Lupus Anticoagulant (LA) Evaluation with Reflex

The PTT-LA is a sensitive, but nonspecific screening test for LA. It may be prolonged if the patient is on anticoagulant therapy (ie, heparin, warfarin, or thrombin inhibitors).

If the patient is not anticoagulated and the result is ≤ 50 sec then you may want to consider an aPTT with a routine reagent. PTT-LA is a modified aPTT with dilute phospholipids that make it very sensitive to LA. The trade-off for the increased sensitivity is a small percentage of false-positive screens. A compelling clinical history, bleeding, or thrombosis, warrants further investigation.

For warfarin, the screening tests (PTT-LA, dRVVT) may be prolonged, but the confirmatory tests are good at distinguishing LA from warfarin effect.

For unfractionated heparin, the Hexagonal Phase and both dRVVT tests have heparin neutralizers. It is possible to overwhelm the neutralizing capacity resulting in false- positive confirmatory studies. Expect the PTT-LA to be prolonged.

However, many thought leaders–including the Scientific Subcommittee (SSC) of International Society of Thrombosis and Haemostasis (ISTH) on LA–recommend not testing patients for LA while anticoagulated. Pengo, et al.  J Thromb Haemost. 2009;7(10):1737-1740.

The designation of "weak" LA is based on the observation that the Hexagonal Phase Confirm result is just above the cut-off for positivity. This is a laboratory finding only and does not reflect the in vivo thrombophilia potential.

Alter the risk of clinical thrombophilia.

LAs are heterogenous antibodies that differ in titer, avidity, and isotype. This is why the ISTH recommends using two different reagents that evaluate different coagulation pathways (PTT-LA–intrinsic pathway, dRVVT–common pathway) to increase the sensitivity of detection.