This study documents national trends in the probability of aspirin use by CVD risk category among patient visits to office-based physicians and hospital outpatient departments. Some improvements were observed over time in the use of aspirin for both the secondary and primary prevention of CVD. However, the magnitude of those improvements is minimal relative to the substantial gaps between clinical practice and evidence-based recommendations. The gaps observed with secondary prevention are particularly concerning, given the existence of conclusive clinical evidence and unequivocal practice guidelines. The use of aspirin among primary prevention patients, including those with diabetes, also appears to be suboptimal, but additionally may reflect uncertainty about the evidence. Our analysis also suggests that despite aspirin's more favorable cost-effectiveness, statins have been prioritized ahead of aspirin as therapy for reducing CVD risk.
While ample evidence attests to the underuse of aspirin in reducing risks of CVD, this study uniquely provides an 11-y trajectory of aspirin use in US outpatient settings and reveals that improvements have been at best modest. The magnitude of improvements seems particularly small in the context of often-repeated national guidelines and abundant clinical evidence supporting aspirin use for the prevention of CVD, particularly in patients with known CVD. Even in 2003, aspirin use was reported in only one-third of the visits by patients having CVD, which points to widespread under-appreciation of aspirin as an efficacious and cost-effective secondary prevention therapy. The usage was 12% among visits by diabetics, a group at increased cardiovascular risk. This was lower than the 16% found among visits by patients with multiple major cardiovascular risk factors for whom evidence supporting prophylactic aspirin therapy is less definitive. The continued increases in aspirin use since 1999–2000 may reflect heightened awareness of the benefits of aspirin in reducing cardiovascular morbidity and mortality, mediated through intensified dissemination of national guidelines and clinical trial findings [2
]. We did not find evidence of aspirin overuse in low-risk patients.
Compared with clinical practice in Europe [22
], our study results add support to the observation that underuse of aspirin is more problematic in the US. The genesis of this gap is likely multifactorial and open to postulation. For instance, US physicians may face greater pressure than their European colleagues to prescribe newer medications as a result of less-restrictive regulations on drug advertising. Also, direct-to-consumer advertising has been shown to change patient-physician relationships and physician prescribing behavior.
The widespread aspirin underutilization could be partly due to uncertainties in risk assessment. Health care providers show little consistency as to how much risk of excess bleeding is acceptable, which may partly account for the variability in aspirin prescribing [33
]. Data indicate that aspirin use is linked to approximately 2.5%–4.5% of the annual upper gastrointestinal events (symptomatic ulcers) and 1%–1.5% of serious complications, such as severe bleeding, perforation, and obstruction [34
]. These risk estimates should be evaluated in the context of average reductions of 15−40% in cardiovascular events when aspirin is used as a preventive therapy [3
]. Accurate risk assessment can be difficult at the individual patient level, especially when discrepancies arise between verbal and written medical history information [36
]. Aspirin resistance may also limit the rates of aspirin use. However, the frequency of aspirin resistance is less well known and may range from 5% to 60% [37
]. In some patients it may be dose related. Lee et al. [38
] indicate that even a low-dose aspirin of 100 mg or less may increase aspirin resistance in patients with coronary artery disease.
Past research also suggests that physicians may assign lower priority to aspirin therapy than to other cardiovascular risk-lowering therapies [25
], and our evaluations of the co-prescription of aspirin and statins support this assessment. We found that aspirin and statin use was significantly higher when the other therapy was present; however, the incremental use became progressively greater for statins over time. Beginning in 1997–1998, statin use in the presence of aspirin transcended aspirin use in the presence of statins for both the high- and intermediate-risk categories, and the gaps remained wide through 2003. These results suggest that even though statins themselves may be underused, aspirin is given even lower priority for lowering cardiovascular risk. These findings are intriguing because both therapies reduce cardiovascular risk by similar magnitudes but differ vastly in cost; statins are prioritized despite the far greater cost-effectiveness of aspirin [39
]. Also, secondary analyses of clinical trial data indicate that aspirin and statins used in combination may be more effective at reducing the relative risk of CVD events than when used alone [42
Statins are newer and more intensely advertised than aspirin, which may partly explain the preferential use of these drugs. Lipid-lowering medications already ranked the fifth most promoted drug class in the US in 1998 [43
]. Statins are proven effective for both the primary and secondary prevention of CVD, whereas the effectiveness of aspirin in primary prevention is less certain. Also, while they are increasingly used as a prophylactic treatment, statins are still most commonly prescribed to people with hyperlipidemia. In contrast, use of aspirin is not specific to any risk factor in the prevention of CVD and therefore may be neglected by many physicians who are trained to perform in an overly acute-care-centered health care system. In addition, statins may be perceived to have a more favorable side-effect profile than aspirin, which has been shown to increase the risk of severe gastrointestinal and cerebral hemorrhage [34
]. Finally, our comparison of aspirin and statin use is confounded by the likelihood of underreporting of over-the-counter aspirin use by participating physicians and clinical staff.
In agreement with previous findings, lower aspirin use is associated with female gender, younger age, noncardiologist care, and care in the private office setting [15
]. The appropriateness of prophylactic aspirin therapy among women, particularly those under 65 y of age, is yet to be determined in light of the new evidence from the Women's Health Study [7
]. However, variations of aspirin use by physician specialty and type of health care setting raise questions about equity in the process of care. As a result of high penetration of managed care, patients are increasingly less likely to see a specialist such as cardiologist, unless referred by their primary care provider [45
]. Primary care providers, including those who practice in private offices, are expected to adhere more diligently to practice guidelines in this area that was previously the domain of specialists.
Our findings should be interpreted in the context of several limitations of the data sources used. Both NAMCS and NHAMCS are designed to produce national estimates on the basis of patient visits, and they provided no way to link multiple visits by the same patient. The per-patient visit nature of our analysis may lead to overestimation of aspirin use, particularly for high-risk patients, due to more frequent visits by sicker patients and indiscriminate reporting of sporadic and long-term use of aspirin. Individuals who have visited an ambulatory care facility may differ from those who fail to do so or do so less frequently. However, observed aspirin use may underestimate actual administration due to its low-cost, over-the-counter availability, although participating physicians and clinical staff are instructed to record nonprescription medication. In an attempt to indirectly gauge the potential of underreporting of aspirin use, Stafford [16
] studied the reporting of multivitamin use during pregnancy and nonprescription analgesic use for osteoarthritis, and concluded that these surveys capture a reasonably substantial proportion of nonprescription medication use. By limiting the number of medications reported to six or fewer, some medications, particularly those perceived as less critical for the treatment of primary diagnoses, may not be reported. When we compared patient visits in which the maximum number of medications were reported with visits in which fewer were reported, we found no differences in the likelihood of aspirin use. If aspirin is, in fact, under-reported, less clinical attention and priority may be given to aspirin use compared to other therapies.
While these data limitations present certain difficulties in interpreting the absolute usage of aspirin, they should have limited impact on our trend analysis. The extent of under-reporting may have attenuated over time due to increased awareness of its effectiveness in cardiovascular risk reduction, which could partly explain the increasing trends in use that we observed. We have no reason to believe that under-reporting varies so substantially by patient visit characteristics that it could have confounded our multivariate logistic analysis.
In conclusion, improvements in aspirin use for reducing risks of CVD among US outpatients are at best modest, and substantial treatment gaps persist, particularly in secondary prevention, for which definitive evidence of benefits is available. Aspirin is more underused than statins despite its more favorable cost-effectiveness. Marked changes in clinical practice are unlikely to occur unless more aggressive, innovative means are implemented to enhance health care provider and patient adherence to consensus guidelines on aspirin therapy to prevent CVD events. In particular, targeted interventions may be warranted in patient subpopulations in which aspirin use is lower than average, including women, young adults, and ethnic minorities. Targeted continuing medical education for primary care providers especially in solo or small-group practices, may introduce greater consistency into practice by specialty and practice setting.
Aspirin is known to be effective in lessening the chance of heart attack, stroke, or other cardiovascular diseases that may occur when blood vessels are blocked by blood clots. Therefore, guidelines recommend that certain groups of people take aspirin regularly either to prevent such clots forming in the first place, or after such a clot has formed to prevent further clots. However, aspirin may increase the chance of bleeding in some people; hence it is important that the benefits of taking aspirin are balanced against possible side effects.
Why Was This Study Done?
The researchers wanted to investigate temporal patterns of aspirin use among patients who would potentially benefit from taking it, and ask whether there were any particular reasons—for either patients or their health care providers—that influenced such use.
What Did the Researchers Do and Find?
They used data over 11 years from two nationwide surveys in the US that study prescribing patterns in outpatients. Some improvements were observed between 1993 and 2003 in the use of aspirin among patients with known CVD and those without. However, the magnitude of those improvements is minimal relative to the substantial gaps between clinical practice and evidence-based recommendations. From 1997 to 1998 onward, statins were used more frequently compared with aspirin as prophylactic therapy for reducing cardiovascular disease risk. Greater aspirin use was seen most frequently in people of advanced age, who were male, who were being cared for by cardiologists (rather than general physicians or other specialists), and who were being seen in hospital outpatient departments (rather than private practices).
What Do These Findings Mean?
Although there is very good evidence that aspirin is particularly useful when given after a cardiovascular event—so-called secondary prevention—there were only modest increases in the use of aspirin in this period. Aspirin is less frequently used than statins, despite its greater cost-effectiveness. Innovative interventions are needed to enhance patients' and health care providers' understanding of and adherence to the guidelines that have been developed on reducing the risk of cardiovascular disease.
Where Can I Get More Information Online?
MedlinePlus has information on aspirin and related drugs:
Omni is a UK-based free catalog of hand-selected and evaluated Internet resources in health and medicine, including a page of links on aspirin: