Assessment of Platelet Function in Whole Blood by Multiple Electrode Aggregometry: Transport of Samples Using a Pneumatic Tube System

The Journal published an interesting study by Paniccia et al1 regarding the assessment of platelet function in whole blood by multiple electrode aggregometry (MEA) in patients with coronary artery disease (CAD) receiving antiplatelet therapy. We and other groups have confirmed the applicability of this method2,3 and its predictive value for the occurrence of stent thrombosis.4 The authors,1 however, do not address the issue of sample transport inside hospitals. Owing to the analysis of whole blood, eliminating the need for sample centrifugation, and the use of an electronic computer-controlled pipette, MEA is applicable in a near-patient setting. In many medical centers, however, laboratory testing is performed in centralized institutes, which provide for 24-hour services and appropriate documentation and quality control. In modern hospitals, patients’ specimens are delivered using pneumatic tube systems (PTSs) to manage the heavy workload and to reduce turnaround times. Provided that the system is designed for the transport of clinical samples and that specimens have been correctly prepared, this kind of transport has been shown to be safe and without a clinically significant effect on the results.5,6 However, in contrast with many clinical chemical or coagulation analytes, platelets may be activated by excessive shaking or agitation of the sample, which may lead to spurious results. The aim of our study was to test the usability of an in-house pneumatic tube laboratory transport system for measuring platelet function with MEA by comparing it with manual on-foot delivery. A pair of blood samples from 20 consecutive patients scheduled for coronary angiography was collected into tubes containing the anticoagulant hirudin. The tubes had to be filled completely to avoid frothing formation. Of the patients, 19 received a single high dose of 600 mg of clopidogrel more than 2 hours before the procedure. The study was approved by the institutional ethics committee, and patients gave written informed consent for participation. Immediately after collecting the blood sample, one tube was packed into a plastic bag, placed in a pneumatic tube container, and sent to the central laboratory with a transport time of less than 1 minute. The other tube was transported on foot by a human courier. After arrival in the laboratory, both blood samples were allowed to rest at room temperature (22°C) for 30 minutes before measurement. Whole blood aggregation was assessed with MEA on a Multiplate analyzer (Dynabyte, Munich, Germany). After dilution of hirudin-anticoagulated blood with 0.9% sodium chloride, adenosine diphosphate (ADP) in a final concentration of 6.4 μmol/L was added, and aggregation was continuously recorded for 5 minutes. Aggregation measured with MEA is quantified as arbitrary units (AU) and area under the curve (AUC) of arbitrary units (AU*min). Statistical analysis was performed using the Spearman rank correlation coefficient and a method comparison according to Bland and Altman. The AUC results for the manually delivered samples ranged between 17 and 1,196 AU*min; the median value was 220 AU*min. Tubed samples ranged between 22 and 907 AU*min; the median value was 220.5 AU*min. The correlation coefficient was very good (R = 0.95) and highly significant (P < .0001). Figure 1 shows a scatter plot of the results demonstrating a satisfactory agreement between the 2 ways of delivery. Method comparison using the Bland and Altman method showed a mean ratio of 1.05 (−2SD = 0.48 and +2SD = 1.62) of the results by manual transport vs PTS and no significant deviation from linearity. The present study demonstrates that transport of correctly prepared samples by a state-of-the art PTS for platelet aggregation tests using the Multiplate device does not alter the results of platelet function testing. These observations are in agreement with the findings of Dyskiewicz-Korpanty et al,7 who did not find differences after PTS transport compared with conventional transport in ADP-induced platelet aggregation using a whole blood lumiaggregometer (Chrono-log). Concerning the PTS, it is important that acceleration and deceleration phases, particularly at bends, transfer units, and at the terminal stop in the laboratory, are minimized. Different hospitals may use different transport systems and different blood collecting tubes. Therefore, it is recommended that each institute evaluate its own systems. Transport of correctly prepared samples for platelet aggregation tests using the Multiplate device can be done with a state-of-the art PTS. Scatter plot showing the correlation between adenosine diphosphate (ADP)-induced platelet aggregation (in arbitrary units [AU*min]) in whole blood after pneumatic tube transport (abscissa) vs sample delivery by foot (ordinate) measured by multiple electrode aggregometry (MEA). R = 0.95. P < .0001. 2009 We thank Drs Sibbing and colleagues for their interest and comments on our investigation of platelet function assessed on whole blood by MEA in patients with CAD receiving antiplatelet therapy.1 Our group, too, is engaged in assessing platelet function in the laboratory by using different methods, confirming the applicability of MEA and its predictive value for the occurrence of stent thrombosis.2–4 The results of this study have been obtained in the framework of an ongoing project started in October 2004 aimed to investigate the prevalence, pathophysiology, and the clinical implications of residual platelet reactivity in patients with cardiovascular disease undergoing percutaneous intervention (PCI). In particular, we are involved in the assessment of new platelet function tests to use for patients with high-risk CAD (especially after PCI with stenting) as tools to monitor the efficacy of antiplatelet therapies and to identify residual platelet reactivity to predict possible clinical outcomes.5–7 In our investigation, we did not address the description of sample transport inside our hospital, but we focused our attention on the interval from specimen withdrawal to the execution of different platelet function tests (aggregation on platelet-rich plasma and whole blood). In particular, to optimize the quality procedure in our laboratory, we perform platelet aggregometry within 2 hours from specimen withdrawal.8 For laboratory quality, the standardization of platelet aggregometry is one of the main objectives.9 The University Hospital of Careggi in Florence, Italy, consists of different pavilions scattered in a wide area (about 2 km2), and the blood samples are hand delivered by a dedicated service with prefixed times to optimize turnaround times (TATs). Only the central laboratory and the emergency department are in the same pavilion, and the samples from the emergency department for screening tests (except blood gas analysis and troponin determination) are delivered using PTSs to reduce the TAT. Ensuring the quality of blood sampling and its delivery is of highest importance before any platelet testing is performed. All of our blood specimens are hand delivered, and, in particular, for platelet function tests, a special emergency delivery service exists to reduce TAT to about a half hour (similar to the time for PTS). At the Department of Heart and Vessels of Careggi there is a collaboration of intent between coronary care units and our laboratory, but also a physical proximity exists to allow a reduction of TAT. A mean of 20 blood specimens for the evaluation of platelet function is transported daily. This organization allows the reduction of errors in the preanalytic phase.10 The interval from specimen withdrawal to testing should be standardized because the loss of carbon dioxide from the blood sample results in an increase of pH that amplifies platelet responsiveness to aggregating agents. The formation of air bubbles and damage of the erythrocyte membrane should be prevented to avoid hemolysis because these cells liberate ADP, an important platelet-aggregating agent. With regard to the interest of Drs Sibbing et al in seeing data relating to the effect of PTS on the platelet activation function, we appreciate their pilot study in which no effect on platelet function was found by the shaking of tubes due to this type of transport, and we recognize the importance of these data.11 Since evaluation of platelet function is of crucial importance in the management of high-risk patients receiving antiplatelet therapy, improved ability to assess platelet function by point-of-care testing in nonspecialized laboratories in a timely and efficient manner is essential.