Fact Sheet Number 106
Complete Blood Count (CBC)
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PrintCOUNT (CBC)
The most common laboratory test is the complete blood count (CBC).
It examines the components of blood, including red and white blood
cells and platelets. Most test results are reported as amounts
in a sample of blood (for example, cells per milliliter) or as
a percentage. Other laboratory tests are discussed in Fact
Sheet 107 and Fact Sheet 108.
All blood cells are made in the bone marrow, the center of
large bones. Some medications or diseases can damage the bone
marrow. This can reduce the numbers of different types of red
or white blood cells.
Laboratories have different “reference ranges” or
normal values for the results of each test. Most lab reports show
the normal range and highlight any test results outside the normal
range.
For more information on laboratory test results, see the Merck
Manual Home Edition, available online at http://www.merckhomeedition.com
TESTS
Red blood cells carry oxygen from the lungs to cells throughout
the body. This is measured by three main tests. The Red Blood
Cell Count (RBC) is the total number of red blood cells. Hemoglobin
(HGB) is a protein in red blood cells that actually carries
oxygen from the lungs to the rest of the body. Hematocrit (HCT)
measures the percentage of blood volume taken up by red blood
cells.
A high RBC is common for people who live at high altitude.
It’s a way the body adjusts to thinner oxygen.
Very low readings for RBC, hemoglobin and hematocrit can indicate
anemia. With anemia, the cells do not get enough oxygen to function
normally. People with anemia feel tired all the time and might
look pale. See Fact Sheet 551 for
more information on fatigue and Fact
Sheet 552 on anemia.
Mean Corpuscular Volume (MCV) measures the average volume
(size) of individual red blood cells. A low MCV means that the
cells are smaller than normal. This is usually caused by an iron
deficiency or chronic disease. A high MCV can be caused by HIV
medications. This is not dangerous. However, a high MCV can indicate
megaloblastic anemia, where red blood cells are large and pale.
This is caused by a shortage of folic acid.
While the MCV measures the average size of red blood cells,
the RDW (Red Blood Cell Distribution Width) measures the
range of red blood cell sizes. RDW can help diagnose anemia or
some vitamin deficiencies.
Mean Corpuscular Hemoglobin (MCH) and Mean Corpuscular
Hemoglobin Concentration (MCHC) measure the amount and concentration
of hemoglobin in the average cell. The MCH is calculated by dividing
total hemoglobin by the total number of red blood cells.
Platelets (PT) help stop bleeding by forming clots and
scabs. If you don’t have enough platelets, you might get internal
bleeding or you could bruise easily. People with HIV disease sometimes
have a low platelet count, also called “thrombocytopenia.”
Taking HIV medications usually corrects this problem. Platelets
are almost never high enough to cause health problems.
TESTS
White blood cells (also called leukocytes) help fight infections
in the body.
White Blood Cell Count (WBC) is the total number of
white blood cells. A high WBC usually means that the body is fighting
an infection. A very low WBC can be caused by problems with the
bone marrow. This condition, called cytopenia or leukopenia, means
that your body is less able to fight off infections.
The Differential counts five types of white blood cells:
neutrophils, lymphocytes, monocytes, eosinophils and basophils.
These are reported as a percentage of the WBC. The percentages
are multiplied by the WBC to get “absolute” counts.
For example, with 30% lymphocytes and a WBC of 10,000, the absolute
lymphocyte count is 30% of 10,000, or 3,000.
Neutrophils or polymorphonuclear cells (Polys)
fight bacterial infections. They normally account for 55% to 70%
of WBCs. If you have a very low count, you could get a bacterial
infection. This condition is called neutropenia. Advanced HIV
disease can cause neutropenia. So can some medications including
ganciclovir, a drug used to treat cytomegalovirus (see Fact
Sheet 501) and the anti-HIV drug AZT.
There are two main types of lymphocytes (lymphs). “T
cells” attack and kill germs, and help regulate the immune
system. “B cells” make antibodies, special proteins
that attack germs. Lymphocytes are normally 20% to 40% of WBCs.
A regular CBC does not give T-cell counts. Most people with HIV
infection get special T-cell tests (see Fact
Sheet 412). However, the results of a CBC are needed to calculate
T-cell counts, so both tests are done at the same time.
Monocytes or Macrophages (Monos) make up 2% to
8% of WBCs. They fight infections by “eating” germs
and telling the immune system what germs they have found. Monocytes
circulate in the blood. When monocytes settle in various tissues
they are called macrophages. A high count usually indicates a
bacterial infection.
Eosinophils (Eos) are normally 1% to 4% of WBCs. They
are involved with allergies and reactions to parasites. Sometimes,
HIV disease can cause a high eosinophil count. A high count, especially
if you have diarrhea, gas or stomach bloating, may indicate the
presence of parasites.
Basophils (Bas) are not well understood, but they are
involved in long-term allergic reactions such as asthma or skin
allergies. They are usually less than 1% of WBCs.
May 8, 2002
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