From Michelle Fahs, Mercy Hospital, St. Louis. Hi, we are going to start performing lupus anticoagulant testing and are planning to perform the PTT–LA and dilute Russell viper venom time (DRVVT). We use Stago instrumentation. We are still determining our testing algorithm from that point and I have a few questions for you.
1. Do you recommend performing a DRVV mix on the samples that have a positive DRVV screen even if we are performing a PTT–LA mix? My first thought was any factor deficiencies or inhibitors that prolong the DRVVT would also prolong the PTT–LA and be detected under the PTT–LA mix, but I am not sure that is necessarily a correct assumption.
2. What is the advantage of performing the STACLOT-LA and the DRVVT confirm rather than just performing the DRVVT confirm? Is it just to give more interpretive information about the possibility of a factor VIII inhibitor?
3. Regarding the mixing tests for DRVVT and PTT–LA, do you have a recommended resource for me that I can access those procedures from? Should the immediate and 2 hour incubation be performed on both? I also noticed different labs use different methods for determining if the mix corrected or not. Shouldn’t “% correction” be used instead of normal ranges when determining if the mix corrected? I have reviewed your PowerPoint presentations on lupus anticoagulant testing and greatly appreciated the information you provided in them. I would be grateful to you for helping me answer these questions.
Hello, Michelle, and thank you for your question. The article that provides the basis for lupus anticoagulant (LA) testing is 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.
Although there are several options, I recommend you perform your mixing study using only your standard partial thromboplastin time (PTT) assay as a means of distinguishing between a coagulopathy (coagulation factor deficiency) and an inhibitor, be it LA or a specific inhibitor such as anti-factor VIII. A summary protocol is provided in the Pengo article, but most laboratory directors prefer a 1:1 mix with pooled normal plasma and an immediate and 2-hour incubated assay when the immediate is negative.
Though opinions vary, I recommend that your choose <10% as your mixing study decision point. If the mix is shortened to within 10% of the control value, correction has occurred, if >10%, there may be an inhibitor present. Some use the upper limit of normal or some other decision point, but basing your decision on the value of the control used in your test system is the most defensible approach.
When performing the PTT–LA, Sta-Clot LA or the DRVV check, do not perform an additional mixing study using the specific screening reagents. You may neutralize, and thus miss, a weak LA using the normal plasma in the mix. If the screen is prolonged, go right on to the confirmatory assay.
Both the PTT-based and DRVV-based assays are necessary, as they are not confirmatory. An LA may be found using one, the other, or both. Many lab directors choose to do one first, and if positive, cancel the second to save resources.