PTT-LA
The PTT-LA test is based on measuring the plasma recalcification time in the presence of cephalin and activator. The test is a variant of the activated partial thromboplastin time (APTT) that uses a low phospholipid reagent optimized for the detection of Lupus Anticoagulants (LA). It is sensitive to the same intrinsic pathway coagulation factors as the regular APTT test (shown in yellow in the diagram).
LA inhibit phospholipid-dependent coagulation tests such as the PTT-LA, and the presence in the test plasma of LA therefore prolongs the clotting time. In order to make a diagnosis of LA, it is necessary to demonstrate (a) prolongation of a phospholipid-dependent clotting test; (b) the presence of an inhibitor activity and (c) neutralization by excess phospholipid.
A prolonged PTT-LA may not necessarily be due to the presence of LA in the test plasma, since the presence of heparin, intrinsic pathway factor deficiencies and the presence of (non-LA) factor inhibitors will also prolong the clot time.
On finding a prolonged PTT-LA, the Hemostasis and Thrombosis Laboratory will first exclude the possibility of heparin in the plasma sample by performing a thrombin time. If the sample contains heparin, this is removed by Hepzyme treatment before testing.
If the PTT-LA remains prolonged, the test will be repeated on a 1:1 mixture of patient plasma and normal plasma. Correction of the PTT-LA is defined as a clot time within 5 seconds of the normal plasma alone. In addition, we will test to see if the clot time is normalized by the addition of hexagonal phase phospholipid (Staclot-LA test). If so, the diagnosis of LA is confirmed.
It should be noted that not all LA are detected by the PTT-LA test and it is recommended that all samples suspected of having LA be screened by three independent clotting methods. In the Hemostasis and Thrombosis Laboratory, these are currently PTT-LA, LA-Ratio (DRVVT) and Kaolin Clot Time.
The presence of LA has been increasingly associated with a variety of problems such as unexplained thrombosis and recurrent fetal loss. We recommend that LA screening be performed as part of the routine evaluation of such cases.
The normal range for the PTT-LA in the Hemostasis and Thrombosis Laboratory is 30.9 - 45.5 seconds.
