The U.S. Preventive Services Task Force and the Guide to Community Preventive Services (USPSTF)
The USPSTF originally adopted a “hierarchy of evidence” which lists a range of study designs ranked in order of decreasing internal validity (see Exhibit 2).
Exhibit 2. Hierarchy of Evidence Used by the USPSTF to Evaluate Clinical Studies
- Category I: Evidence from at least one properly randomized controlled trial.
- Category II-1: Evidence from well-designed controlled trials without randomization.
- Category II-2: Evidence from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.
- Category II-3: Evidence from multiple times series with or without intervention or dramatic results in uncontrolled experiments such as the results of the introduction of penicillin treatment in the 1940’s.
- Category III: Opinions of respected authorities, based on clinical experience, descriptive studies and case reports, or reports of expert committees.
Recognizing the limitations of a strictly hierarchical approach, the USPSTF subsequently developed a set of criteria for assessing internal validity that allows them to categorize individual studies and an overall body of evidence as good, fair, or poor (see Exhibit 3).55
Exhibit 3. USPSTF Criteria for Grading the Internal Validity of Individual Studies
Study design: Systematic reviews
Criteria:
- Comprehensiveness of sources/search strategy used
- Standard appraisal of included studies
- Validity of conclusions
- Recency and relevance
Study design: Case-control studies
Criteria:
- Accurate ascertainment of cases
- Nonbiased selection of cases/controls with exclusion criteria applied equally to both
- Response rate
- Diagnostic testing procedures applied equally to each group
- Appropriate attention to potential confounding variables
Study design: Randomized controlled trials (RCTs) and cohort studies
Criteria:
- Initial assembly of comparable groups (For RCTs: adequate randomization, including concealment and whether potential confounders were distributed equally among groups. For cohort studies: consideration of potential confounders with either restriction or measurement for adjustment in the analysis); consideration of inception cohorts
- Maintenance of comparable groups (includes attrition, crossovers, adherence, contamination)
- Important differential loss to follow-up or overall high loss to follow-up
- Measurements: equal, reliable, and valid (includes masking of outcome assessment)
- Clear definition of interventions
- All important outcomes considered
- Analysis: adjustment for potential confounders for cohort studies, or intention-to-treat analysis for RCTs
Study design: Diagnostic accuracy studies
Criteria:
- Screening test relevant, available for primary care, adequately described
- Study uses a credible reference standard, performed regardless of test results
- Reference standard interpreted independently of screening test
- Handles indeterminate results in a reasonable manner
- Spectrum of patients included in study
- Sample size
- Administration of reliable screening test
The USPSTF has also recognized the importance of external validity (i.e., how generalizable the results are to the relevant populations). Their systematic reviews now assess the internal validity (i.e., does the study provide an accurate result), external validity (i.e., is the study relevant to the setting of interest), the magnitude of effect (i.e., for the benefit of a service to be considered substantial there must be either a small relative impact of a frequent condition with a substantial population burden, or a large impact on an infrequent condition that poses a significant burden at the individual patient level), and certainty (i.e., degree of assurance that recommended services actually produce more benefit than harm).56
The USPSTF uses tables that take into account the level of certainty and magnitude of effect (see Exhibit 3) to translate the evidence of reviews into suggestions for practice (see Exhibits 4 and 5).
There is recognition that a simple hierarchy of evidence based on internal validity provides only a limited assessment of the interventions for the community-based settings in which public health interventions occur.57
Exhibit 4. USPSTF Recommendation Grid: Letter Grade of Recommendation or Statement of Insufficient Evidence Assessing Certainty and Magnitude of Net Benefit for Population Level Interventions
| Certainty of Net Benefit |
Magnitude of Net Benefit |
| Substantial |
Moderate |
Small |
Zero/negative |
| High |
A |
B |
C |
D |
| Moderate |
B |
B |
C |
C |
| Low |
Insufficient |
Exhibit 5. What the USPSTF Grade Means and Suggestions for Practice
| Grade |
Definition |
Suggestions for Practice |
| A |
The USPSTF recommends the service. There is high certainty that the net benefit is substantial. |
Offer or provide this service. |
| B |
The USPSTF recommends the service. There is high certainty that the net benefit is moderate or there is moderate certainty that the net benefit is moderate to substantial. |
Offer or provide this service. |
| C |
The USPSTF recommends against routinely providing the service. There may be considerations that support providing the service in an individual patient. There is at least moderate certainty that the net benefit is small. |
Offer or provide this service only if other considerations support the offering or providing the service in an individual patient. |
| D |
The USPSTF recommends against the service. There is moderate or high certainty that the service has no net benefit or that the harms outweigh the benefits. |
Discourage the use of this service. |
| I Statement |
The USPSTF concludes that the current evidence is insufficient to assess the balance of benefits and harms of the service. Evidence is lacking, of poor quality, or conflicting, and the balance of benefits and harms cannot be determined. |
Read the clinical considerations section of USPSTF Recommendation Statement. If the service is offered, patients should understand the uncertainty about the balance of benefits and harms. |
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