Activated protein C resistance assay performance: improvement by sample dilution with factor V-deficient plasma.

OBJECTIVE

To evaluate a modification of a commercially available reagent kit (COATEST APC Resistance Kit) for functional activated protein C (APC) resistance testing, and to determine the ability of the modified assay to demonstrate APC resistance in patients receiving warfarin.

DESIGN

Functional APC resistance testing was performed using both the first-generation COATEST APC Resistance Kit and a modified, or second-generation, version of the COATEST assay that uses predilution of the patient sample with factor V-deficient plasma. Factor V genotyping for APC resistance (FV R506Q) was performed using a well-characterized polymerase chain reaction-restriction fragment length polymorphism method.

SETTING

University medical center.

PATIENTS

Seventy-three individuals referred for hypercoagulability testing who were not receiving warfarin therapy and 29 patients with a history of venous thrombosis who were receiving warfarin therapy.

MAIN OUTCOME MEASURE

Sensitivity and specificity as determined by comparing functional APC resistance to the FV R506Q genotype.

RESULTS

In 73 patients referred for hypercoagulability testing, but not receiving warfarin therapy, a sensitivity of 0.86 and a specificity of 0.75 were obtained with the first-generation COATEST assay. In contrast, a sensitivity and specificity of 1.0 were obtained when the second-generation COATEST assay was employed. In 29 patients receiving warfarin, the first-generation assay exhibited a sensitivity and specificity of 0.88 and 0.95, respectively, whereas the sensitivity and specificity for the second-generation assay was 1.0.

CONCLUSIONS

Predilution of patient plasma with factor V-deficient plasma results in improved sensitivity and specificity of the COATEST APC Resistance Kit, thus offering a simple modification to enhance APC resistance determination in the routine clinical laboratory setting.