Screening with the activated protein C resistance assay yields significant savings in a patient population with low prevalence of factor V leiden.

Testing for factor V Leiden can be performed with a molecular assay or a test for activated protein C resistance. We noted that physicians in our institution tended to order the molecular test 80% of the time, but the prevalence of the mutation in our patient population was less than 10%. Consequently, we decided to introduce the activated protein C resistance assay in house and consistently use it for screening before the more expensive genetic test and to negotiate a discounted charge for the latter at a reference laboratory. After 6 years since these interventions began, the prevalence of an abnormal screening test result remained low (202/2,475 [8.2%]), even among white patients (10.9%). With this simple approach, the cost to test patients for factor V Leiden decreased by more than 90%, while the productivity of our laboratory increased by the introduction of a high-volume, fully automated assay.

The effect of lupus anticoagulant in the second-generation assay for activated protein C resistance.

The activated protein C resistance (APCR) assay is the test of choice to screen for factor V Leiden. We evaluated the effect of lupus anticoagulant on the baseline clotting time of the second-generation APCR assay with plasma samples from 54 patients to determine whether a falsely low APCR ratio could be predicted. We also assessed whether a modification of the assay could make it more reliable in the presence of strong lupus anticoagulants. Of 54 plasma samples, 5 yielded a false-positive APCR ratio, and all 5 had a prolonged baseline clotting time. Further dilution (1:40) of the plasma samples in factor V-deficient plasma led to correction of the APCR ratio and did not affect the sensitivity of the test for factor V Leiden. Our data support that the baseline clotting time is a good predictor of a false-positive APCR test result and should be checked before calculating the ratio. The modified APCR assay reliably identified the false-positive ratios and could be used to screen for factor V Leiden in samples with strong lupus anticoagulant.