From “Sunny,” What is the reason for not using prothrombin time (PT) in investigating lupus anticoagulant (LA)? It is also a phospholipid-dependent assay. Thanks.
Hello, and thank you for your question. Although a particularly avid LA may on occasion prolong the PT, PT reagent (thromboplastin) phospholipid is typically concentrated enough to neutralize plasma LA so that it has no, or limited, influence on the assay. The tissue thromboplastin inhibition time (TTI, also called dilute PT) test is a PT modification in which the reagent thromboplastin is diluted, and hence the phospholipid concentration is reduced, see Triplett DA, Brandt J. Annotation, laboratory identification of the lupus anticoagulant. Br J Haematol 1989;72:139. Although the LA panel used by most laboratories features the dilute Russell viper venom time (DRVVT) test and LA-sensitive (low phospholipid) partial thromboplastin time (PTT), the TTI has been used successfully by many laboratories. In 2009, however, the TTI failed to retain the recommendation of the International Society for Thrombosis and Haemostasis Subcommittee on Lupus Anticoagulant/Phospholipid-dependent Antibodies. See Pengo V, Tripodi A, Reber G, et al. Update of the guidelines for lupus anticoagulant detection. J Thromb Haemostas 2009;7:1737. The Subcommittee offered the opinion that the TTI suffers from variability in thromboplastin reagents. This recommendation is up for continued debate, and many laboratories continue to offer the TTI as a means for detecting LA.