Anesthesiology 2006; 105:676 – 83 Copyright © 2006, the American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc. Quantifying the Effect of Antiplatelet Therapy A Comparison of the Platelet Function Analyzer (PFA-100 ® ) and Modified Thromboelastography (mTEG ® ) with Light Transmission Platelet Aggregometry Seema Agarwal, F.R.C.A.,* Margaret Coakely, F.R.C.A.,† Kalpana Reddy, F.R.C.A.,* Anne Riddell, M.Sc., F.I.B.M.S.,‡ Susan Mallett, F.R.C.A.§ Background: Antiplatelet therapy with aspirin and clopi- dogrel is known to confer protection against ischemic events. Increasing numbers of patients are presenting for surgery while taking these drugs. This may lead to an increase in peri- operative blood loss, particularly in those who have a height- ened response to the drugs. Identifying these patients preoper- atively would allow us to plan appropriate management. Methods: The antiplatelet effect of aspirin and/or clopi- dogrel was measured using two point-of-care monitors: the platelet function analyzer (PFA-100 ® ; Dade, Miami, FL) and the modified thromboelastograph (mTEG ® ; Haemoscope Corp., Niles, IL). This was compared with optical light transmission aggregometry. Results: All people taking aspirin displayed a definitive aspirin effect on aggregometry (n  20). Ninety percent of these were identified by modified thromboelastography (n  18). Seventy percent were identified by the platelet function ana- lyzer (n  14). Fifty percent of people taking clopidogrel dis- played a definitive response to the drug on aggregometry. Sev- enty percent of these were identified on modified thromboelastography (n  7). None were identified by the platelet function analyzer. There was good agreement between the results of the aggregometry and modified thromboelastog- raphy in clopidogrel patients (  0.81). Conclusion: The search for a point-of-care monitor of plate- let function has been the focus of much research. This study has shown that the modified thromboelastograph can be used for monitoring the effect of clopidogrel as well as aspirin. It poten- tially has a wide scope to be used for the monitoring of effec- tiveness of therapy as well as a possible predictor of perioper- ative bleeding. ANTIPLATELET therapy with aspirin and/or clopidogrel is known to protect against vascular occlusive events including myocardial infarction, acute coronary syn- drome, and stroke. 1 In addition, some studies have shown an additional benefit when the drugs are taken together. 2,3 Numerous patients presenting for surgery will be tak- ing one or both of these drugs. It is common practice to stop them 7–10 days before surgery because of a per- ceived increase in perioperative bleeding. A number of studies (mainly in cardiac and vascular surgery) in pa- tients taking aspirin or clopidogrel have demonstrated a trend toward increased blood loss and transfusion re- quirements. 4–7 However, the evidence for this increase is not consistent or universal with further studies unable to demonstrate a significant effect. 8 –11 It is now becoming clearer that there is a spectrum of response to these drugs, with some patients having min- imal change in platelet function (resistance) whereas others are “hyperresponders.” 12–14 This variability in pa- tient response may account for the fact that, although trends to increased blood loss are evident, not all pa- tients seem to have the same bleeding risk when taking the same dose of antiplatelet drugs. There is also some concern that stopping antiplatelet therapy before surgery and allowing the recovery of platelet function may put the patient at an increased risk of ischemic events. In 2002, the French Society of An- aesthesiology recommended that “the common practice of withdrawing antiplatelet agents [be] challenged be- cause [of] an increased incidence of myocardial infarc- tion in patients in whom treatment was interrupted.” 15 The American College of Chest Physicians has also em- phasized the need to continue preoperative medication including antiplatelet drugs in the perioperative period. In addition to this, there is a growing population with drug-eluting coronary stents who are at high risk for stent occlusion. They are usually recommended to con- tinue taking both aspirin and clopidogrel in the year after stenting even when undergoing high-risk surgery. 16 In light of this, it would be advantageous to have an effective method of monitoring the effects of antiplatelet drugs. Patients with profound inhibition of platelet func- tion could be identified before surgery, and appropriate management could be planned in advance, including Additional material related to this article can be found on the ANESTHESIOLOGY Web site. Go to http://www.anesthesiology- .org, click on Enhancements Index, and then scroll down to find the appropriate article and link. Supplementary material can also be accessed on the Web by clicking on the “Arti- clePlus” link either in the Table of Contents or at the top of the Abstract or HTML version of the article.  * Specialist Registrar in Anaesthesia, § Consultant in Anaesthesia, Royal Free Hospital. † Consultant in Anaesthesia, University Hospital of Wales, Cardiff. ‡ Senior Scientist, Research and Development, Department of Haemophilia, Royal Free Hospital. Received from the Department of Anaesthesia, Royal Free Hospital, London, United Kingdom. Submitted for publication February 24, 2006. Accepted for publication May 25, 2006. Monetary funding for the purchase of PFA-100 ® cartridges was provided by the Vascular Anaesthesia Society of Great Britain and Ireland, London, United Kingdom. TEG ® consumables were provided by Medi- cell Ltd., Hendon, Middlesex, United Kingdom. Interim analysis presented at the Annual Meeting of the American Society of Anesthesiologists, Atlanta, Georgia, October 21–26, 2005, and the Vascular Anaesthesia Society of Great Britain and Ireland, Oxford, United Kingdom, September 19 –20, 2005. Address correspondence to Dr. Mallett: Department of Anaesthesia, Royal Free Hospital, Pond Street, London NW3 2QG, United Kingdom. susanv.mallett@royalfree.nhs.uk. Individual article reprints may be purchased through the Journal Web site, www.anesthesiology.org.  5th Consensus of the American College of Chest Physicians, July 2000. Available at: www.chestnet.org. Accessed May 11, 2006. Anesthesiology, V 105, No 4, Oct 2006 676