Aspirin Resistance: Is it Real? Is it Clinically Significant?

Laboratory Evidence of Aspirin Resistance 

The gold standard for determining aspirin’s effect on platelet reactivity is optical aggregometry, also called light transmission aggregometry.9, 10 This test directly measures the inhibition of platelet COX-I activity by aspirin.11 It measures the response of platelet-rich plasma to exposure to a platelet agonist, arachidonic acid. As platelets aggregate in response to the agonist, light transmission increases. An increase in light transmission indicates persistent platelet activity. Aspirin resistance is defined as an increase in light transmission due to platelet aggregation despite aspirin therapy. This test is time consuming and cannot be done at the bedside.

Two cartridge-based platelet function tests that can be performed in minutes at the bedside are available and FDA approved. The Platelet Function Analyzer (PFA-100; Dade-Behring, Deerfield, Ill) simulates platelet-based hemostasis in vitro.12, 13 The time to cessation of high shear blood flow (closure time) through a capillary by a platelet plug is determined. Collagen plus ADP or epinephrine is used as an agonist. Aspirin resistance is defined as a normal closure time.

The VerifyNow rapid platelet function assay (RPFA-ASA; Accumetrics, San Diego, Calif) is an additional cartridge-based platelet function test that can be performed at the bedside.4 Whole blood is exposed to fibrinogen-coated beads and platelet agonists. If platelets aggregate, light transmission will increase. An increase in light transmission indicates the absence of an aspirin effect on platelets and is taken as evidence of aspirin resistance.

In addition to these platelet function tests, aspirin resistance has been defined as failure to suppress thromboxane generation. This is determined by measuring serum thromboxane B27, 14 or by measuring urinary 11 dehydrothromboxane B2 levels, a marker of in-vivo thromboxane generation.6, 14, 15

Prevalence of Aspirin Resistance Depends on the Test Utilized 

Several studies have reported “reasonable” correlations among the 3 most commonly used tests of platelet function.15, 16, 17 However, Topol et al9 reported a poor correlation between PFA-100 and optical platelet aggregation in detection of aspirin resistance. Patrono et al18 also reported poor correlations among these tests. Steinbuhl8 and Wang19 have suggested that the platelet response to aspirin might be continuous rather that binary. This raises the issue of what percent response to aspirin in these tests should be considered significant.

The prevalence of aspirin resistance as defined by these 3 tests varies widely, as shown in Table 1. The reported prevalence of aspirin sensitivity using the PFA-100 test ranges from 9.5% to 35%.12, 20, 21, 22 When the RPFA-ASA test is used, the prevalence ranges from 7% to 27%.4, 21, 23, 24, 25 However, using the “gold standard,” optical aggregometry, the prevalence is much lower: 0.4% to 9%.11, 24, 26, 27

Table 1. Prevalence of Aspirin Resistance

	Study	Population	Test Used	Aspirin⁎	Resist
	Grundman et al22	53 HX stroke	PFA-100	100	34%
	Macchi et al12	72 CAD	PFA-100	160	29%
	Anderson et al23	202 history AMI	PFA-100	161	35%
	Gum et al20	325 stable CVD	PFA-100	325	9.5%
	Chen et al4	151 PCI	RPFA	80-325	19%
	Wang et al21	422 adults⁎	RPFA	80-325	23%
	Lee et al25	468 stable CAD	RPFA	80-325	27%
	Malinin et al24	141 risk factors	RPFA	325	7%
	Schwartz et al26	190 HX AMI	LTA	81-325	9%
	Tantry et al11	223 CAD	LTA	325	0.4%
	Schwarz et al26	190 HX MI	LTA	325⁎⁎	0.5%
	Gum et al23	326 stable CVD	LTA	325	5.2%
	Malinin et al24	141 risk factors	LTA	325	0.7%

⁎ Aspirin, mg/d. ⁎⁎ Dose was observed.

Depending upon which of these 3 tests is used, the number of Americans on long-term aspirin therapy who are aspirin resistant could be as few as 120,000 or as many as 10.5 million.

Prevalence of Aspirin Resistance May Depend on the Aspirin Dose 

In addition to the type of test used, there is evidence that the dose of aspirin affects the prevalence of aspirin resistance. As noted in Table 1, when the PFA-100 test is used the prevalence of resistance varied from 29% to 34% when the aspirin dose was 161 mg/day or less.12, 22, 23 However, in the report by Gum et al20 the incidence was 9.5% when the aspirin dose was 325 mg/day.

The same apparent dose effect is noted in the reports using the RPFA-ASA test. In 4 studies where the aspirin dose was 80 to 325 mg, the prevalence was reported to be 19 to 27%.4, 21, 25 However, Malinin et al24 reported a prevalence of 7% when the aspirin dose was 325 mg/day.

In reports using optical aggregometry, the highest prevalence (9%) was reported when the aspirin dose was 81 to 325 mg.26 In a study by Schwartz et al26 of 191 patients with CAD, aspirin was discontinued for 7 days, and then aspirin resistance was measured 2 hours after a nurse observed each patient chew a 325 mg aspirin tablet. Only 1 of 190 (0.5%) patients was aspirin resistant, and that patient admitted taking an NSAID 12 hours before taking aspirin. Two other studies using optical aggregometry reported a prevalence of aspirin resistance of less than 1% in patients taking 325 mg aspirin/day.11, 24

Lee et al25 used RPFA-ASA to measure aspirin resistance in 468 patients with stable CAD who were taking 80 to 325 mg aspirin/day. The percent resistant was 30% in 384 patients taking 80-100 mg, 17% in 72 taking 150 mg, and 0% in 12 patients taking 300 mg/day. Hart et al14 reported that the degree of thromboxane inhibition increased as the dose of aspirin was increased.

These studies strongly suggest that aspirin resistance is dose-dependent, and they are consistent with the findings of a recent review28 that reported that aspirin doses less than 160 mg/day are less effective in preventing myocardial infarction and stroke than doses of 160 mg or more.

It is possible that aspirin resistance and the failure of aspirin to prevent myocardial infarction and stroke have the same cause: an inadequate dose of aspirin. Given the therapeutic implications, additional studies to determine if aspirin resistance is dose dependent are sorely needed.

Non-Compliance May be the Major Cause of Aspirin Resistance 

In a study reported by Schwartz et al,26 17 of 190 (9%) patients taking 81 to 325 mg aspirin/day were aspirin resistant by light aggregometry. When requestioned, 10 of the 17 admitted that they had not been taking aspirin. When retested after being observed ingesting 325 mg of aspirin, none of the 17 were aspirin resistant. Tantry et al11 studied 203 patients undergoing percutaneous coronary intervention (PCI) who had been prescribed 325 mg/day of aspirin. Seven of the 203 (3.4%) were found to be aspirin resistant by aggregometry. On further questioning, all 7 patients admitted to being non-compliant. When the test was repeated after they were given 325 mg of aspirin in the hospital, none of the 7 were aspirin resistant. Cotter et al29 measured thromboxane B2 production in 73 acute myocardial infarction survivors. Thromboxane production was not suppressed in 21 patients (29%), indicating aspirin resistance. When questioned, 12 of the 21 admitted that they were not taking their aspirin.

In these 3 reports,11, 26, 29 45 patients were found to be aspirin resistant; when requestioned, 29 of the 45 (64%) admitted that they were not taking the prescribed aspirin. Twenty four of these patients were retested after ingesting 325 mg of aspirin; none were aspirin resistant.

It is very apparent from these 3 studies that aspirin resistance is very uncommon in compliant patients. Non-compliance may be the major cause of aspirin resistance. Since aspirin is an over-the-counter medication, compliance cannot be determined by pharmacy records. Bhatt et al30 have suggested that measurement of salicylate levels might be required to confirm compliance.

Other Factors That may Influence the Prevalence of Aspirin Resistance 

Several studies have reported that the prevalence of aspirin resistance is higher in women4, 25 and that resistance increases with age.12, 25 There are other reports of increased resistance in patients with acute coronary syndromes,19 CHF,19 renal insufficiency,20 low hemoglobin.25 Insulin-dependent diabetes,20 smoking,12 interaction with NSAIDS,19 and non-absorption19 are less consistent. In addition to these clinical factors, a wide range of genetic factors that might affect aspirin resistance have been proposed.5, 19, 31

Clinical Correlates of Aspirin Resistance 

Four prospective studies have reported clinical outcomes during follow-up of patients in whom aspirin resistance has been determined by laboratory testing.

Grotemeyer et al32 in 1993 were among the first to report a relationship between aspirin resistance, as defined by platelet function tests and adverse vascular events. They measured platelet reactivity in 90 stroke survivors. The patients were then treated with 500 mg aspirin 3 times per day and followed for 24 months. Major end-points (stroke/myocardial infarction/vascular death) occurred during follow-up in 4.4% of aspirin responders and 40% of aspirin non-responders (P<.0001).

A prospective study of aspirin resistance in 326 patients with stable cardiovascular disease was reported by Gum et al 2003.27 They assessed aspirin resistance by optical platelet aggregation and by PFA-100 in patients who had taken 325 mg aspirin/day for>7 days. The prevalence of aspirin resistance was 5.2% by platelet aggregation and 9.5% by PFA-100. After 1.9 years follow-up, the major end-point, death/myocardial infarction/stroke, occurred in 4 of 17 (24%) aspirin-resistant patients as determined by optical aggregation and 30 of 309 (10%) patients who were not aspirin-resistant (P=.03). However, when they used PFA-100 to define aspirin resistance, there was no significant difference in end-points between those aspirin-resistant (15.1%) and those who were aspirin sensitive (12.9%).9 Compliance with aspirin therapy was determined by interview; salicylate levels were not measured.

Anderson et al23 reported that the prevalence of aspirin resistance, as assessed by PFA-100 was 35% in 71 myocardial infarction survivors. They were then treated with aspirin, 160 mg/day, and followed for 4 years. The incidence of clinical events during follow-up among those who were aspirin resistant was not significantly greater (P=.28) than in those not resistant.

Aspirin resistance was related to the outcome of elective percutaneous coronary intervention (PCI) in 151 patients by Chen et al.4 Aspirin resistance as determined by RPFA-ASA was present in 29 (19.2%). The aspirin dose varied from 80 to 300 mg/day. All patients also received clopidogrel. The incidence of myonecrosis as measured by CK-MB and by troponin I after PCI was higher in those who were aspirin resistant than in those who were not resistant (OR 2.9; 95% CI, 1.2 to 6.9; P=.015). The possible relationship between aspirin dose (80 to 300 mg/day) and the incidence of myonecrosis was not assessed.

Three of these reports4, 27, 32 indicate an increased incidence of adverse vascular events in patients who were determined to be aspirin resistant by platelet function tests. The number of patients in 223, 32 of these 3 reports was quite small (71 and 90). In the study by Gum et al27 there were only 4 adverse events in the 17 aspirin-resistant patients.

It should be noted that compliance with aspirin therapy was not confirmed by salicylate levels in any of these studies. It is possible that both the initial diagnosis of aspirin resistance and the subsequent incidence of adverse clinical events in these studies were due to non-compliance.

In addition to these prospective studies, a large case-control study was reported by Eikelboom et al.6 They measured urinary 11-dehydrothromboxane B2 levels, a marker of in-vivo thromboxane generation in patients enrolled in the Heart Outcomes Prevention Evaluation (HOPE) study.33 They compared the urinary levels at baseline in 488 patients treated with aspirin who had myocardial infarction/stroke or cardiovascular death during 5 years of follow-up with 488 age and sex-matched controls who received aspirin but did not have a vascular event during follow-up. Patients with levels of urinary 11-dehydrothromboxane B2 in the upper quartile, indicating relative aspirin resistance, had a higher risk of a vascular event during follow-up then those in the lowest quartile (OR 1.8, 95% CI, 1.2 to 2.7; P=.009). The dose of aspirin in this study was not stated, and salicylate levels were not determined. As with the previous studies, the results could have been affected by non-compliance.

Conclusions 

There is no widely accepted definition of aspirin resistance. The most frequent definition is the failure of aspirin to inhibit platelet aggregation by inhibiting thromboxane A2 generation. Several different tests have been proposed to assess aspirin’s effect on platelets. Unfortunately the prevalence of aspirin resistance varies from 0.4%11 to 34%,22 depending upon which test is used. The correlations among these tests are not clear.

There is considerable indirect evidence that aspirin resistance might be dose-dependent. If this is confirmed by appropriately designed studies, the treatment of aspirin resistance is self-evident.

The most common cause of aspirin resistance appears to be non-compliance. When compliance is assured by observation of aspirin ingestion, the prevalence of aspirin resistance is miniscule. Studies of aspirin resistance in patients in whom compliance has been documented by salicylate levels are needed.

The clinical importance of aspirin resistance is uncertain. Several studies have suggested that patients with aspirin resistance, as defined by laboratory tests, are at increased risk of myocardial infarction and stroke. Most of these studies were relatively small, and none have documented that the patients were compliant with the prescribed aspirin therapy.

Given the widespread prevalence of cardiovascular disease and the proven efficacy of aspirin in the prevention of vascular events, additional studies to determine the true incidence and cause of aspirin resistance are vitally needed.

If the prevalence is less than 1%, as reported in 3 studies,11, 24, 26 it is clear that recurrent vascular events in patients on aspirin therapy are rarely due to aspirin resistance. These adverse events are due to non-compliance or an inadequate dose of aspirin.