From Lorna Bogertman, Valley Health: Hello George, I have a question concerning lupus anticoagulant (LA) testing. We have had several requests from physicians to perform hexagonal phase LA testing only. Is it valid to perform this test without the rest of the LA profile? We have found that if we performed Stago’s LA-sensitive partial thromboplastin time (PTT–LA) and the dilute Russell viper venom time (DRVVT), the LA would have been reported as negative, however in this case the hex phase test was positive. How should this be reported? If the profile is really negative, what are the other causes of a positive hex phase test other than LA?
Hello, Lorna, and thank you for your question. I’m in Rochester attending the Mayo Laboratories Clinical and Laboratory Update in Thrombosis, Anticoagulant, and Vascular Medicine, and had the opportunity to raise your question with several colleagues, who agree that the DRVVT and the hex-phase assay are not confirmatory, but rather must be employed in paraallel to test for LAs of different specificity. Thus, if the DRVVT is positive for LA and the hex-phase assay is negative, it is appropriate to report that LA is present, and likewise, if the hex-phase alone is positive, you can report LA. This of course assumes you’ve already ruled out a factor deficiency, factor-specific inhibitor, and heparin.
Stago’s StaClot LA kit is the system that is based on the hex phase reagent. The kit uses an LA-sensitive PTT reagent and a reagent that consists of phospholipids suspended in the hexagonal phase conformation. Hex-phase phospholipids bind and neutralize LA. You incubate the test plasma with the hex-phase phospholipid and compare the PTT performed on the mixture to the PTT performed on the plasma alone. If the mixture PTT result is shortened by a specified interval, usually 8 seconds, the test is positive for LA. DRVVT kits function similarly, but may detect LAs of slightly different specificity, which is why both are used.
The International Society on Thrombosis and Haemostasis guidelines for LA testing are available at Pengo V, Tripodi A, Reber G, et al. Update of the guidelines for lupus anticoagulant detection. Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibody of the Scientific and Standardisation Committee of the International Society on Thrombosis and Haemostasis. J Thromb Haemost. 2009; 7:1737–40.