HEMATOCRIT TEST SCREEN FOR ANEMIA
Category: Internal Medicine
Abstract : The hematocrit is a test that measures the percentage of blood that is comprised
of red blood cells.
Purpose The hematocrit is used to screen for anemia, or is measured on a
person to determine the extent of anemia. An anemic person has fewer or smaller
than normal red blood cells. A low hematocrit, combined with other abnormal
blood tests, confirms the diagnosis. The hematocrit is d
The hematocrit is a test that measures the percentage of blood that is comprised
of red blood cells.
Purpose
The hematocrit is used to screen for anemia, or is measured on a
person to determine the extent of anemia. An anemic person has fewer or smaller
than normal red blood cells. A low hematocrit, combined with other abnormal
blood tests, confirms the diagnosis.
The hematocrit is decreased in a variety of
common conditions including chronic and recent acute blood loss, some cancers,
kidney and liver diseases, malnutrition, vitamin B12 and folic acid
deficiencies, iron deficiency, pregnancy, systemic lupus erythematosus,
rheumatoid arthritis and peptic ulcer disease.
An elevated hematocrit is
most often associated with severe burns, diarrhea, shock, Addisons disease, and
dehydration, which is a decreased amount of water in the tissues. These
conditions reduce the volume of plasma water causing a relative increase in
RBCs, which concentrates the RBCs, called hemoconcentration. An elevated
hematocrit may also be causedby an absolute increase in blood cells, called
polycythemia. This may be secondary to a decreased amount of oxygen, called
hypoxia, or the result of a proliferation of blood forming cells in the bone
marrow (polycythemia vera).
Critically high or low levels should be immediately called to the attention
of the patients nurse or doctor. Transfusion decisions are based on the results
of laboratory tests, including the hematocrit. Generally, transfusion is not
considered necessary if the hematocrit is above 21%. The hematocrit is also used
as a guide to how many transfusions are needed. Each unit of packed red blood
cells administered to an adult is expected to increase the hematocrit by
approximately 3% to 4%.
Precautions
Fluid volume in the blood affects hematocrit values.
Accordingly, the blood sample should not be taken from an arm receiving IV fluid
or during hemodialysis. It should be noted that pregnant women have extra fluid,
which dilutes the blood, decreasing the hematocrit. Dehydration concentrates the
blood, which increases the hematocrit. In addition, certain drugs such as
penicillin and chloramphenicol may decrease the hematocrit, while glucose levels
above 400 mg/dL are known to elevate results. Blood for hematocrit may be
collected either by finger puncture, or sticking a needle into a vein, called
venipuncture. When performing a finger puncture, the first drop of blood should
be wiped away because it dilutes the sample with tissue fluid. A nurse or
phlebotomist usually collects the sample following cleaning and disinfecting the
skin at the site of the needle stick.
Description
Blood is made up of red blood cells, white blood cells (WBCs),
platelets, and plasma. 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 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 hematocrit may be measured manually by centrifugation. A thin capillary
tube called a microhematocrit tube is filled with blood and sealed at the
bottom. The tube is centrifuged at 10,000 RPM (revolutions per minute) for five
minutes. The RBCs have the greatest weight and are forced to the bottom of the
tube. The WBCs and platelets form a thin layer, called the buffy coat, between
the RBCs and the plasma, and the liquid plasma rises to the top. The height of
the red cell column is measured as a percent of the total blood column. The
higher the column of red cells, the higher the hematocrit. Most commonly, the
hematocrit is measured indirectly by an automated blood cell counter. It is
important to recognize that different results may be obtained when different
measurement principles are used. For example, the microhematocrit tube method
will give slightly higher results than the electronic methods when RBCs of
abnormal shape are present because more plasma is trapped between the cells.
Aftercare
Discomfort or bruising may occur at the puncture site. Pressure
to the puncture site until the bleeding stops reduces bruising; warm packs
relieve discomfort. Some people feel dizzy or faint after blood has been drawn,
and lying down and relaxing for awhile is helpful for these people.
Risks
Other than potential bruising at the puncture site, and/or
dizziness, there are no complications associated with this test.
Normal results
Normal values vary with age and sex. Some representative
ranges are:
• at birth: 42-60%
• six to 12 months: 33-40%
• adult
males: 42-52%
• adult females: 35-47%
Hit: 474 times
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