The VerifyNow Aspirin Assay was previously marketed as the Ultegra Rapid Function Platelet Assay -- ASA. It is FDA-approved "to aid in the detection of platelet dysfunction due to aspirin ingestion...."[26] It is an in vitro semiquantitative measurement of ASA-dependent aggregation that requires whole blood collected in 3.2% sodium citrate. The cartridge contains arachidonic acid, which is thought to be more specific for ASA. The instrument tests aggregation of activated platelets through binding to human fibrinogen-coated beads. The numerical result is reported in aspirin-response units (ARU) with the ASA resistance cut-off defined as ≥ 550 ARU (aspirin-related platelet dysfunction not detected).[26] Its sensitivity as a screen for ASA-induced platelet dysfunction is approximately 91%, according to the manufacturer's product information.[26]
There are primarily 2 limitations to the VerifyNow Aspirin Assay. The first is that, according to the manufacturer, this test cannot be used in patients with inherited platelet defects or in patients receiving many other anti-platelet drugs. The second is that the few evaluation studies to date are flawed. In some studies, ASA resistance has been correlated with biochemical cardiovascular injury, but there has been no systematic follow-up of those patients -- outcomes data.[27] In those studies that did attempt to correlate test results with clinical outcomes, there were confounding variables, such as other anti-platelet drugs.[28]
PlateletWorks is FDA-approved to detect platelet dysfunction due to inhibition secondary to diet, ASA, and/or other drugs. It is an in vitro quantitative measurement of platelet activation that requires whole blood collected in 3.2% sodium citrate. This assay uses separate tubes containing collagen and ADP. This method uses traditional cell counting principles (electronic impedance). Both pre- and postactivation platelet counts are obtained and the numerical result represents percent inhibition. The formula is:
(agonist platelet count) x 100
baseline platelet count
PlateletWorks also has limitations that need to be addressed. There is a very short time allowed -- 10 minutes -- between sample collection and assay. Some studies have indicated that the assay's precision may be a cause for concern. Some laboratories, Dr. Peterson said, have found an unacceptably high false-positive rate because of interference by dietary substances such as chocolate and red wine. Dr. Peterson emphasized that obtaining a dietary history that includes chocolate, wine, herbal medicines, and prescribed and over-the-counter medications is prudent before performing any of these assays.
AspirinWorks is FDA-approved to detect ASA-induced inhibition of TX metabolites. It is an in vitro quantitative measurement of ASA-induced inhibition of TXA2 generation. The assay requires 10 mL of freshly voided urine collected in a preservative. The urine sample may be frozen for assay later. The test, either a radio-immunoassay (RIA) or an enzyme immunoassay (EIA), measures levels of 11-dihydro-thromboxane-B2, a relatively stable breakdown product of TXA2. The assay therefore indirectly measures in vivo activity of TXA2. Reduced levels are interpreted as due to ASA effect. The numerical results are expressed in quartiles. The first quartile is less than 132 pg/mg creatinine, the second quartile 133 to 193 pg/mg creatinine, the third quartile 194 to 296 pg/mg creatinine, and the fourth quartile greater than 296 pg/mg creatinine. Different quartiles represent differing degrees of risk for clinically significant events. A patient whose results are in the first quartile has a relative risk of an untoward cardiovascular event defined as 1. A patient whose results are in the second quartile has a 1.3 times greater risk of myocardial infarction (MI) than a patient in the first quartile. A patient whose results are in the third quartile has a 1.5 times greater risk, and a patient in the fourth quartile has twice the risk of MI.[9]
There are 2 potential limitations with the AspirinWorks assay. The first is that reference ranges of RIA assays differ from those of EIA assays. Similar to tests for lipid levels, the method for a particular patient should be the same or the results may not be interpretable. The second major possible limitation is that the results interpretation depends on a demarcation of the quartiles. However, patients whose results comprised the second, third, and fourth quartiles appeared to be different from patients in the control population. Specifically, the cardiovascular risk factors were higher in the case group than in the control group.[9] Despite this possible limitation, the study authors did conclude that a "clear association" existed between urinary 11-dihydro-thromboxane-B2 levels and cardiovascular outcomes.
Conclusion
So where does the laboratory stand? There are no perfect assays for a clinical phenomenon for which there is no agreed-upon definition. Because we are not certain what ASA resistance is, should we be performing these tests? What we cannot overlook is the most important requirement of any diagnostic test -- that it be clinically useful. In some settings, these assays may provide better answers than laboratories were able to provide before. For example, if a surgeon wants to know preoperatively if a patient has ingested ASA, these screening assays may be more useful than the bleeding time test. If a physician wanted to ascertain whether a patient was compliant with prescribed ASA therapy, these screening assays (PFA-100, VerifyNow, PlateletWorks) might provide an immediate answer. But those uses are not the same as asking whether the laboratory tests that are currently available assess ASA resistance or predict clinical outcomes. Although there is no unequivocal evidence that they do, there is evidence to suggest they might. As explained previously, in appropriate circumstances, such as screening for the presence of ASA or as a substitute for the highly flawed bleeding time test, these assays may be useful. The laboratory should also be prepared to offer guidance on interpreting the results of any of these assays.
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