A Practical Guide to Laboratory Haemostasis


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.

Reference Ranges

Normal Healthy Patients:
LA+ve individuals:
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.

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.