The risk of false results in the assessment of platelet function in the absence of antiplatelet medication: comparison of the PFA-100, multiplate electrical impedance aggregometry and verify now assays.

OBJECTIVES

Evaluation of aspirin (ASA) responsiveness with platelet function tests varies by the choice of blood mixture and functional test and cut off values for defining the the treatment used. Addition to that we also aimed to determine agreement between three tests and to research whether there is any necessity to measure baseline platelet activity.

METHODS

The study group comprised of 52 patients with multiple risk factors receiving primary prophylaxis of ASA (100 mg/day). For each patient inhibition of platelet aggregation with aspirin was determined using three different whole blood tests: Multiplate electrical impedance aggregometry, Verify Now Aspirin, and collagen-epinephrine closure time PFA-100. Platelet aggregation was assessed with multiplate electrical impedance aggregometry,and was defined as the area under curve (AUC,AUxmin). Maximal 6,4 microM collagen-induced AUC were used to quantify platelet aggregation due to ASA. The ASA response was defined as >30 % reduction in basal platelet aggregation with multiplate electrical impedance aggregometry. Collagen induced platelet aggregation at the Verify Now Aspirin assay quantitated the ASA-induced platelet inhibition as aspirin reaction units (ARU). According to manufacturer insert ARU>550 indicates aspirin resistance. ASA platelet function studies were assessed twice at baseline (pre-aspirin), and after 7 day(post-aspirin) were performed.

RESULTS

After ASA intake none of the patients was found aspirin resistant with PFA-100. (CEPI-CT (129+/-36 vs 289+/-18 ). None of the patients was found aspirin resistant with PFA-100. As>30 % reduction in basal platelet aggregation with multiplate electrical impedance aggregometry is selected all of the patients have been stratified as responders.(COL TEST 688+/-230 vs 169+/-131 AU) None of the patients with Verify Now Aspirin found resistance to ASA(594+/-62 vs 446+/-43).Prior to ASA intake 15 of all patients with VN(501+/-16) and 2 of all patients with multiplate electrical impedance aggregometry (223+/-40 AUC )aggregation levels below the cut off label before ingestion of ASA.None of the patients was above the cut off label with PFA -100 (129+/-36).

CONCLUSIONS

Verify Now ASA assay, multiplate electrical impedance aggregometry and PFA-100 seem to be reliable tests in reflecting ASA effect on platelets. Cut off labels for the defining the responsiveness given by manufacturer may show significant interindividual variability with Verify Now ASA assay and multiplate electrical impedance aggregometry, and these test may show platelet inhibition despite the absence of ASA intake. Consideration of the pretreatment values may eliminate the risk of overestimation in the assessment of platelet inhibition by ASA.