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Lupus Anticoagulant (LA) Evaluation with Reflex

Test code(s) 7079X

Question 1. How is the Lupus Anticoauglant (LA) Evaluation with Reflex performed?

Question 2. The PTT-LA is prolonged, yet the interpretive comment states, “No Lupus Anticoagulant Detected.” Is this significant?

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 seconds, 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 warrant further investigation.

Question 3. Can I test my patient while he/she is anticoagulated with heparin or warfarin?

For warfarin, the screening tests (PTT-LA, dRVVT) may be prolonged, but the confirmatory tests are good at distinguishing LA from the 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 markedly 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 anticoagulated1

Question 4. What is the significance of a “weak” positive Hexagonal Phase Confirm?

The designation of "weak" LA is based on the observation that the Hexagonal Phase Confirm result is just above the cutoff for positivity. This is a laboratory finding only and does not indicate a lower potential to cause a thromboembolic event.

Question 5. The interpretation states that “a Lupus Anticoagulant is detected because 1 of 2 confirmatory tests is positive.” How can this be?

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.

Question 6. Can the newer anticoagulants (ie, factor Xa inhibitors such as rivaroxaban, apixaban, and endoxaban and direct thrombin inhibitors such as dabigatran and argatroban) cause lupus anticoagulant to be misclassified as being positive?

Yes, false-positive lupus anticoagulant results may occur when patients are taking either factor Xa inhibitors or direct thrombin inhibitors. If a patient is known to be taking one of these medications at the time of specimen collection, the lupus anticoagulant test result should be considered indeterminate.

See the Quest Diagnostics Nichols Institute publication, “Bleeding and Thrombosis: Better answers from high-quality testing.


  1. 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. 209,7;1737-1740.
This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
Document FAQS.1 Version: 3
Version 3 effective 06/27/2107 to present
Version 2 effective 09/18/2013 to 06/27/2017
Version 1 effective 01/21/2013 to 09/18/2013
Version 0 effective 08/01/2011 to 01/21/2013