When
you are prescribed warfarin, your doctor will advise of the levels recommended
for
your INR. Generally, it will be in the range 2.0
to 3.0, although both higher and lower
levels are
not uncommon.
So what is INR and how is it measured?
Well, it seems as though the
laboratory measures what is called the prothrombin test time".
The prothrombin time is most commonly
measured using blood plasma. Blood is drawn into a test tube containing liquid citrate which
acts as an anticoagulant by binding the calcium in a sample. The blood is mixed,
then centrifuged to separate blood cells from plasma. In newborns a
capillary whole blood specimen is used.
The plasma is analyzed on an automated
instrument at 37°C .
After initial processing, tissue factor (also known as factor III) is
added, and the time the sample takes to clot is measured optically.
The result (in seconds) for a prothrombin
time performed on a normal individual will vary according to the type of
analytical system employed. This is due to the variations between different
batches of manufacturer's tissue factor used in the reagent to perform the
test.
]International
normalized ratio (INR)
The INR was devised to standardize the
results to take into account the variations in prothrombin time as a
result of different batches of manufacturer's tissue factor.
A high INR level such as an INR of 5 indicates that there is a
high chance of bleeding, whereas if the INR is 0.5 then there is a high
chance of having a clot. Normal range for a healthy person is 0.9 –1.3,
and for people on warfarin therapy, generally 2.0 –3.0, although the
target INR may be higher in particular situations.
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