a bottie of plastic and dispensed in cryotubes of
l
mi. All tubes containing plasma pool were frozen
simultaneously and stored at 80
oc.
In order to establish a reference value of the ISI
calibrator, the preparation was tested by The
Thrombosis andHaemostasisResearchUnit inLei–
den,TheNederlands.ltwas testedbymanual tech–
nique against four international reference prepa–
rations of thromboplastins, BCT/099 (human
plain), OBT/79 (bovine combined), RBT/79 (rab–
bit plain) andCRM 149R (rabbitplain).Therewere
some discrepancies in the values related to refe–
rencematerialsofdifferent species.Themean INR
values found with the four materials were respec–
tively 2.54, 2.77, 2.48 and 2.62.
Amean reference value of2.60 INRwas adop–
ted. This value was fairly close to the consensus
value of2.47 INRobtained by preliminary testing
in 26Danish laboratories.
The first national ISI calibrator has been repla–
ced with subsequently preparation, the first time
in 1994 and the seeond time in 1996with the 3rd.
national ISI calibrator, which is still in use.
The later calibrators are produced andassigned
with values in the samemanner as the first, i.e. the
mean value obtained by testing against internatio–
nal reference preparations of thromboplastins and
compared with the previous calibrator. The pre–
sent calibrator is intended to be usedwhen a labo–
ratory changes their reagent batch or shifts to an–
other coagulameter. A4th. ISI calibrator is now in
preparation and will be available in the autumn of
1999.
Determination of local ISI value
The ISI calibrator and the mean normal plasma
coagulation time (MNPT) are used to calculate
the ISI value of the thromboplastin reagent in the
local analytical setup of the prothrombin time.
U
sing the assigned INR value of the ISI cali–
brator, the method for the practical use of the ca–
librator to establish the ISI value of local analyti–
cal system isderived from the following equation:
ISI =log INRcal x (/!log PT ratio),
where
JNRCal
is INRvalue of the calibrator and
PTratio
is equal
to coagulation time of the calibrator divided by
MNPT.
62
Normal Plasma Calibrator
The expression of prothrombin time in INR valu–
es is basedon the coagulation time ofnormal plas–
mas. Additionally the mean normal prothrombin
time has to be used in the recalculation of the Jocal
ISI value. However, many laboratories do not use
a sufficient number of fresh normal samples to
determineMNPT.
A normal plasma calibrator was introduced to
the Danish laboratories as a further improvement
of the standardisation in ordertoreplace the diffe–
rent commercial and local preparations of plasma
pools. The Danish normal calibrator is a pooled
plasma preparation from 40 healthy persons wit–
hout medication, 20 maJe and 20 females. The
collection, virus testing and storing of blood fol–
Jowed the same Iines ofdirectionwhich was used
for the collection and storing of the ISI calibrator.
The Danish normal calibrator was designaled
with a "consensus" value of 1.00 INR, and for a
non-anticoagulation purpose a value of 1.00 arbi–
trary units.The normal calibrator is intending to
beused as analternativeway to determine themean
normal prothrombin time and simultaneouslywith
the ISI calibrator for calculating the local ISI va–
lue of the reagent in use. It can also be used as
100% activity reference plasma when expressing
the prothrombin time in arbitrary units as a part of
a "Liver Profile".
The frozen normal calibrator has recently been
tested by The Thrombosis and Haemostasis Re–
search
U
nit inLeiden,TheNederlands.
It
was tes–
ted against three international reference prepara–
tions of thromboplastins, rTF/95 (human recom–
binant), OBT/79 (bovine combined) and CRM
149S (rabbit plain). The INR value was found to
0.98 with a CV of 3.1%. The calculation was re–
ferring to an MNPT deterrnined by use of fresh
plasma from a normal Dutch population.
Extern quality
assessment
materials
Externa! quality assessment schemes on coagula–
tion tests inDenmark go back to 1989.Liquid fro–
zen materials at four INR levels are used. All ma–
terials are testedfor anti-HIV, HbsAg, anti-HCV
and dispensed in cryotubes and frozen at -80
oc.
The program takes place five times a year and in–
cludes eight coagulation parameters besides the
prothrombin time. Leve] l is unmodified plasma
Klinisk Kemi
i
Norden 3. 1999
