Oxygen Saturation
Test
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What is the
test?
Your red blood cells must carry sufficient oxygen
through your arteries to all of your internal
organs to keep you alive. Normally, when red
blood cells pass through the lungs, 95%-100%
of them are loaded, or "saturated," with oxygen
to carry. If you have lung disease or other types
of medical conditions, fewer of your red blood
cells may be carrying their usual load of oxygen,
and your oxygen saturation might be lower than
95%. Your blood oxygen level can be measured
in two ways.
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How
do I prepare for the test?
There is no preparation necessary.
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What
happens when the test is performed?
An estimate of your oxygen saturation can be
made easily and painlessly with a clip that fits
on your finger. This clip shines a light through
one side of your finger; a detector measures
the light that comes through the other side.
This machine can make a good guess about your
oxygen saturation because blood cells that are
saturated with oxygen absorb and reflect light
differently than those that are not. Blood cells
are a bright red when they are loaded with oxygen,
and they change to a bluish color when they are
no longer carrying a full load.
This machine cannot give a perfect measurement
of your oxygen saturation; it can give only a
rough estimate, and its measurement can be affected
by things as simple as red nail polish on your
finger.
A better test for measuring your oxygen saturation
is an arterial blood gas test. For this test,
a small sample of blood must be drawn directly
out of an artery. Most routine blood tests use
blood that is drawn out of a vein, so this test
is a little different. The artery that is sampled
most often is the radial artery in your wrist,
the one you can feel when you take your pulse.
To draw blood from this artery, your doctor or
a technician feels your pulse before inserting
the needle. Some patients find that it hurts
a little more to have blood taken from an artery
instead of a vein, but the procedure takes only
a few seconds. Your arterial blood can be directly
tested for its oxygen level, and other tests
(such as the level of carbon dioxide and the
pH of the blood) can be done as well.
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What
risks are there from the test?
Measurement made with a fingerclip has no risks.
The risks of an arterial blood gas test are very
small. Even temporary injury to your artery is
unlikely to cause a problem, because most patients
pump blood to their hand through more than one
artery.
Before drawing your blood, your doctor may do
a brief physical examination to make sure that
you still get good blood flow to your hand even
when one wrist artery is blocked. To do this
test, the doctor presses down first on both sides
of your wrist to block blood flow, until your
hand becomes pale. Then he or she lifts off the
pressure from one side to see if that is enough
to let your hand turn pink again.
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Must
I do anything special after the test is over?
You will need to have pressure held over the
artery for a few minutes after the blood is drawn,
because arteries are more likely than veins to
bleed afterward.
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How
long is it before the result of the test is
known?
The results of the arterial blood gas test are
processed very quickly and are available within
15 minutes in most laboratories. The fingerclip
estimate of oxygen is available immediately.
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