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More on False Positive Hex-phase Phospholipid Tests

On November 29 I attempted to answer the following question from G Pihan of Beth Israel Deaconess Medical Center in Boston: “What is the incidence of false positive hexagonal phospholipid neutralization tests in the presence of a strong (400-800 BU) factor VIII inhibitor?” Click on False Positive Phospholipid Neutralization to see the discussion, which was lengthy and inconclusive.

I had distributed this question to several colleagues. One  was Marlies Ledford-Kraemer, MBA, BS, MT(ASCP)SH, who manages CLOT-ED, Inc., in association with theECAT Foundation. Marlies responded January 6 with a helpful reference:

From Marlies: I can refer you to an article published by Doug Triplett‘s group on the hexagonal phase phospholipid assay: Triplett DA, Barna LK, Unger GA. A hexagonal (II) phase phospholipid neutralization assay for lupus anticoagulant identification. Thromb Haemost 1993;70:787-93.  They made note of one presumed false positive in a patient sample with a high titer FVIII inhibitor, though they did qualify their comments a bit: “The patient was a severe hemophiliac who had been exposed to a wide variety of factor VIII replacement therapy. Although the patient result is presumed to be a false positive, we cannot rule out the presence of an underlying LA in this sample.” Later in the article they write, “A potential source of a false positive Staclot LA in the setting of a factor VIII inhibitor may be due to greater decrease of VIII activity in the diluent tube compared to the hexagonal (II) phase PL tube.” This, of course, still fails to answer G Pihan’s question, however it is clear this concern existed early in hexagonal phase phospholipid neutralization tests.

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