Specificity of an Assay Based on a Factor V-depleted Plasma in Patients Carrying the Arg 506 GIn Mutation

Activated protein C resistance, the factor V Leiden mutation, and a laboratory testing algorithm

OBJECTIVES

To present the current understanding of factor V Leiden and activated protein C resistance, and to propose a laboratory testing algorithm.

DATA SOURCES

Publications on MEDLINE with the terms factor V Leiden or activated protein C resistance through mid 2001, as well as publications in the authors' files, were screened for inclusion in this report.

STUDY SELECTION

Original studies that report a novel finding on testing or clinical features of activated protein C resistance or factor V Leiden are included. Data Extraction.-The novel or key findings from the selected studies are analyzed.

DATA SYNTHESIS

Protein C and protein S are the integral components of an anticoagulation pathway that limits fibrinogen conversion to fibrin through the degradation of factors Va and VIIIa. When factor Va is resistant to degradation by activated protein C, this anticoagulation pathway does not operate properly, and patients have an increased risk for thrombosis. This report describes the protein C/protein S pathway, the significance of activated protein C resistance and the factor V Leiden mutation, and the clinical testing used to detect activated protein C resistance and the factor V Leiden mutation. A proposed laboratory testing algorithm is also provided.

CONCLUSIONS

Factor V Leiden is a risk factor for venous thrombosis and it is particularly common in white populations. A laboratory testing algorithm is proposed.

Modification of the ProC Global assay using dilution of patient plasma in factor V-depleted plasma as a screening assay for factor V Leiden mutation

The activated protein C (APC) resistant-factor V (factor V Leiden) has emerged as the most common inherited risk factor for thrombosis in the Caucasian population. Beside DNA analysis, the laboratory diagnosis is often based on the detection of a poor anticoagulant response to exogenous APC. The ProC Global assay (Dade Behring, Marburg, Germany) is a global clotting assay, which was primarily developed to evaluate the functionality of the protein C anticoagulant pathway. It is based on the ability of endogenous APC, generated by activation of protein C by an extract from Agkistrodon contortrix contortrix venom, to prolong an activated partial thromboplastin time. It was previously found to be highly sensitive for the factor V Leiden mutation and for protein C deficiency, but only moderately sensitivity for protein S deficiency. Here, we evaluated the performance of a modification of the ProC Global assay using a 1 : 5 pre-dilution of patient plasma in factor V-depleted plasma in the screening of the factor V Leiden mutation-related APC resistance. For that purpose, we investigated selected frozen plasma samples from 341 patients with a history of venous thromboembolism. The sensitivity for the factor V Leiden mutation of the modified assay was found to be 100%, as all the carriers of that mutation (five homozygotes and 77 heterozygotes) had a decreased response to the assay, i.e. a normalized ratio below 0.80. Its specificity was also 100% since none of the other tested patients had a decreased response, i.e. isolated protein C (n = 3) or protein S deficiency (n = 50), or without any abnormality of the protein C pathway (n = 143), even those on oral anticoagulant treatment (n = 76). However, it would be preferable that each laboratory defines both its reference range and its cut-off level. Finally, even if larger-scale multicentre studies are needed before definite recommendations could be made, these results suggest that the ProC Global performed using a 1 : 5 pre-dilution of the patient plasma in factor V-depleted plasma could be validly used as a screening assay of the factor V Leiden mutation-related APC resistance in patients with a history of thrombosis.

Laboratory evaluation of hypercoagulable states

The number of well-characterized hereditary and acquired hypercoagulable conditions is increasing, such that in many thrombophilic patients, the laboratory can now identify a hypercoagulable condition. This review describes the currently known hypercoagulable states that predispose patients to venous, and in some instances, arterial thrombosis. For each condition, the discussion includes the incidence, magnitude of the thrombotic risk in the general population in comparison with symptomatic families, synergistic interactions among the various hypercoagulable conditions, molecular pathogenesis, and interpretation of laboratory test results. In addition, recommendations for laboratory testing are summarized.

Activated protein C resistance and anticardiolipin antibodies in patients with venous leg ulcers

BACKGROUND

Hypercoagulable states, which include the presence of anticardiolipin antibodies (ACAs), have been associated with skin ulceration. Resistance to activated protein C (APC), resulting from the factor V Leiden mutation, is a common risk factor for venous thrombosis. Its prevalence among patients with venous leg ulcers is not known.

OBJECTIVE

Our purpose was to determine the prevalence of factor V Leiden and ACA in patients with venous leg ulceration.

METHODS

Twenty-nine consecutive patients with venous leg ulcers were studied. Resistance to APC was first determined by functional assay based on the partial thromboplastin time. Patients with an abnormally low APC ratio were then subjected to molecular analysis for confirmation of factor V Leiden. Measurements of ACA were performed by enzyme-linked immunosorbent assay.

RESULTS

APC resistance was detected in 11 of 26 patients. However, only 2 of these 11 patients (7.7% overall) were found to be heterozygous for the factor V Leiden mutation. ACA was present in neither patient with the Leiden mutation but was found in 6 of 21 patients tested (29% overall).

CONCLUSION

The factor V Leiden mutation, unlike ACA, may not be more prevalent in patients with venous leg ulcers than in the general population. Our results emphasize the importance of molecular analysis for factor V Leiden in patients with APC resistance.

Evaluation of a new chronometric assay for factor V Leiden-dependent APC resistance

Since the first description by Dahlbäck et al (1) of a hereditary defect in the plasma of three unrelated thrombophilic patients characterized by a poor anticoagulant response to activated protein C (APC), many reports have shown a high prevalence of the APC resistance in both the general and thrombophilic populations (2-6). In almost all cases APC resistance is due to a single point mutation in the factor V gene, (a G to A substitution at nucleotide position 1691), which predicts the synthesis of a factor V molecule (FVQ506 or FV Leiden) in which the Arg 506 in one of the APC cleavage sites is replaced by Gln (7), rendering factor Va resistant to inactivation by APC. The FVQ506 mutation is the most frequent hereditary risk factor for venous thrombosis (2, 5, 6, 8-10) making an assay for APC resistance with a high predictive value for the mutation highly desirable, especially when molecular diagnosis at the gene level is not possible. The aim of this study was to evaluate the performance of a new factor Xa-based assay for the FV Leiden-dependent APC resistance.