Background: Resistance to activated protein C (APC resistance) is a thrombophilic abnormality characterized by a normal plasma level of protein C and an inherited defect in the coagulative response. This condition is believed to be caused by a point mutation in factor V, the so-called factor V Leiden, and is inherited as an autosomal dominant trait.
Purpose: A case-control study was carried out to evaluate the prevalence of APC resistance and factor V Leiden in patients with retinal vein occlusion (RVO) and in control subjects.
Methods: Eighty-four consecutive RVO patients and 70 controls were tested for APC resistance with a commercial assay (Chromogenix). The first 30 patients and 47 controls were also studied for factor V Leiden. In addition, a repeat APC-resistance test was performed in 40 RVO patients and in 9 controls with a second-generation assay done to compare the reliability and reproducibility of the tests.
Results: Results of testing for APC resistance with the first-generation assay revealed positive results in 38 (45%) of the study patients and 6 (9%) of the controls. The difference in frequencies of APC resistance in patients and controls was statistically significant (P < 0.0001). In the patients tested for factor V Leiden, one (3%) was a heterozygous carrier of the Arg506GIn mutation and one (2%) of the controls was a heterozygous carrier. No homozygous individuals were identified in either the study or the control groups. The difference in frequencies of factor V Leiden in study patients and controls was not statistically significant (P = 1). The repeat APC-resistance assay using factor V-deficient plasma in 40 RVO patients and 9 controls did not show any significant difference between study patients and controls or an association between APC resistance and the determination of the factor V Leiden mutant.
Conclusion: The first-generation commercial assay for APC resistance is not a useful screening test. The molecular test for factor V Leiden is the only definitive method. Furthermore, no significant association was found between factor V Leiden and retinal vein occlusion. Accordingly, routine testing for the presence of the factor V Leiden mutant is not advisable for patients with retinal vein occlusion.