University of Massachusetts Medical School Faculty Publications


Hematocrit alters VerifyNow P2Y12 assay results independently of intrinsic platelet reactivity and clopidogrel responsiveness

UMMS Affiliation

Department of Medicine, Division of Cardiovascular Medicine

Publication Date


Document Type



Adenosine Diphosphate; Aged; Blood Platelets; Female; *Hematocrit; Humans; Male; Middle Aged; Platelet Aggregation Inhibitors; Purinergic P2Y Receptor Antagonists; Receptors, Purinergic P2Y12; Ticlopidine


Cardiology | Cardiovascular Diseases | Hematology


BACKGROUND: The VerifyNow P2Y12 assay assesses the adequacy of clopidogrel therapy by measuring ADP-induced platelet activation in whole blood. Low hematocrit is associated with high clopidogrel on-treatment platelet reactivity (HTPR) defined by this assay.

OBJECTIVES: To characterize the effect of hematocrit on VerifyNow values and determine if it is due to hematocrit-dependent changes in intrinsic platelet reactivity or an in vitro assay phenomenon.

PATIENTS/METHODS: Adenosine diphosphate-induced platelet activation was measured using the VerifyNow P2Y12 assay, whole blood impedance and light transmission platelet aggregometry (LTA) before and after clopidogrel loading in 113 patients undergoing elective cardiac catheterization. Iso-TRAP-induced platelet activation was additionally measured using the VerifyNow device. Multivariate modeling employing clinical and laboratory variables was used to investigate the association between hematocrit and VerifyNow values.

RESULTS: VerifyNow P2Y12 reaction units (PRU) and iso-TRAP Base units before and after clopidogrel loading, but not their relative change, exhibited strong negative correlation with hematocrit (P < /= 0.0005 for both). While hematocrit remained a strong predictor of post-clopidogrel PRU (P = 0.001) in multivariate modeling, it was independent of post-clopidogrel ADP-induced platelet reactivity as measured by LTA (P = 0.001). Correcting for the effects of hematocrit resulted in a 15-39% reduction in the prevalence of HTPR defined by thresholds of 208-236 PRU.

CONCLUSIONS: The effect of hematocrit on VerifyNow PRU values is an in vitro phenomenon that is independent of intrinsic change in ADP-induced platelet reactivity and clopidogrel responsiveness. Correcting for hematocrit when using this assay may more accurately identify patients with HTPR that may benefit from alternative antiplatelet therapy.

DOI of Published Version



J Thromb Haemost. 2013 Oct;11(10):1814-22. doi: 10.1111/jth.12376. Link to article on publisher's site

Related Resources

Link to Article in PubMed

Journal/Book/Conference Title

Journal of thrombosis and haemostasis : JTH

PubMed ID