Estimating short- and long-term reference change values and index of individuality for tests of platelet function
Introduction
Long-term mechanical circulatory support (MCS) is used as a bridge to cardiac transplant, a bridge to recovery in patients with myocardial damage, and a destination therapy for end-stage heart failure patients who are not candidates for transplant [1]. These patients are both prothrombotic and exhibit a tendency to bleed. During the perioperative period, up to 60% of patients experience excess bleeding; while historically 5–20% have experienced thromboembolism or stroke [1]. Although some newer continuous flow LVAD devices demonstrate lower rates of systemic thromboembolism and bleeding, device thrombosis continues to be a problem [2].
To balance the risk of perioperative bleeding with device thrombosis and thromboembolism, protocols for MCS placement call for titration of antiplatelet agents using laboratory tests of platelet function. Tests of arachidonic-induced platelet function are used to titrate aspirin and adenosine diphosphate (ADP)-induced platelet function to titrate agents such as dipyrimidole and clopidogrel [3], [4], [5]. However there is little evidence to suggest that platelet function testing (PFT) is precise or reliable enough to allow for monitoring of platelet function over short periods of time.
Thromboembolic complications are also the most common cause of morbidity after neurointerventional procedures, especially with use of intra-arterial flow diverters such as the pipeline embolization device (PED). Dual antiplatelet therapy with aspirin and clopidogrel is nearly universal before and after PED placement; with protocols to monitor response with PFT increasingly common [6], [7], [8], [9]. Some studies demonstrated that monitoring platelet function and tailoring antiplatelet agents using PFT results improved outcomes after PED procedures [6], [7], [8], [9]. One investigator has also demonstrated wide dynamic variability in individual response to clopidogrel after PED placement, requiring repeat measurement and titration to remain in desired therapeutic ranges [7], [9]. However other studies have found that PED outcomes in centers that did not monitor PFT were better than those that did [10]; or that risk of bleeding or thrombosis was not higher in patients with higher on-treatment platelet function [11].
In these and other critical care settings, PFT may be monitored over days or weeks to determine whether a patient’s risk for coagulopathy/bleeding has increased, or to determine whether antiplatelet medications have affected platelet function. In order to use PFT in this context, it is necessary to know whether these tests can be interpreted relative to a fixed clinical cut-off or reference interval; or whether only “delta” values (changes relative to a patient’s baseline value) should be used to interpret results. Traditionally, this question can be answered by studying biological variation (CVI) and calculating parameters such as index of individuality (II) and reference change value (RCV) [12]. While biological variability studies are performed on healthy individuals, the data obtained allow both definition and comparison (between tests) of the minimum detectable changes in defining disease states or changes in patient condition (e.g. the troponin RCV or “delta” used to suspect acute coronary syndrome). In this study, we used a repeated measurement design [13], [14] in order to estimate short- and long-term RCV and II for VerifyNow and Multiplate tests of arachidonic acid-induced and ADP-induced platelet function. The results help determine how changes in PFT should be interpreted over days and months.
Section snippets
Subjects and samples
Sixteen apparently healthy volunteers (8 male and 8 female) who denied taking aspirin-containing or nonsteroidal anti-inflammatory products for at least 10 days, and had no history of abnormal bruising or bleeding were recruited for the study. Blood samples were collected by venipuncture into 2.0 mL 3.2% sodium citrate tubes (Greiner Bio-One) for VerifyNow testing and Multiplate 3.0 mL hirudin tubes for testing on the Multiplate instrument. Study volunteers had blood drawn on 3 occasions on the
Results
Each donor in the study had 6 blood collections, assayed in duplicate on both testing platforms yielding 12 analytical results per platelet function test per person. Three specimens were collected on the first study day, and were used to estimate the short-term biologic variability and RCV. More than 56% of volunteers’ data followed a Gaussian distribution with no transformations needed, and the means of volunteers’ data were also sufficiently normal (all p > 0.11). Analytic precision of the
Discussion
In order to balance the risk of bleeding and thrombosis, protocols for both MCS and neurointerventional procedures recommend monitoring platelet function frequently and titrating antiplatelet agents based upon these results [3], [4], [5], [6], [7], [8], [9]. However in order to effectively utilize these tests in these settings, it is important to understand how changes in platelet function over a given period of time should be interpreted—relative to a fixed decision limit or population-based
Conclusion
For VerifyNow ARU, very low biologic and between-subject variability, combined with low analytic imprecision, allows interpretation of results relative to a fixed cut-off or population-based reference interval. Very small (less than 5%) changes in VerifyNow ARU represent changes in platelet function for an individual patient. In contrast, VerifyNow PRU and Multiplate ASPI and ADP tests demonstrated greater between-subject variability; resulting in II < 1.4 and confounding interpretation of
Acknowledgements
This publication was made possible by CTSA Grant Number UL1 TR002377 from the National Center for Advancing Translational Sciences (NCATS), a component of the National Institutes of Health (NIH). Its contents are solely the responsibility of the authors and do not necessarily represent the official view of NIH. Brooke M. Katzman, Amy M. Wockenfus, Renee J. Scott, Sandra C. Bryant, Allan S. Jaffe and Brad S. Karon declare no conflicts of interest and have no financial disclosures relevant to the
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