The numbers, size, and shape of red blood cells, white blood cells, and platelets are recorded. Blood cells with unusual shapes or sizes can help diagnose many blood diseases, such as leukemia, malaria, or sickle cell anemia.
A complete blood count is done on a sample of blood taken from a vein.
Why It Is Done
A complete blood count may be done as part of a routine physical examination. A blood count can provide valuable information about the general state of your health.
A complete blood count also may be done to:
Investigate the cause of certain symptoms such as fatigue, weakness, fever, bruising, or weight loss.
Detect anemia or determine the severity of blood loss.
Help diagnose an infection.
Help diagnose diseases of the blood, such as leukemia.
Monitor the response to some types of drug or radiation treatment.
Investigate a history of abnormal bleeding.
How To Prepare
No special preparation is needed before having this test.
How It Is Done
The health professional drawing blood will wrap an elastic band around your upper arm to temporarily stop the flow of blood through the veins in your arm. This makes it easier to put the needle into a vein properly because the veins below the band get larger and do not collapse easily.
The needle site is cleaned with alcohol and the needle is inserted. More than one needle stick may be needed if the needle does not get placed correctly or if the vein cannot supply enough blood.
When the needle is properly placed in the vein, a collection tube will be attached to the needle and blood will flow into it. Sometimes more than one tube of blood is collected.
When enough blood has been collected, the band around your arm will be removed. A gauze pad or cotton ball is placed over the puncture site as the needle is withdrawn. Pressure is applied to the puncture site for several minutes and then a small bandage is often placed over it.
How It Feels
You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. However, many people do not feel any pain (or have only minor discomfort) once the needle is positioned in the vein. The amount of pain you feel depends on the skill of the health professional drawing the blood, the condition of your veins, and your sensitivity to pain.
There is very little risk of complications from having blood drawn from a vein. You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
Rarely, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
Continued bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medications can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medication, tell the health professional before your blood is drawn.
Normal values can vary from lab to lab.
Normal values for the complete blood count (CBC) tests vary, depending on age, sex, elevation above sea level, and type of sample. The health professional may use all the CBC values to evaluate a condition. For instance, the red blood cell (RBC) count, hemoglobin (Hgb), and hematocrit (HCT) are the most important values needed to help decide if a person has anemia, but the red blood cell indices and the blood smear also help guide the diagnosis and suggest a possible cause for the anemia.
To assess white blood cell (WBC) count, your doctor will look at both the number (WBC count) and the WBC differential. To determine whether there are too many or too few of a certain type of cell, the doctor multiplies the total count by the percentage of that particular cell. There are normal values for the total number of each type of white cell.
White blood cell (WBC) count 4,000–10,000 WBCs per microliter (µL)
White blood cell types (WBC differential) Neutrophils:
Red blood cell (RBC) count Men:
4.6–6.2 million RBCs per microliter (µL)
4.2–5.4 million RBCs per µL
4.6–4.8 million RBCs per µL
Hematocrit (HCT) Men:
Hemoglobin (Hgb) Men:
14–18 grams per deciliter (g/dL)
In general, a normal hemoglobin level is about one-third the value of the hematocrit.
Red blood cell indices Mean corpuscular volume (MCV):
80–100 femtoliters (fL)
Mean corpuscular hemoglobin (MCH):
28–32 picograms (pg)
Mean corpuscular hemoglobin concentration (MCHC):
32–36 grams per deciliter (g/dL)
Red cell distribution width (RDW) Adults:
Platelet (thrombocyte) count 140,000–450,000 platelets per microliter (µL)
Blood smear Normal:
Blood cells are normal in shape, size, and number.
Greater than normal values may mean
A high white blood cell count can indicate infection, inflammation, damage to body tissues (such as a heart attack), severe physical or emotional stress (such as a fever, injury, or surgery), kidney failure, or diseases such as cancer. Very high levels of white blood cells can sometimes indicate leukemia.
The number of neutrophils can be increased by infection, heart attack, burns, severe stress, and the use of corticosteroid medications. Neutrophils are decreased by exposure to radiation, some kinds of infection (such as HIV infection), lack of vitamin B12 or folic acid, or an autoimmune disease such as systemic lupus erythematosus (SLE).
Band neutrophils increase in response to severe infection, especially infection caused by bacteria.
Lymphocytes are increased by some viral infections, diseases that affect the immune system, and some types of leukemia. Lymphocytes can be decreased by the use of corticosteroid medications, conditions that affect the immune system (such as AIDS), malnutrition, and severe, long-term illnesses.
Monocytes can be increased by infection with a virus or fungus (like yeast), some types of cancer and leukemia, and tuberculosis (TB).
Eosinophils can be increased by allergies, some infections (especially those caused by parasites), skin diseases, reactions to some medications, some types of leukemia and cancer, diseases that affect the immune system (such as systemic lupus erythematosus), and underactive adrenal glands. Eosinophils can be decreased by severe stress or Cushing's syndrome.
Basophils can be increased by some types of leukemia and cancer or a poorly functioning thyroid gland. Basophils are decreased by pregnancy, severe stress, and an overactive thyroid gland.
A high red blood cell count means the blood contains too many red blood cells. High values can be caused by a lack of oxygen (which can occur from living at high altitude), smoking, exposure to carbon monoxide, long-term lung disease, kidney disease, certain forms of heart disease or a rare disorder of the bone marrow (polycythemia vera).
A high RBC count can also be caused by too little water in the body (dehydration), a result of drinking too little; other causes include frequent diarrhea or vomiting, excessive sweating, severe burns, and the use of diuretics.
A high hematocrit value means the blood contains too many red blood cells (a condition called polycythemia). High values can be caused by a lack of oxygen (which can occur from living at high altitude), smoking, exposure to carbon monoxide, long-term lung disease, certain forms of heart disease, kidney disease, certain types of cancer, or polycythemia vera, a rare disorder of the bone marrow.
A high hematocrit value can also be caused by too little water in the body (dehydration), a result of drinking too little; other causes include frequent diarrhea or vomiting, excessive sweating, severe burns, and the use of diuretics.
A high hemoglobin value means the blood contains too many red blood cells. High values can be caused by a lack of oxygen (which can occur from living at high altitude), smoking, exposure to carbon monoxide, long-term lung disease, certain forms of heart disease, kidney disease, or polycythemia vera, a rare disorder of the bone marrow.
A high hemoglobin value can also be caused by too little water in the body (dehydration), a result of drinking too little; other causes include frequent diarrhea or vomiting, excessive sweating, severe burns, and the use of diuretics.
A high mean corpuscular volume (MCV) value may indicate alcoholism, liver disease, a lack of folic acid or vitamin B12 that is causing anemia (such as pernicious anemia), or certain bone marrow disorders.
A high number of platelets can be caused by severe bleeding, infection, strenuous exercise, pregnancy, removal of the spleen, iron deficiency, polycythemia vera, rheumatoid arthritis, some types of cancer and leukemia, or a condition of unknown cause in which platelets are overproduced (called essential thrombocytosis).
Lower than normal values may mean
A low white blood cell count can mean that the body is not producing enough white blood cells. For example, this may occur late in pernicious anemia or after cancer chemotherapy or radiation therapy. An allergic reaction to a medication may cause a low white blood count.
A low white blood cell count can also result from chemotherapy, bone marrow failure (aplastic anemia), viral infections, malaria, alcoholism, AIDS, and other conditions.
Low values for the red blood cell count, hematocrit, or hemoglobin tests mean the blood does not contain enough healthy red blood cells. This leads to an inability to supply the body with enough oxygen (anemia).
Anemia can be caused by a large loss of blood. Some conditions that can result in blood loss include severe menstrual bleeding, stomach ulcers, colon cancer, inflammatory bowel disease, or a bladder tumor.
Anemia can be caused by a decrease in red blood cell production, reactions to some chemicals and medications, tumors, Addison's disease, or by the absence of some essential nutrient such as iron, folic acid, or vitamin B12.
Anemia can also be caused by increased destruction of red blood cells due to conditions such as hemolytic anemia or sickle cell disease.
Low MCV and MCHC values can indicate anemia caused by a lack of iron, thalassemia, lead poisoning, or long-term infection. Certain chronic diseases (such as diabetes or arthritis) may cause low MCV and MCHC values.
A low number of platelets can be caused by infection, lack of vitamin B12or folic acid, severe internal bleeding, some types of cancer, systemic lupus erythematosus (SLE), leukemia, certain conditions involving the immune system (such as idiopathic thrombocytopenic purpura), or conditions in which the platelet count appears low because too many platelets are being stored in the spleen.
Abnormal size or structure of blood cells may mean
Red blood cells may be unusually large (macrocytes) or small (microcytes). Macrocytes may indicate some types of anemia, a liver problem, or a vitamin B12 or folic acid deficiency. Microcytes may indicate iron deficiency, thalassemia, or lead poisoning.
The red blood cells may have an abnormal shape. Sickle-shaped cells may indicate sickle cell anemia. Oval cells may indicate liver disease or a stomach ulcer.
The structures inside the red blood cell may look abnormal. Some conditions that can cause an abnormal appearance of red blood cells include malaria, lead poisoning, and some types of anemia and leukemia.
What Affects the Test
The values of the red blood cell count, hematocrit, and hemoglobin may be increased by the prolonged use of a tourniquet while drawing the blood sample.
Children normally have higher WBC counts than adults.
Many medications can increase WBC counts.
Many medications can cause low platelet levels, including steroids, some antibiotics, thiazide diuretics, chemotherapy medications, quinidine (Quinaglute Dura-Tabs, Cardioquin, Quinidex), and meprobamate (Equanil, Miltown, Meprospan).
A very high white blood cell count or high levels of a type of fat (triglycerides) can cause falsely high hemoglobin values.
The blood platelet count can appear low, even though the number of platelets may be normal, if the spleen is enlarged (due to certain types of cancer, for instance). In this case, the platelets are concentrated in the spleen instead of in the blood.
Pregnancy normally can cause low red blood cell, hematocrit, and hemoglobin values.
Rough handling, contamination, or inadequate refrigeration of the blood sample can cause inaccurate test results.
What To Think About
The white blood count can vary by as much as 2,000 WBCs per microliter (µL) as a result of exercise, stress, or smoking.
Other red blood cell tests that may be done include:
Erythrocyte sedimentation rate (ESR). An ESR test measures how quickly red blood cells (erythrocytes) settle in a test tube. When inflammation in the body is present (such as from an infection or cancer), red blood cells may settle more quickly than normal. An ESR may help detect certain inflammatory diseases when CBC results are normal. See Sedimentation Rate test.
Reticulocyte count. This test counts the number of immature red blood cells (reticulocytes) in a blood sample. Generally, only a few reticulocytes are present in the blood in relation to mature red blood cells. However, recent bleeding or destruction of mature cells can cause increased production of red blood cells. It can help identify some types of anemia and monitor a person's response to treatment.
Hematocrit measurements can vary widely, depending on the method and type of instrument used to determine the value.
Author Renée Spengler, RN, BSN
Associate Editor Daniel Greer
Primary Medical Reviewer Steven L. Schneider, MD
- Family Practice
Specialist Medical Reviewer Douglas A. Stewart, MD
- Medical Oncology
Last Updated January 21, 2003