first ISI calibrator. The interlaboratory variation at
both the therapeutical and the moderate reduced
leve! has been within the interval 4 -8
%.
The accuracy of levels obtained by use of the
.ISI calibrator is questionable because the stated
INR value of the calibrators is established by only
one reference laboratory. In order not to change
theDanish level by replacing of one national cali–
brator with a new one, the leve! has been tested
against the previous calibrators before use. There
is agreement among the currently three national
calibrators. No shifts have been observed. Occasi–
onally comparison of the Danish leve] with cali–
bration according to the test kit manufactures has
shown that it fit very well with their average.
We do not have much knowledge of the situa–
tion at the high dose leve!, due to that only freeze
dried control materials have been tested. We have
not yet been able to establish a calibrator at a high
INR level.
The plan for improvement of the standardisa–
tian in Denmark is replacement of the present ISI
calibrator not with one but with two frozen pools
of plasma, one at the therapeutical leve] 2-3 INR
and one at a higher leve!, hopefully about 4 INR.
The last pool is planned to be collected from pa–
tients with artificial heart valves.
CONCLUSION
It
has been possible to produce a stable calibrator
without matrix effects and create a mode! for cal–
culationof local ISI valuesof thromboplastins from
various species used in the prothrombin time.
The calibrators are stable for several years sto–
red at -80 °C.
It
is possible to ship the frozen plas–
ma materials stored on dry ice to laboratories all
over Denmark.
66
The recalibration of ISI values of throm–
boplastin reagents used in the local setup of the
prothrombin time has had a profound effect. The
overall interlaboratory variation in Danish extern
quality schemes has been reduced to approxima–
tely 6 percent without altering the mean leve! of
the INR values.
Acknowledgements
Thanks are due to Dr.
A.M.H.P.
van Besselaar,
Leiden, the Netherlands for testing the Danish ca–
librators against the international reference prepa–
rations of thromboplastins.
References
L.
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