Ratio of factor V activities in PT and APTT assays as a new diagnostic marker of lupus anticoagulant.
Lupus anticoagulant (LA) is an antibody that interferes with phospholipid-dependent coagulation tests. We investigated the usefulness of the ratio of factor V activity determined by the Simplastin auto test (PT assay) to factor V activity determined by the Platelin Excel LS test (APTT assay) for detection of LA in plasma samples obtained from 276 patients with haematological and non-haematological disorders and 73 healthy subjects. This ratio was significantly higher in the 15 LA-positive (4.82 +/- 3.34) than in samples from healthy subjects (1.09 +/- 0.10) and was > 1.4 in 10 of the remaining 261 patient samples. The ratio was particularly low in the 54 samples from warfarin-treated patients. These findings suggest that determination of this ratio may be useful as a routine laboratory test for detection of LA. This test requires no specific antigens and can be applied in patients receiving anticoagulants such as warfarin and heparin.