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LAC Testing on Warfarin Specimens

A colleatue writes that we frequently encounter cases in which the physician orders a lupus anticoagulant [LC] profile despite a normal PTT. We use Siemens ACTIN FSL and it returns a positive. Of course with a normal PTT, there is no place for mixing study. How to explain this?

Also, when taahe patient is on warfarin we do a 1:1 mix with pooled normal plasma[PNP]. then perform the DRVVT screen and confirm steps and the test is interpreted as positive when exceeding the cut off. Is this mixing enough to fullfil the criteria for LAC testing? I know that LAC testing during warfarin therapy is not encouraged but we cannot simply reject the test, that’s why we adopt the mixind step to overcome the inactive coag factors.

Hello, and thank you for your question. Many facilities employ a PTT reagent with intermediate LAC sensitivity for heparin monitoring and coagulopathy screening. If the physician finds a clinical indication for thrombophilia such as unexplained thrombosis or a series of spontaneous abortions, he/she may request a LAC profile even though the “routine” PTT assay result is normal. In this instance, the laboratory employs a separate, LAC-sensitive reagent such as Actin-FSL or PTTLA. The LAC-sensitive PTT reagent may return a positive LAC result even though the routine LAC reagent returned a normal PTT. There is no need to follow up with a mixing study.

While some DRVVT kit manufacturers claim LAC accuracy in the presence of vitamin K antagonists, most practitioners avoid the DRVVT assay on patients who are known to be on warfarin because both the screen and confirm values are prolonged, introducing possible inaccuracy. Diluting with pooled normal plasma prior to testing may dilute the avidity of the LAC, causing a false negative. Some operators perform the DRVVT on the undiluted patient specimen and follow up equivocal results with a pooled normal plasma mixing study to confirm LAC.

There are several ways to deal with LAC testing under various conditions of avidity and interferences, and we invite comments on these recommendations. [Geo]

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