Factor VIII coagulant activity (VIII: C) has been shown by several investigators to
exhibit increased stability in vitro when physiological levels of plasma ionized calcium
are maintained by anticoagulation with heparin rather than citrate. An increase in
initial activity of VIII: C in heparin over that of VIII: C in citrate has been reported
but this has not been confirmed. In order to assay VIII :C in heparinized plasma,
the heparin anticoagulant effect must be excluded without interfering with the validity
of the assay. A one-stage clotting assay for VIII: C has been developed where heparin
is neutralized by Polybrene®, a synthetic polymerized quaternary ammonium salt. VIII :C may be accurately measured
by this method which satisfies the requirements for a valid assay of parallelism and
linearity.
Key words
VIII:C assay - Heparin neutralization in vitro - Polybrene
