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A Practical Guide to Laboratory Haemostasis

 

Factor V Ratio



Introduction

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.

Principles

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.

Method

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.

Interpretation

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:
1.09±0.1
LA+ve individuals:
4.82±3.34
LA+ve individuals on warfarin:
3.9±1.2

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.