Impedance Aggregometry platelet aggregation is dependent on platelet count
Mohamed Soliman, Jürgen Peters, Matthias Hartmann
Klinik für Anästhesiologie und Intensivmedizin, Universität Duisburg-Essen, Universitätsklinikum Essen, Essen, Germany.
Introduction: Impedance aggregometry (Multiplate ®) has proved its usefulness in the prediction of thrombotic complications after coronary and cerebrovascular stent interventions (1-3). However, although the evidence is poor, it is often taken for granted, that the platelet count when within the physiological range does not affect platelet aggregation. We hypothesized that 1) impedance aggregometric values and platelet count are highly correlated and 2) the aggregation/platelet count ratio expresses platelet function independent of platelet count. 
Methods: Following ethics committee approval, platelet rich plasma, derived by centrifugation with 110 g for 5 minutes at 24oC of blood samples from 8 different patients, was diluted with platelet poor plasma creating different concentrations of platelets. Thereafter, platelet count was measured and samples were subjected to impedance aggregometry using thrombin receptor-activating peptide (TRAP) for platelet activation. Platelet poor plasma was obtained by centrifugation at 1800 g for 10 minutes at 24oC. Aggregation and platelet count were correlated and aggregation was compared in undiluted and 2-fold diluted samples. Statistics: Pearson correlation and Student`s t-test for paired sample. 
Results: In every single experiment, platelet count and impedance aggregometry highly correlated (mean correlation coefficient: 0.91 ± 0.07), and, in all measurements (n=32), the correlation between evoked platelet aggregation and platelet count was (r= 0.78, p= 0.0001), with an aggregation unit/platelet count correlation of 0.16 (p= 0.3). Moreover, there was a significant difference when comparing platelet aggregation in undiluted and 2-folds diluted samples (n=19, 94.3 AU vs 41.8, p= 0.0001). In contrast, there was no difference in the aggregation/count ratio between undiluted and 2-folds diluted samples (n=19, 0.33 AU/103 platelet mm-3 vs 0.27, p= 0.13).
Conclusion: In individuals, there is a strict proportionality between platelet count and aggregation, and, therefore, this variable might be indicative of the number of platelets aggregating when evoked by a particular activator. Conversely, normalization of impedance aggregometry findings can be achieved by simply dividing aggregation by platelet count.
References: 1. Siller-Matula, J Thromb Haemost. Feb;8(2): 351-9, 2. Straub, N. Thromb Haemost. Oct 24;111(2). 3. Müller-Schunk S. Am J Neuroradiol. 2008;16:786–791. doi: 10.3174/ajnr.A0917.