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Lupus Anticoagulant (LA)

Definition

  • LAs are heterogeneous IgG or IgM autoantibodies that inhibit phospholipid-dependent assays of blood coagulation. Because phospholipid is essential for several steps in the coagulation cascade, the presence of LAs can prolong various phospholipid-dependent clotting times, such as PTT, PT, and the dilute Russell viper venom time (dRVVT).

Use

  • None of the tests mentioned in the Definition discussion is sufficiently sensitive to detect all LAs; therefore, two screening tests are required before LA can be excluded.
  • The most commonly used screening tests are PT (1:100 diluted) and dRVVT. (Kaolin or micronized silica clotting time is no longer in use.) A positive screening test (prolonged dilute PT or dRVVT) requires confirmation by adding excess phospholipids in the test.

Interpretation

  • A normalization of the clotting time in either test confirms the presence of LA but requires that the tests be repeated in 12 weeks, because frequently the LA is a temporary phenomenon (Fig. 16.1).

Figure 16 " 1Algorithm for testing lupus anticoagulant antibodies.View OriginalFigure 16 " 1Algorithm for testing lupus anticoagulant antibodies.View Original

Limitations

  • There is considerable interlaboratory variation with the performance of the LA assays, especially dRVVT. In recent surveys, there was a false-positive detection of LAs in 24% of samples and a false-negative result of 18.5% in participating centers. One of the factors that may contribute to a false-positive result is contamination with heparin. Preanalytical variables, such as improper plasma preparation, may lead to false-negative results because of contamination with platelets.
  • It is recommended that assessment for LA not be undertaken while the patient is on oral anticoagulants, if at all possible (see Fig. 16.1).

Suggested Readings

1Giannakopoulos B, Passam F, Ioannou Y How we diagnose the antiphospholipid antibody syndrome. Blood. 2009;113:985 " 994. 2Moffat KA, Ledford-Kraemer MR, Plumhoff EA Are laboratories following published recommendations for lupus anticoagulant testing? Thromb Haemost. 2009;101:178 " 184.