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Use of Oral Antithrombotic Agents Among Health Maintenance Organization Members With Atherosclerotic Cardiovascular Disease
Jonathan B. Brown, MPP, PhD;
Thomas E. Delea, MSIA;
Gregory A. Nichols, PhD;
John Edelsberg, MD, MPH;
Patricia J. Elmer, PhD, MS;
Gerry Oster, PhD
Arch Intern Med. 2002;162:193-199.
ABSTRACT
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Background Numerous randomized trials document the value of antithrombotic agents
for the treatment of cardiovascular disease (CVD). Although antithrombotic
agents are often prescribed at hospital discharge after CVD-related events,
much less is known about the ongoing use of such agents.
Methods We examined the use of oral antithrombotic agents among a random sample
of 2500 persons with atherosclerotic CVD who were enrolled in Kaiser Permanente
Northwest Region, a not-for-profit group-model health maintenance organization.
Study subjects were identified based on a diagnosis of coronary heart disease,
ischemic stroke or transient ischemic attack, or peripheral arterial disease
in outpatient problem lists, visit records, and hospital discharge abstracts.
Use of prescription antithrombotic agents was identified from pharmacy dispensing
records. Regular use of aspirin, recall of aspirin advice and education, and
other patient characteristics were ascertained by mail survey.
Results Among the 1844 subjects who returned the survey and answered the question
regarding aspirin use, 84% were using either aspirin (72%) or a prescription
antithrombotic agent (12%), typically warfarin sodium. Antithrombotic therapy
was relatively underused in persons with peripheral arterial disease (75%
used an antithrombotic agent and 62% used aspirin). Use of antithrombotic
agents, including aspirin, did not differ by age but was higher among men
(87%, 76%) than women (81%, 67%). Nearly all subjects reported having received
aspirin education (94%) or advice (81%); recall of education or advice was
associated with a dramatically higher likelihood of using antithrombotic agents.
To a lesser extent, so was contact with a cardiologist or vascular surgeon
during the prior year.
Conclusions High rates of use of antithrombotic agents can be achieved among persons
with CVD in integrated not-for-profit health systems with mechanisms in place
to encourage such use, including guidelines, messages to clinicians, nurse
care management, alerts and routines embedded in electronic medical records,
and direct mailings to patients. Continued efforts should be made in all settings
to optimize the use of antithrombotic therapy among persons at an elevated
risk of atherothrombotic events.
INTRODUCTION
NUMEROUS randomized controlled trials1-2
demonstrate the value of oral antithrombotic therapy in preventing myocardial
infarction (MI), ischemic stroke, and vascular death in patients with clinical
atherosclerotic cardiovascular disease (CVD). Two types of oral antithrombotic
agents are available—antiplatelet agents and anticoagulants. Aspirin
is the most extensively studied and by far the most frequently used antiplatelet
agent. Regular use of low-dose aspirin for the secondary prevention of atherothrombotic
events in patients with established CVD is widely recommended.3-4
In addition to aspirin, 3 other oral antiplatelet agents—clopidogrel
bisulfate, ticlopidine hydrochloride, and dipyridamole—and 1 oral anticoagulant—warfarin
sodium—are used in the United States. While studies5-6
consistently report that most patients with acute MI receive orders for aspirin
on hospital discharge, data on the long-term use of antithrombotic agents
among outpatients with CVD are limited and inconsistent. In the Atherosclerosis
Risk in Communities Study,7 only 53% of middle-aged
subjects with a self-reported history of MI, and 37% of those with a self-reported
history of stroke, took aspirin. More recently, Cook et al8
found that only 43% of respondents to the 1996 National Family Opinion Mail
Panel survey with a self-reported history of CVD reported taking aspirin regularly.
Similarly, using data from the 1996 National Ambulatory Medical Care Survey,
Stafford9 found that only 34% of physicians'
office visits for coronary heart disease (CHD) included a report of aspirin
or another antithrombotic agent as a new or continuing medication. In contrast,
O'Connor et al10 reported that 71% of patients
clinically diagnosed as having CHD in a nonprofit, mixed-model, midwestern
health maintenance organization (HMO) took aspirin regularly. These studies
suggest that the use of oral antithrombotic agents among outpatients with
atherosclerotic CVD may be suboptimal in many settings.
Kaiser Permanente Northwest (KPNW) is a not-for-profit group-model HMO
located in metropolitan Portland, Ore. Starting in 1995, multiple initiatives
at KPNW have promoted the use of aspirin among patients diagnosed as having
atherosclerotic CVD. In 1995, a clinical practice resource (similar to a clinical
practice guideline) that encourages the use of aspirin was developed and publicized.
This was subsequently reinforced by electronic mail and hard-copy communications.
Independently, in 3 population-based case management programs for members
of the HMO with diabetes, congestive heart failure, and recent MI, nurses
promote the use of aspirin. Finally, cardiovascular specialists at the health
plan have for many years tried to ensure that members hospitalized for atherosclerotic
CVD are discharged with a prescription for aspirin.
In this study, we examined the use of antithrombotic agents among KPNW
members diagnosed as having atherosclerotic CVD, focusing attention on factors
associated with the use of these agents and the type of antithrombotic therapy
received.
PARTICIPANTS AND METHODS
RESEARCH SETTING
Kaiser Permanente Northwest provides comprehensive prepaid coverage
to about 430 000 persons, or approximately 20% of the population of metropolitan
Portland. Plan members are similar in age, sex, ethnicity, and economic status
to the area population as a whole.11
Kaiser Permanente Northwest maintains administrative and clinical electronic
end-user (searchable) databases containing information on member demographics;
inpatient admissions; pharmaceutical prescriptions; sales of prescription
drugs from Kaiser Permanente pharmacies; outpatient contacts, including visits,
diagnoses, problem lists (patient-specific cumulative lists of diagnoses),
outside claims, and referrals; laboratory tests; and measurements, such as
weight and blood pressure. During this study, however, aspirin use could not
be documented from these sources.
POPULATION AND SAMPLE
To identify all KPNW members diagnosed as having atherosclerotic CVD
as of March 31, 1999, we searched all outpatient problem lists for the prior
3-year period, and all inpatient discharge abstracts for the prior 13-year
period, for diagnoses of CHD (International Classification
of Diseases, Ninth Revision, Clinical Modification codes 410.xx, 411.0,
411.1, 411.8x, 412, 413.xx, 414.0x, 36.0x, and 36.1x), ischemic stroke or
transient ischemic attack (TIA) (International Classification
of Diseases, Ninth Revision, Clinical Modification codes 433.xx, 434.xx,
and 435.x), and peripheral arterial disease (PAD) (International
Classification of Diseases, Ninth Revision, Clinical Modification codes
440.2x and 443.9). Excluding members who died or left the health plan before
March 31, 1999, a total of 19 502 persons met the selection criteria.
After ascertaining that a simple random sample of 2500 persons with CVD would
capture 200 persons with PAD, our smallest subgroup of interest, we used SAS
statistical software (SAS Institute Inc, Cary, NC) to select such a sample
from a file containing the entire eligible population.
QUESTIONNAIRE
We developed a 5-page questionnaire that included questions about regular
aspirin use, adverse effects, and reasons for not using aspirin. We ascertained
aspirin use based on an affirmative response to the question, "Do you take
aspirin or an aspirin-containing product regularly? (Regularly means at least
every other day.)" To assist respondents in answering this question accurately,
the question was followed with a list of aspirin and aspirin-containing products
and a list of products that do not contain aspirin (eg, acetaminophen [Tylenol]
and ibuprofen [Motrin]). We also asked members whether they had received education
about aspirin ("Has a doctor or other health professional ever told you that
taking aspirin or an aspirin-containing product could help prevent heart attack
or stroke?") or advice to use it ("Has a doctor or other health professional
prescribed or suggested that you take aspirin or an aspirin-containing product
regularly?"). The questionnaire also included the Medical Outcomes Study 12-Item
Short-Form Health Survey,12 supplemented with
all 10 items from the Physical Function scale of the Medical Outcomes Study
36-Item Short-Form Health Survey.13-14
We also asked members about self-care, smoking status, educational level,
race or ethnicity, and household income. Finally, we asked members about any
adverse effects of aspirin that they had experienced. A copy of the questionnaire
may be obtained at http://www.kpchr.org/info/present/antiplatelet-brown.pdf.
Cover letters, questionnaires, and prepaid return-mailing envelopes
were sent via US Post Office Priority Mail to all study subjects during May
1999. A second complete priority mailing was sent during June 1999 to all
those who did not respond to the first mailing.
ADMINISTRATIVE DATA
We ascertained the age and sex of all study subjects from HMO enrollment
records. We identified persons who had received prescription antithrombotic
agents (clopidogrel, dipyridamole, ticlopidine, or warfarin) using electronic
pharmacy dispensing records. We labeled subjects as users of prescription
antithrombotic therapy if they purchased any of these agents during the 3
months preceding the initial mailing of the survey. To determine whether patients
had other comorbid conditions known to be associated with antiplatelet and
anticoagulant use (atrial fibrillation, valvular heart disease, or venous
thromboembolism) or with increased risk of CVD (diabetes mellitus), we searched
inpatient discharge diagnoses and outpatient problem lists and visit diagnoses.
Prestudy use of services (prescription drugs and ambulatory visits) was ascertained
from administrative databases for the 1-year period spanning July 1, 1998,
through June 30, 1999. Prior CVD-related events were identified from hospital
discharge summaries.
STATISTICAL ANALYSES
We compared the baseline characteristics of survey respondents with
those of nonrespondents using the t test for differences
in means and the Pearson  2 test for differences in proportions.
We calculated the percentage of patients with a prescription for an antithrombotic
agent in the 3 months before the survey, the percentage reporting regular
use of aspirin, and the percentage who were not using an oral antithrombotic
agent of any type (ie, no prescription antithrombotic or aspirin use). We
calculated the percentage of subjects receiving antithrombotic therapy in
subsets defined by demographic characteristics, comorbidities, and medical
and self-care characteristics. We also examined the proportion of regular
aspirin users who reported experiencing adverse effects of aspirin therapy.
All analyses were performed using SAS statistical software, version 6.12.
RESULTS
CHARACTERISTICS OF THE STUDY SUBJECTS
Of the 2500 subjects to whom questionnaires were mailed, 548 could not
or did not respond. Among this 548, 30 questionnaires were returned as undeliverable
and 28 were sent to persons who had recently died. Ninety-two subjects returned
their questionnaires blank (indicating refusal to participate, per an instruction
in our cover letter), and 398 did not respond at all. Among the 1952 members
(78%) who returned nonblank questionnaires, 108 (6%) did not answer the question
concerning the use of aspirin ("aspirin-item nonrespondents"). We focused
attention in our analyses on the remaining 1844 subjects who answered the
aspirin use question (our analysis sample).
Approximately 81% of the subjects in the analysis sample had a history
of CHD; 20%, ischemic stroke or TIA; and 11%, PAD (not mutually exclusively)
(Table 1). Other concomitant medical
conditions included atrial fibrillation (17%), valvular heart disease (10%),
and venous thromboembolism (2%). Questionnaire nonrespondents were slightly
younger than those in the analysis sample, less likely to have CHD, and more
likely to have diabetes or ischemic stroke or TIA. Aspirin-item nonrespondents,
on the other hand, were almost 5 years older than the analysis sample (on
average), were less likely to be men, and were more likely to have PAD, atrial
fibrillation, valvular heart disease, and venous thromboembolism. Use of prescription
antithrombotic agents was similar among questionnaire nonrespondents, aspirin-item
nonrespondents, and those in the analysis sample, as was the time since the
last CVD-related event (approximately 2 years in all 3 groups).
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Table 1. Characteristics of Survey Respondents and Nonrespondents*
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USE OF ANTITHROMBOTIC AGENTS BY DEMOGRAPHIC CHARACTERISTICS AND COMORBIDITIES
Among the 1844 subjects in the analysis sample, 84% were using an oral
antithrombotic agent, including 72% who reported taking aspirin regularly
and 12% who were receiving a prescription antithrombotic agent (Table 2). Included among those who purchased prescription agents
were 70 persons (4% of the analysis sample) who also reported regular use
of aspirin. The most commonly used prescription agent was warfarin (9%), followed
by clopidogrel (1%), ticlopidine (1%), and dipyridamole (<1%).
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Table 2. Use of Antithrombotic Agents by Demographic Characteristics
and Comorbidities*
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Almost one quarter of the subjects with PAD were not using an antithrombotic
agent, nearly twice the proportions among those with histories of CHD and
ischemic stroke or TIA. Persons with a history of ischemic stroke or TIA were
much more likely to be using a prescription agent than those with a history
of CHD. Persons with histories of atrial fibrillation, valvular heart disease,
and venous thromboembolism also used prescription agents more frequently.
Across age groups, use of antithrombotic agents was similar but not
identical. Persons younger than 75 years were more likely to report use of
aspirin than those who were older, and those 75 years and older were more
likely to be receiving prescription agents. As a result, the mean age was
higher for persons using prescription antiplatelet agents (72.2 years) than
for those using aspirin (69.3 years). Women were less likely than men to take
aspirin, but only slightly more likely to have purchased a prescription agent.
Overall, antithrombotic use was higher among men (87%) than among women (80%).
Members of minority ethnic groups appeared infrequently in the study
sample; estimates of their use of antithrombotic agents are, therefore, unreliable.
Educational level was not associated with the use of these agents, but household
income was, with less use at the highest and lowest levels of annual income
(<$10 000 and  $100 000).
Many subjects recalled having received education and advice regarding
the use of aspirin (Table 3).
Among the 1517 subjects who answered the question concerning aspirin education,
84% remembered receiving education. A slightly lower proportion of those who
answered the aspirin advice question (81% of 1821) recalled receiving such
advice. Recall of patient education and advice regarding aspirin was strongly
associated with higher rates of use of antithrombotic agents. More than 80%
of those who recalled receiving aspirin education or advice reported that
they used aspirin regularly, compared with 12% among those who did not recall
receiving aspirin advice and 31% among those who did not recall receiving
aspirin education. Among the 342 subjects who did not recall receiving advice,
52% were not using an antithrombotic agent, and those who were using antithrombotic
agents were 3 times more likely to be using a prescription agent than aspirin.
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Table 3. Use of Antithrombotic Agents by Medical and Self-care Characteristics*
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Among the 284 subjects who were not using an antithrombotic agent and
who answered the question concerning aspirin advice, 62% did not recall ever
having received such advice. Of the 108 persons who were not using an antithrombotic
agent who did recall being advised to take aspirin, 80% reported at least
1 reason for avoiding the medication (data not shown). The most commonly cited
reasons were "taking too many pills" (36%), allergy or side effects (34%),
and forgetfulness (29%). One fifth said they "don't like taking medication
or pills," and a few (5%) said they could not afford the cost.
The use of aspirin vs prescription agents was associated with measures
of overall health, physical function, and comorbidity. Of the subjects reporting
poor overall health, 26% were receiving prescription agents compared with
3% of those reporting excellent health. The mean Medical Outcomes Study 36-Item
Short-Form Health Survey Physical Function scale score was 61.1 among aspirin
users vs 47.6 among those using prescription agents, but the Medical Outcomes
Study 12-Item Short-Form Health Survey Mental Health scale scores were similar
among the 2 groups.
Contact with a cardiologist or a vascular surgeon during the previous
year was associated with greater use of aspirin and prescription antithrombotic
agents. A neurologic visit was also associated with greater use of prescription
agents (22%), but not of aspirin. Persons using prescription agents had more
total pharmacy distributions in the year before the survey (43.8 vs 28.8)
and more ambulatory care visits (16.4 vs 12.2). Persons whose most recent
hospitalization for CVD occurred 6 or more years before the survey were much
less likely to be using an antithrombotic agent than those who were hospitalized
for CVD during the preceding year (67% vs 86%).
Approximately 44% of the persons who used aspirin regularly reported
1 or more adverse effects from the medication (data not shown). The most frequently
reported adverse effects were bruising (18%), upset stomach/heartburn (17%),
ringing in the ears (16%), epistaxis or other bleeding (8%), and asthma or
shortness of breath (7%). Reports of black tarry stools (2%), severe allergic
reactions (such as rash, hives, and swelling of the tongue) (1%), and other
adverse effects (including ulcer) (1%) were rare. The number of persons reporting
adverse effect data was 1397.
COMMENT
The rate of aspirin use that we observed is substantially higher than
the rates reported in the 3 large community-based studies7-9
of antithrombotic use among US patients with CVD. However, it is similar to
the rate recently reported for members of another integrated not-for-profit
HMO.10 Three factors probably explain these
differences and this similarity.
First, the 3 community-based studies may underestimate the use of aspirin
among patients with CVD in the United States. The Atherosclerosis Risk in
Communities Study7 was conducted from 1987
to 1989, a period during which the use of aspirin for secondary prevention
was less well established and before the advent of intensified media discussion
of aspirin's benefits (including a major advertising campaign for Bayer).
In a more recent study by Cook et al,8 CVD
was self-reported, so their sample may have included patients who had never
actually had CVD and, accordingly, were not instructed to take aspirin or
other antithrombotic agents (CVD was self-reported in the Atherosclerosis
Risk in Communities Study also, but this study was based on detailed interviews
by trained interviewers, not on a mail survey). Based on a physician survey,
Stafford9 reported rates of aspirin use that
were less than half of those observed in our study, which relied on patient
self-report. Estimates of the use of prescription antithrombotic agents were
virtually the same (11%) in the 2 studies, however. At our study site, while
72% of respondents said that they were taking aspirin prophylaxis, the electronic
medical record documented only about 20% (a subsequent campaign by the HMO
dramatically improved documentation, to 69%). These findings suggest that
many patients take aspirin without their physicians' knowledge or that physicians
often fail to record aspirin use when they are aware of it. Therefore, studies
that rely on physician report, such as the one by Stafford, may underestimate
the use of aspirin.
Second, we believe that the multiple initiatives to promote aspirin
use that were undertaken at KPNW in the 4 years before the survey contributed
to the relatively high rates of antithrombotic use that we observed. These
included developing a guideline, reinforcing it with letters to physicians,
implementing aspirin-aware case management programs, and ensuring aspirin
initiation after hospitalization. The effectiveness of one of these initiatives—the
effort to ensure that members hospitalized for CVD were discharged with a
prescription for aspirin—is suggested by our finding that antithrombotic
use was more common among persons whose last CVD-related hospitalization occurred
after 1995. More than 90% of KPNW members with CVD recalled receiving aspirin
education or advice, and these members were much more likely to use an antithrombotic
agent than those who did not.
Last, we believe that the culture, structure, and resources of a large,
integrated, not-for-profit HMO may have contributed to the high rates of use
seen in our study and in the study by O'Connor et al10
at HealthPartners in Minneapolis, Minn. At KPNW and similar institutions,
clinicians and patients interact within an organizational structure and culture
that focus on population-based preventative care. Primary care and CVD specialists
communicate frequently, not only through patient referrals but also via telephone
consultations and electronic mail (in addition to hard-copy and intranet-based
guidelines and alerts). Use of a single integrated medical record (electronic
since 1995 at KPNW) facilitates documentation and communication of medication
use history. For example, the previously mentioned increase in aspirin documentation
between 1999 and 2000 was achieved by alerts and processes embedded in the
electronic medical record, by a groupwide financial incentive, and by leadership
from a physician-led management structure.
Whether similar results could be obtained in less integrated clinical
settings remains to be seen. Many of the investments made at KPNW could be
duplicated in decentralized HMO and fee-for-service settings, including aspirin-aware
case management services for persons with diabetes and CVD and programs to
ensure that all persons discharged from the hospital with CVD receive aspirin.
A simple mailing to clinically appropriate persons identified from hospital
and drug dispensing records may increase aspirin use at a nominal cost.15 The future development of Internet-based and/or "smart
card" integrated medical records may help.
Several limitations of our study should be noted. First, we relied on
patient self-report to measure aspirin use. In an effort to give the right
answer, some respondents may have overestimated the regularity of their use
of aspirin (social desirability bias). We attempted to minimize this problem
by explicitly defining regularly in the question regarding use of aspirin
and by listing aspirin- andnon–aspirin-containing products. Our cover
letter also emphasized that answers would not be shared with clinicians and
would not affect respondents' health plan benefits or medical care. We cannot
directly measure social desirability bias, but it is encouraging that only
4% of subjects said they were taking aspirin regularly, yet had purchased
a prescription antithrombotic agent during the previous 3 months. Switching
among agents and the simultaneous prescription of warfarin and aspirin could
account for much or all of this overlap.
A second limitation of any questionnaire-based study is survey and item
nonresponse. Persons with a history of ischemic stroke or TIA were significantly
less likely to return our survey, as were younger persons and those with diabetes.
Some persons who had experienced stroke may have been physically unable to
complete and return the survey. In the KPNW membership, younger persons are
generally less likely to return health surveys (and less likely to adhere
to medical advice). The lower rate of response among persons with diabetes
may be related to competition from diabetes' surveys that were concurrently
fielded by other researchers. We found no evidence that persons using prescription
antithrombotic agents were less likely to complete the survey or to answer
the aspirin use question. The rate of use of prescription agents was 14% among
nonrespondents, not significantly different from the 12% rate of use among
respondents (P = .42). Similarly, the rate of use
of prescription agents among the 108 survey respondents who failed to answer
the question concerning aspirin use was 15%, again not significantly different
from the rates found in other subgroups. Nevertheless, we are unsure whether
nonrespondents had the same high rate of antiplatelet use as respondents.
In a 1997 survey of persons with diabetes in the KPNW membership, otherwise
similar nonrespondents were less likely to make and keep primary care appointments
or to monitor their blood glucose level at home, although their adherence
to recommended antihyperglycemic drug regimens was similar to that of respondents
(J.B.B., unpublished data, June 1998). Fortunately, in the present study,
the 80% survey response (excluding persons who had moved or died) and 95%
item response limit the potential impact of nonresponse bias.
Finally, while members of KPNW are generally similar in age, sex, ethnicity,
and economic status to the Portland metropolitan area population, they do
not duplicate the US population structure. In particular, members of minority
ethnic groups are underrepresented. This may limit the generalizability of
our findings.
Despite their limitations, our results document that it is possible
to achieve a high use of antithrombotic agents in persons diagnosed as having
atherosclerotic CVD, at least in integrated medical systems. Continued efforts
should be made in all settings to optimize the use of aspirin and of other
antithrombotic agents among all persons at high risk of atherothrombotic CVD–related
events.
AUTHOR INFORMATION
Accepted for publication May 8, 2001.
This study was supported by Sanofi-Synthelabo, New York, NY, and Bristol-Myers
Squibb Co, Princeton, NJ.
Corresponding author and reprints: Jonathan B. Brown, MPP, PhD, Center
for Health Research, 3800 N Interstate Ave, Portland, OR 97227-1110 (e-mail: jonathan.brown{at}kpchr.org).
From the Center for Health Research, Portland, Ore (Drs Brown, Nichols,
and Elmer); and Policy Analysis Inc, Brookline, Mass (Mr Delea and Drs Edelsberg
and Oster). Policy Analysis Inc was a paid consultant to Sanofi-Synthelabo,
New York, NY, and Bristol-Myers Squibb Co, Princeton, NJ, on the development
of a health-economics program for the prescription antiplatelet agent, clopidogrel
bisulfate (Plavix), while this study was conducted.
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