Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (900 page)

   Hb, lipid, and bilirubin do not typically interfere.

DILUTE RUSSELL VIPER VENOM (dRVVT) ASSAY
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   Definition

   The dRVVT assay detects the presence of LA. This test is helpful for the diagnosis of the antiphospholipid antibody syndrome and acquired thrombophilia.

   Use

   The dRVVT assay consists of three stages:

   The screening reagent initiates plasma clotting by directly activating factor X, thereby bypassing both the intrinsic and extrinsic pathways of coagulation. LA antibodies prolong the clotting time. If the clotting time is not prolonged in the presence of the dilute venom, LA is not present, and the second stage of the assay is omitted.

   If the clotting time is prolonged (>20% of the control), a PTT analysis on a 1:1 incubation of normal plasma with the patient’s plasma to discriminate between an inhibitor or a clotting factor deficiency (correction of the clotting time to <44 seconds) is routinely done. If there is no correction, and the clotting time remains prolonged, an inhibitor has been demonstrated, and the laboratory proceeds with the next step.

   A “confirmation” reagent containing a high concentration of phospholipid is added to the plasma under study. If the clotting time in the first phase has been prolonged by LA antibodies, the reagent neutralizes the antibodies and the clotting time becomes shorter, similar to that of the control. If the clotting time elongation in the first stage is not due to LA but a different inhibitor, the clotting time in the presence of the confirmation reagent remains elongated. Additional studies to rule out other etiologies for the initially prolonged clotting time are then indicated.

   Results are expressed as a ratio: The clotting time of the screening reagent is divided by the clotting time of the confirmation tests.

   Interpretation

   Ratio >2.0: LA strongly present.

   Ratio of 1.6–2.0: LA moderately present.