Summary
Six commercially available reagents for the determination of the activated partial
thromboplastin time have been evaluated and compared with respect to their sensitivity
to the coagulation factors VIII, IX and XI and to their response to heparin. Some
variation was observed among the reagents regarding their sensitivity to factor XI
and even greater differences were obtained with factors VIII and IX. It was also clear
that none of the reagents was sensitive to the same extent to the factors tested.
The sensitivity to heparin shows considerable variation, in terms of time as well
as mode of response to increasing heparin levels. In four reagents this response is
linear, it is logarithmic in one and the remaining one is yet again different.
It seems unlikely that any standardization of the APTT determination is at present
possible with the reagents studied.
