A Guide to Your Complete Blood Count (CBC) Screen
What is a Complete Blood Count?
The complete blood count is one of the most commonly ordered blood screening
tests and is used to present a general picture of a person’s overall
health. The 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).
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:
• White Blood Cell Count (WBC)
• White Blood Count Differential
• Red Blood Cell Count (RBC)
• Hemoglobin Concentration (HGB)
• Hematocrit Value (HCT)
• RBC Indices - which includes mean corpuscular volume (MCV), mean
corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration
(MCHC), and red cell distribution width (RDW)
• Platelet Count (PLT) - including mean platelet volume (MPV)
White Blood Cell Count (WBC)
White blood cells (leukocytes) are important for your body’s defense
system. White blood cells are larger than red blood cells, but there are
fewer of them.
• Optimal Range: 3.7-10.7 K/mm3
When you have an infection, an increased number of white blood cells are
sent form the bone marrow to attack the bacteria or virus that is causing
the infection. An increased number of white blood cells may occur with
mild infections, appendicitis, pregnancy, leukemia, allergic reactions,
hemorrhage, and hemolysis. Strenuous exercise, emotional distress, and
anxiety can also cause an increase in WBC. A low white blood cell count
makes it harder for your body to fight off an infection. People with a
low WBC are more likely to catch colds or other infectious diseases. Low
WBC counts may be seen in overwhelming infections like mumps, lupus, cirrhosis
of the liver, HIV, and cancer. In addition, radiation therapy and certain
types of drug therapy tend to lower the WBC.
Red Blood Cell Count (RBC)
Red blood cells (erythrocytes) are the most common type of cell in the
blood. Red blood cells are continuously produced by the bone marrow in
healthy adults. 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. The cells contain hemoglobin,
which carries oxygen and carbon dioxide throughout the body.
• Optimal Range: 3.95- 5.35 M/mm3
If the concentration of red blood cells and/or the amount of hemoglobin
in the blood drops below normal, 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. Some examples that produce a low RBC include:
• Acute or chronic blood loss
• RBC destruction (e.g., hemolytic anemia, etc.),
• Nutritional deficiency (e.g., iron deficiency, vitamin B12 or folate
• Bone marrow disorders or damage, chronic inflammatory disease
• Chronic kidney disease
Though not as common, some conditions that cause high RBC are:
• Excessive dehydration
• Pulmonary disease
• Congenital Heart Disease
• Kidney or tumor that produces excess erythropoietin
• Living at high altitude
• Certain genetic disorders
Hemoglobin is the iron-containing protein found in all red blood cells
(RBCs) that gives the cells their characteristic red color. Hemoglobin
enables RBCs to bind to oxygen in the lungs and carry it to tissues and
organs throughout the body. The oxygen is used by the cells to produce
energy. It also helps transport a small portion of carbon dioxide, a product
of cell metabolism, from tissues and organs to the lungs, where it is exhaled.
• Optimal Range: 12.0-16.0 gm/dL
Hemoglobin results usually mirror RBC results and causes. People with a
low hemoglobin level most often have anemia and usually have a low red
blood cell count and a low hematocrit. Signs of symptoms of anemia - paleness,
shortness of breath, fatigue - will start to show when the hemoglobin
is too low. Hemoglobin increases with altitude adaptation. In general,
females have lower red blood cell counts and hemoglobin values than men.
A hematocrit is a test that measures the proportion of a person's blood
that is made up of red blood cells (RBCs). Blood consists of RBCs, white
blood cells (WBCs), and platelets suspended in a fluid portion called
plasma. The hematocrit is a ratio of the volume of red blood cells to
the volume of all these components together, called whole blood. The value
is expressed as a percentage or fraction.
• Optimal Range: 36.0-48.0%
Hematocrit measurement is useful in identifying anemia, and red cell production
within the circulatory system. Hematocrit increases with altitude training
or dehydration. Women generally have lower hematocrit values than men.
What are Indices?
Indices are values which measure hemoglobin, hematocrit, and platelet components
found in red blood cells. These indices include:
MCV: Mean cell volume, measures the average size of the red blood cells.
• Optimal Range: 81.0-101.0 fl
MCH: Mean cell hemoglobin, reflects the average weight of hemoglobin found
in the red blood cell.
• Optimal Range: 27.9-33.3 PG
MCHC: Mean cell hemoglobin concentration, reflects the average amount of
hemoglobin in the red blood cell.
• Optimal Range: 31.9-35.9 g/dL
RDW: Red cell distribution width is a histogram (visual), which reflects
the distribution of the size of the red blood cell population.
• Optimal Range: 10.5-14.5%
MPV: Mean platelet volume, reflects the average volume of platelets.
• Optimal Range: 5.7-11.7 fl
Platelets, also called thrombocytes, are special cell fragments that play
an important role in normal blood clotting.
• Optimal Range: 140-410 K/mm3
Platelet counts are often done if you are prone to bruising or if you are
about to have surgery. A person who does not have enough platelets may
be at an increased risk of excessive bleeding and bruising, while an overabundance
of platelets can cause excessive clotting. The platelet count may change
with bleeding disorders, heart disease, diabetes, inflammatory disorders,
What is a Differential Blood Count?
There are five different types of white cells that make up the differential
blood count. White blood cells (leukocytes) come in several shapes and
sizes and can be identified by the laboratory instrument known as a hematology
analyzer, or under a microscope.
Neutrophils: the most populous of the circulating white cells, they are also the shortest
lived in circulation. After the production and release by the marrow,
they only circulate for about eight hours before proceeding to the tissues,
where they live for about a week.
• Optimal Range: 41.4-74.4%
A high neutrophil count may be seen in infections, some cancers, arthritis,
and sometimes when the body is under stress (for example after surgery,
trauma, or a heart attack). A decreased neutrophil count may indicate
certain viral infections, severe, overwhelming infections, leukemia, lupus,
drug reactions, anaphylactic shock, enlarged spleen, and damage to bone marrow.
Lymphocytes: normally makes up 25% of total white blood cells. They occur in three
forms (the differential does not distinguish among them): B cells, which
function primarily to produce antibodies associated with immunity, T cells,
which recognize foreign substances and process them for removal and natural
killer cells, which directly attack and kill abnormal cells such as cancer
cells or infected cells. They are the second most populous of the circulating
white blood cells.
• Optimal Range: 24.0-44.0%
An increased number of lymphocytes may be produced with a viral infection,
bacterial infections, acute stress, chronic inflammatory disorders, and
leukemia’s. A decreased number of lymphocytes may occur with chemo
therapy and HIV.
Monocytes: Monocytes are produced by the marrow, circulate for five to eight days,
and then enter the tissues. When entering the tissues they can either
transform into macrophages (cells that eat bacteria, viruses and parasites
and are able to present them to other immune cells) or dendritic cells
(process antigens and present them to T-cells).
• Optimal Range: 0.0-10.0%
High levels of monocytes may indicate chronic infections, infections within
the heart, collagen vascular diseases (lupus, rheumatoid arthritis), and
leukemia’s. Low levels of Monocytes may indicate bone marrow damage,
Eosinophils: eosinophils play two roles in your immune system: destroying foreign
substances and regulating inflammation.
• Optimal Range: 0.0-4.7%
A high eosinophil count often indicates allergies, asthma, skin diseases,
drug reactions, inflammatory disorders (celiac disease, inflammatory bowel
disease), parasitic infections, and some cancers, lymphomas, and leukemia’s.
Low levels of eosinophils are usually not medically significant. Low levels
may indicate stress or acute inflammatory states.
Basophils: the basophils are the least numerous of the white cells. They are easily
recognized by their very large deep purple granules. Basophils serve as
mediators of inflammatory responses, especially hypersensitivity reactions.
• Optimal Range: 0.0-1.6%
A high basophil may indicate rare allergic reactions, inflammation (rheumatoid
arthritis, ulcerative colitis), and some leukemia’s. Low levels
of basophils are usually not medically significant. Low levels may indicate
stress or acute inflammatory states.