Clinical Impact of Increasing the Sensitivity of Activated Partial Thromboplastin Time (aPTT) Reagent for Lupus Anticoagulant.

Background: Lupus Anticoagulants (LA) are antibodies that interfere with phospholipids dependent coagulation pathways. First indication of the presence of LA is usually provided by prolonged aPTT. The sensitivity of various commercial aPTT reagents for LA varies. For some years, trend has been to shift towards aPTT reagents that are more sensitive to LA. Starting from May 2004, our coagulation laboratory changed aPTT reagent from Thrombosil by Hemoliance to aPTT-SP by Hemosil. During transition, for quality control purpose, we continued to perform aPTT with Thrombosil in patients found to have prolonged aPTT with aPTT-SP. The aPTT-SP is considered to be more sensitive for LA than Thrombosil. Since then we found increased frequency of prolonged aPTT being discovered during screening coagulation tests. As per our laboratory protocol, all the samples with prolong aPTT were further evaluated with mixing studies and Dilute Russel Viper Venom Test (DRVTT) for LA.

We did a retrospective study to evaluate the clinical significance of prolonged aPTT by aPTT-SP.

Method: Retrospective chart review of all the patients with prolonged aPTT with aPTT-SP from May 2004 to April 2005 was done. Information regarding patient’s age, sex, laboratory data, history of any thrombo-embolic phenomenon and other significant medical conditions was collected.

Results: In 1 year 50 patients (28 females and 22 males) with age ranging from 19 to 80 years, with prolonged aPTT with aPTT-SP reagent were identified. 29 had prolonged aPTT by both reagents and 21 had prologed aPTT by aPTT-SP only. 13 of 21 patients with prolonged aPTT with aPTT-SP (more sensitive reagent) and normal aPTT with Thrombosil (less sensitive reagent), had positive LA. Only three had vascular thrombosis. Most institution including ours, routinely tests for LA during thrombophilia work up regardless of aPTT result. Thus LA would have been diagnosed in these three patients despite normal aPTT by Thrombosil. At the same time diagnosing LA in the absence of thrombosis does not lead to change in clinical management.

Results are shown in table.

Conclusion: Use of aPTT reagent more sensitive to LA, does not provide any clinical advantage. In fact it led to extra work-up and delay in invasive procedures. It also had the potential to lead to extra intervention, like administration of FFP in emergency situations, before LA as a cause of prolonged aPTT could be determined.