Factor V Ratio
Lupus anticoagulants have been associated with activated Protein C [APC] resistance related to FV activity. it is possible, therefore, that lupus anticoagulants may interfere with Va activity.
The Factor V ratio relies on the finding that the PT is usually normal or only slightly prolonged in individuals with a LA whereas the APTT [with appropriate reagents] is or may be prolonged. This test derives a ratio of the FV activity using a PT-based assay to that derived using an APTT-based assay.
Factor V activity is measured in a 1-stage APTT and a 1-stage PT assay and the ratio of the two is then derived.
The ratio of the PT FV to the APTT FV is very tight in normal healthy individuals with a mean value of 1.09±0.1. The ratio was significantly higher in individuals with a LA - 4.82±3.34. In patients with a LA and on warfarin the ratio was similar at 3.9±1.2.
|Normal Healthy Patients:||
|LA+ve individuals on warfarin:||
What Test Next
In any test for a LA, some form of confirmatory test is necessary to demonstrate PL-dependence of the antibody.
In addition all positive tests should be repeated in 12 weeks.
Not all tests will detect all LAs and this test should be combined with a second test using different methodology e.g. DRVVT.
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