Why to they have to draw so much blood?
This is one of the questions most frequently asked by patients and
family members. This section answers the question and explains basic
laboratory tests.
Health or illness of many of the body's important systems is
reflected in the blood stream. Because changes occur rapidly in
critical illness, blood is sampled frequently in order to make
adjustments to these changes.
Since repeated needle sticks are uncomfortable for anyone, the
ICU team may draw the patient's blood through tubes (catheters)
inserted into arteries or veins. Not all patients have or require
these tubes. However, if they are being used, these tubes may stay
in place for several days so that the number of needle sticks is
reduced dramatically.
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Blood being drawn through tubing (called a
catheter). |
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Laboratory tests have been so improved recently that today less
blood needs to be drawn than in the past, often less than a
teaspoon. Newer technologies now allow many blood tests to be
performed right at the bedside. This is called point of care
testing. Some newer methods don't remove blood at all. Instead,
blood is tested right in the blood stream or a small amount of blood
is withdrawn, tested and then returned to the patient.
Many tests can be performed on the blood. Only the most common
tests are described here.
| Arterial Blood
Gas (ABG) |
| One of the hallmarks of critical illness
or injury is an abnormality in the body's ability to take in
(inhale) enough oxygen and to give off (exhale) enough
carbon dioxide. The amount of oxygen and carbon dioxide in
the blood stream can be directly measured from a small blood
sample drawn from an
artery. This is usually done from a small artery at the
wrist or in the upper part of the leg.
In addition this test provides information about the
acid/base balance of the body. The body's systems function
best within a certain narrow acid/base range. Too much acid
or too much base may cause serious problems. The doctor and
nurse may follow this closely when the patient is critically
ill or injured.
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| Basic Blood
Chemistry |
The basic blood chemistry is a common
blood test performed on many ICU patients. The test provides
information regarding the patient's electrolyte balance
(potassium and sodium) and measures the kidney function.
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| Complete Blood
Count (CBC) |
The CBC provides information about the
numbers of red blood cells, the amount of hemoglobin in the
blood, number and type of white blood cells and the number
of platelets. Anemia is the term that describes a patient's
condition when the red blood cells are low. Hemoglobin is
the substance in the red blood cells that carries oxygen to
the body. The white blood cells are the cells that help
fight infection in the body. Platelets are involved in the
body's clotting system. By studying the blood cells under a
microscope much can be learned about health and illness.
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| Heart (Cardiac)
Enzymes |
Heart enzymes are proteins found in the
heart muscle cells. When the heart muscle is damaged (such
as heart attack) certain enzymes or proteins leak out of the
cells from the heart muscle into the blood stream. These
enzymes may be measured over time (usually 3-4 times in 24
hours) and can help make the diagnosis of heart attack.
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| Lactic Acid |
Elevated
lactic acid levels may indicate that the body is not
receiving enough oxygen or that it cannot use the available
oxygen. Poor lung function, poor heart function, very low
blood levels, very low blood pressure, or poisons can be
associated with elevated lactic acid levels. This is a very
dangerous situation and a persistently elevated lactic acid
level is usually a bad sign.
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| Other Blood
Tests |
Many other blood tests are performed. A
sick
liver or
pancreas can be diagnosed and followed with blood tests.
Foreign substances such as drugs and poisons can be directly
measured in the blood stream. Studies of the blood's ability
to clot may be performed.
Hormone levels can be measured. The list of other blood
tests available is extensive. Speak with your doctor or
nurse if you have specific questions.
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Basic Blood Tests
