Transcript Chapter 7: The Hierarchy of Evidence

Chapter 7
The Hierarchy of Evidence
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Chapter Overview
• The purpose of this chapter is to explore the concept
of “evidence” as it applies to the advancement of
health care practice and patient care.
• The growing focus on evidence-based medicine has
been fueled by the advances in information
technology.
• Clinical experience, combined with appraisal of the
best available literature, should drive patient care
decisions and the advancement of health care.
Medical decisions become evidence based.
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Evidence Across Patient Care
• Patient care includes: screening examinations,
diagnostic procedures, providing a prognosis,
developing a treatment plan, and implementing
prevention efforts.
• Research efforts in each of these areas can inform and
improve clinical practice.
• Keep in mind that differences in research methods and
data analysis across patient care can result in
differences in how the hierarchy of evidence is
described.
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Levels of Evidence
• Not all evidence is created equally.
• Differences in the strength of evidence come from two
sources: sampling and research methods.
• Sampling: Larger samples are more likely to estimate
true population values and result in narrower
confidence intervals than small samples.
• Research methods: There are two models of a
research method hierarchy.
– A hierarchy of strength of evidence for treatment
decisions
– The Oxford Center for Evidence-Based Medicine
hierarchy
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A Hierarchy of Strength of Evidence for
Treatment Decisions
• Within patient, randomized treatment order trials
• Systematic review of clinical trials with random
assignment to treatment
• Clinical trial with random assignment to treatment
• Systematic review of non-random assignment of
treatment trials
• Single trial with non-random assignment of treatment
• Laboratory studies related to physiological and
biomechanical mechanisms underlying disease, injury, or
treatment
• Opinion developed through informal clinical observations
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The Oxford Center for Evidence-Based
Medicine Hierarchy
Level of Evidence
1.
a. Systematic review of RCTs
b. Individual RCTs
c. All-or-none studies
2.
a. Systematic review of RCTs
b. Individual cohort studies
c. Outcomes research
Case-control studies
3.
a. Systematic reviews of case-control
studies
b. Individual case-control studies
Case series / Case study
4.
a. Includes poorly designed cohort and
case-control studies
5.
a. Animal research
b. Bench research
c. Unpublished clinical observations
Randomized controlled trials
Cohort studies
Anecdotal evidence
(Adapted with permission from http://www.cebm.net/index.aspx?o=1025. Oxford Center for EvidenceBased Medicine--Levels of Evidence (March 2009). Accessed May 10, 2010.)
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Within Patient, Randomized
Treatment Order Trials
• Within patient, randomized treatment order trials are
referred to by Guyett et al. as N-of-1 randomized trials.
• Within patient, randomized treatment order trials provide
the strongest evidence.
• They are studies in which patients receive all interventions
under consideration in random order over the duration of
the investigation.
• They provide strong evidence and minimize the number of
patients needed as there are no comparison groups.
• They minimize the influence of differences between patients
in terms of responsiveness to intervention.
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Limitations of Within Patient, Randomized
Treatment Order Trials
 Trials require that the condition being treated is
relatively stable and that the effect of an
intervention does not influence the response to the
other interventions being investigated.
 Trials are not common because few conditions
treated by therapists and athletic trainers are stable
across long periods of time and because the natural
history and interventions directed at change are not
reversible.
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Systematic Reviews of Randomized Trials
• Systematic reviews: Research efforts in which data
are acquired from existing literature through a planned
and thorough search process.
• Data acquired through systematic review may undergo
statistical analysis through a process called metaanalysis.
• Because data from multiple studies are combined,
statistical power is increased providing for narrower
confidence limits and greater confidence in the
conclusions drawn from the analysis.
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Randomized-Controlled Clinical Trials (RCTs)
• These studies prospectively assign patients to
treatment groups and measure specific outcomes in
order to assess the benefit of a particular
intervention.
• These trials are prospective, in contrast to case
series or case studies, in which patients are identified
after a course of care for analysis.
• Prospective designs allow for greater control over
factors that confound the identification of cause–
effect relationships.
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Cohort Studies
• Cohort studies may be prospective or retrospective in
design.
• They involve the study of groups based upon exposure or
intervention, and they assess for differences in outcomes.
• The principle difference between an RCT and a
prospective cohort is that in the RTC, individuals are
randomly assigned to interventions, whereas in a cohort
study, groups are investigated.
• Random assignment in retrospective cohort studies is not
possible because researchers identity groups based on
exposure or intervention at some point in the past and
then follow the groups forward.
• Prospective cohort designs can maximize efficiency, reduce
research costs, and yield important information.
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Outcomes Studies and Case-Controlled Studies
Outcomes Studies
• Population-based “outcomes research” is similar to
cohort studies.
• These studies seek to understand the end results of
particular health care practices and interventions.
Case-Controlled Studies
• These studies are similar to retrospective cohort studies
or imbedded within a prospective cohort study.
• In these studies, comparisons are made between groups
of subjects based on an outcome rather than an
exposure or intervention.
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Case Series and Case Studies
• Case series and case studies provide detailed
descriptions of a series or a single case.
• By design, these reports provide no statistical
comparison but rather describe the course of care and
the outcome of one or more cases.
• Because only a single or a few cases are reported on, it
is not possible to make inferences with regard to cause
and effect or to generalize the outcome of the cases
with confidence.
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Bottom of the Evidence Hierarchies
• Guyatt et al. label one type “physiological studies.”
• Oxford Center for Evidence-Based Medicine identifies
these studies as bench research and animal
research.
• The other type is identified as unsystematic clinical
observations, anecdotal evidence, or unpublished
clinical observations.
• Conclusions drawn from bench (that is, basic)
science and animal research cannot be generalized
directly to human patients.
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Hierarchy of Evidence for Diagnosis
• Oxford Centre for Evidence-Based Medicine identified five levels of evidence
of diagnostic studies paralleling those of treatment and prevention studies.
(Note: Diagram on Chapter 7, slide 6).
Level 1:
• Level 1a evidence consists of systematic reviews of high- quality individual
trials with homogeneous findings and clinical decision rules (CDRs).
• Level 1b studies consist of cohort studies and CDRs validated in a single
setting.
• Level 1c studies reveal “absolute” SPIN or SNOUT.
• SpPIN refers to tests with near-perfect specificity (high specificity rules in).
• SnNOUT refers to tests with near-perfect sensitivity (high sensitivity rules
out).
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Hierarchy of Evidence for Diagnosis
(continued)
Level 2:
• Level 2a evidence consists of systematic reviews of 2b
evidence.
• Level 2b evidence consists of exploratory (small sample)
reports with good reference standards and CDRs not
validated prospectively at one or more centers.
Level 3:
• Level 3a evidence consists of systematic reviews of 3b
reports.
• Level 3b reports involve the enrollment of non-consecutive
patients or fail to consistently apply reference standards.
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Hierarchy of Evidence for Diagnosis
(continued)
Level 4:
• Level 4 evidence comes from case-control
studies or diagnostic studies without
independent review of the reference standard.
Level 5:
• Level 5 evidence stems from expert opinion and
bench and animal science.
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Hierarchy of Evidence for Prognosis
• Similar to hierarchy of diagnostic studies.
• Strong similarities in Level 1 evidence, with all or no
case series replacing absolute SpPIN or SnNOUT in
level 1c.
• Level 2 differences consist of “outcomes research” at
level 2c, which does not exist for diagnostic studies.
• No Level 3 evidence exists for studies of prognosis.
• Level 4 consists of case series studies.
• Level 5 evidence is generally consistent across all
categories of research.
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Relevance of Evidence to Problem
• Ask the following about the papers you read:
– Are the results accurate, correct, and unbiased?
– Do the results apply to the patient I am
treating?
– Will the results help me?
• Consider the whole of the patient being treated
when applying the results of students.
• Problems occur when:
– The evidence is less strong or conflicting.
– The patient differs from those described in the
research.
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Chapter Summary and Key Points
• Confidence gained from experience and continued study of the
literature helps the clinician cope with the stresses of providing
optimal care and eases the anxiety of the patients we treat.
• N-of-1 trials and “Level 1” evidence should be weighted more
heavily in clinical decisions than sources lower on the hierarchy.
• Hierarchies of evidence relate the strength of evidence in
making decisions about patient care, not the sophistication of
the research methods and data.
• Differences in the strength of evidence come from two sources:
sampling and research methods.
• Poor methods increase the likelihood of biased data, threatening
internal as well as external validity.
• Clinicians must carefully consider how strongly the available
research will influence their decisions.
• All evidence is worthy of consideration for the patients seeking
our care.
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