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Clinical Chemistry 52: 383-388, 2006. First published January 19, 2006; 10.1373/clinchem.2005.059535
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Right arrow Hemostasis and Thrombosis
(Clinical Chemistry. 2006;52:383-388.)
© 2006 American Association for Clinical Chemistry, Inc.

Hemostasis and Thrombosis

Determination of Clopidogrel Resistance by Whole Blood Platelet Aggregometry and Inhibitors of the P2Y12 Receptor

Boris T. Ivandica, Philipp Schlick, Peter Staritz, Kerstin Kurz, Hugo A. Katus and Evangelos Giannitsis

Department of Medicine III, University of Heidelberg, Heidelberg, Germany.

aAddress correspondence to this author at: Innere Medizin, Abt. III, Universitätsklinikum Heidelberg, Otto-Meyerhof-Zentrum, Im Neuenheimer Feld 350, 69120 Heidelberg, Germany. Fax 49-6221-56-4866; e-mail Boris.Ivandic{at}med.uni-heidelberg.de.

Background: Inhibition of platelet aggregation by clopidogrel may be insufficient in up to 30% of users. These nonresponders carry an increased risk of cardiovascular events. We reported here a simple assay to study clopidogrel responsiveness.

Methods: Electrical impedance aggregometry was performed in diluted whole blood in the presence of 5 and 20 µmol/L ADP. Some samples were incubated with 0.1 mmol/L methyl-S-adenosine monophosphate (MeSAMP), a P2Y12 receptor blocker, to maximize inhibition of aggregation before aggregometry. To validate the assay, we analyzed 6-min impedance in 21 healthy probands and 244 patients with coronary artery disease (CAD).

Results: At 5 µmol/L ADP, the imprecision of the assay was 11%. Mean (SD) impedance of the healthy cohort was 12.2 (2.2) {Omega}. The mean – 3 SD was used to define the cutoff for clopidogrel responsiveness: responders and nonresponders exhibited a 6-min impedance ≤5 {Omega} and >5 {Omega}, respectively. Samples from nonresponders were incubated with MeSAMP and analyzed again to distinguish pharmacokinetic and pharmacodynamic types of resistance. Sixteen percent of CAD patients were classified as nonresponders (38 and 2 cases of pharmacokinetic and pharmacodynamic resistance, respectively). Female sex was strongly associated with clopidogrel resistance (P = 0.0002, Fisher exact test). A higher clopidogrel loading dose (P = 0.0353, Mann–Whitney U-test) was given to responders (median, 450 mg) than nonresponders (median, 300 mg). Age and cardiovascular diagnosis showed no significant associations.

Conclusions: Impedance aggregometry using 5 µmol/L ADP is a useful tool for studying clopidogrel responsiveness. MeSAMP allows characterization of responsiveness "on treatment" and may be useful for optimizing clopidogrel dosing.




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