## 4 Discussion  
The results of the present study demonstrate that the platelet aggregation findings as obtained by impedance aggregometry are critically dependent on the platelet count: a strict proportional relation between both variables was demonstrated in samples with both reduced, normal and elevated platelet  
Figure 1: (**A**) The effect of platelet count on the impedance aggregometry reading and (**B**) the ratio of the impedance aggregometry reading and platelet count. Platelet-rich plasma samples were diluted with platelet-poor plasma to obtain different platelet counts. Impedance aggregometry readings were correlated with platelet count (n = 31, r = 0.78, \(p\) = 0.0001, power = 0.99). In contrast, the ratio of the impedance aggregometry reading and platelet count was independent of platelet count and thus specific for platelet function (n = 31, r = 0.16, \(p\) = 0.4). Shown are individual data points as well as linear regression lines and 95% confidence intervals. Figure 2: Platelet function as determined with impedance aggregometry in 8 healthy probands. Shown are individual dilution experiments demonstrating the effect of the platelet count on impedance aggregometry readings (**A**) before and (**B**) after correction for the platelet count. Individual correlation in (**A**) ranged from 0.88 to 0.94 (\(p\) < 0.05). Note, that more than 5-fold differences in aggregation between probands can be observed after correction for the platelet count (**B**). count, respectively. Correction can easily be achieved by formation of a ratio of impedance reading and the correspondent platelet count. The conventional procedure to determine platelet function is the Born aggregometry [1; 17]. The method determines the transmission of light in a platelet-rich plasma sample upon induction of platelet aggregation by an activator. For this procedure, the platelet count is adjusted to a certain value for reasons of comparability of results. The method is technically demanding, requires a specialized laboratory and is not easy to standardize. Impedance aggregometry, in contrast, relies on the aggregation of activated platelets to the surface of electrodes. Measurements are performed in anticoagulated whole blood samples and the samples are not adjusted for platelet count before the assay. Several standardized activators are commercially available; thus, a detailed analysis of platelet aggregation characteristics is feasible. Computerized design and the use of unprocessed whole blood enables the determination at the bedside. The fact that impedance aggregometry uses unprocessed whole blood implicates that there is no correction for platelet count. As shown in the present study, impedance aggregometry readings are dependent on both the platelet function and platelet count. This fact might not be critical when the pattern of aggregation obtained by several activators is compared. For example, clopidogrel leads to a specific reduction of the ADP-induced platelet aggregation, while the response to the remaining activators (arachidonic acid, collagen, thrombin receptor activating peptide) is unaltered. Thus, the differences in impedance aggregometry readings with the different activators might give an impression for eventual action of platelet antagonists. In other clinical situations, the thrombotic risk might likely be defined by both the platelet count and platelet function. However, in other constellations, the interpretation of results might be impossible. For example, either triofiban or thrombocytopenia can decrease impedance aggregometry findings and the diagnosis of etiology is impossible from the readout. In this case, correction of impedance aggregometry finding will lead to the right diagnosis. There are, however, some disadvantages of the correction. At first, the complexity of the procedure increases and the use at the bedside might be impossible in some settings. Moreover, errors of both impedance aggregometry and the platelet count measurement increase the combined error, when using the calculation for correction.