Staclot LA

The Staclot LA is an APTT-based assay that is based on the principle that
Lupus Anticoagulants (LA) can be neutralized by hexagonal phase phosphoplipids (HPP). The
Hemostasis and Thrombosis Laboratory uses this test to confirm the presence of a LA
following the finding of a prolonged PTT-LA that does not correct fully with normal
plasma.

The test plasma is incubated with and without addition of HPP. An APTT is
then performed on both samples using a LA-sensitive reagent. If a LA is present, it is
neutralized by the HPP, resulting in a shorter clotting time than the plasma without HPP.
The Staclot LA is considered to be positive for the presence of LA if the difference in
clotting times between the two tubes is more than 8.0 seconds.

The APTT reagent in this assay contains a heparin inhibitor which makes the
test system insensitive to heparin levels up to 2.0 U/mL. The test procedure also
incorporates a mixing test with normal plasma to exclude factor deficiiences as a cause
of prolonged clotting times.

Due to the heterogeneity of phospholipid antibodies, no single test is
currently able to detect them all. It is therefore necessary to use at least three
independent screening tests to exclude the possibility of a Lupus Anticoagulant. The
Hemostasis and Thrombosis Laboratory currently employ the PTT-LA, LA Ratio (DRVVT) and
Kaolin Clot Time for this purpose.