In our study, the TAS system demonstrated excellent correlation
with laboratory INRs in the therapeutic range. With the variations
in reagent sensitivity, as expressed by the ISI, prothrombin times
were statistically different between the TAS and the other PT methods,
but the INR bias between the TAS and the laboratory and
Coumatrak methods were not statistically significant, except for a
statistically significant bias between the TAS aPTT and the laboratory
method. The normal reference range for the TAS aPTT was
markedly different from that of the laboratory method, but using
aPTT ratios did not improve the correlation between the two systems.
Part of the bias was due to heparin sensitivity, and the TAS
methods correlated more strongly with heparin levels than the
Ciba-Corning 512, but both methods correlated poorly. The HMT
was statistically different from the ACT, but correlated more
strongly with heparin levels. As a result of the bias, use of the HMT
in cardiopulmonary bypass may require different monitoring targets
than the recommended >400 seconds with the ACT.