Summary
The prothrombin time (PT), obtained from a fresh normal plasma pool (FPP), is the
basis both for the establishment of the 100% activity (normal plasma) and for the
ratio calculation used in the International Normalized Ratio (INR) according to the
recommendations of the ICSH/ICTH (6). Today the PT of lyophilized normal plasma pools
are successfully used as reference for the assessment of samples in proficiency studies.
However, a lack of comparability is to be recognized. Therefore the Committee of Hematology
of the German Association of Diagnostics’ and Diagnostic Instruments’ Manufacturers
(VDGH) decided to produce a candidate reference plasma (VDGH Reference Plasma) which
was calibrated against fresh normal plasma pools in an international study.
The basic calibration was performed by using the same certified BCR thromboplastin
(BCT/099) by all participants. The endpoint was determined manually and by using the
coagulometer Schnitger-Gross. In additional testings each participant used his own
routine thromboplastins and methods. Calculating the ratio [PT VDGH Reference Plasma
(sec)/PT fresh normal plasma pool (sec)] the VDGH Reference Plasma showed a deviation
from the average fresh normal plasma pool of 1.05 both with the BCT/099 and with all
thromboplastins. There were obtained some statistical differences between “plain”
and “combined’’ (added factor V and fibrinogen) thromboplastins. No statistical difference
was found between the different endpoint measurement methods (manual, mechanical,
optical).
In spite of these statistical deviations the VDGH Reference Plasma can be used for
the standardization of the PT-normal (100%) value with different ratios for plain
(1.06) and combined (1.02) thromboplastins. The manufacturers will use this VDGH Reference
Plasma for the calibration of their commercially available calibration plasmas, which
allows the user of such a material to calculate a calibrated 100% PT value.
Keywords
Prothrombin time - normal plasma - INR Standardization
