What are the different grades for evidence in medical research?

Different Grades for Evidence in Medical Research

Evidence quality in medical research is commonly classified into hierarchical grades that reflect the reliability and strength of the evidence, with the GRADE system being one of the most widely adopted approaches.

GRADE System (Grading of Recommendations, Assessment, Development and Evaluation)

Quality of Evidence Grades

• High (A): Further research is very unlikely to change our confidence in the estimate of effect. This typically comes from high-quality randomized controlled trials (RCTs) with consistent results 1
• Moderate (B): Further research may have an important impact on our confidence in the estimate of effect and may change the estimate. This often comes from RCTs with limitations or strong observational studies 1
• Low (C): Further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate. This typically comes from observational studies 1
• Very Low (D): Any estimate of effect is uncertain. This comes from case reports, expert opinion, or studies with serious limitations 1

Strength of Recommendations

• Strong (1): The desirable effects of an intervention clearly outweigh the undesirable effects (or vice versa) 1
• Weak (2): The desirable effects probably outweigh the undesirable effects, but the panel is not confident about these trade-offs 1

Combined Grading

The GRADE system combines quality of evidence with strength of recommendation, resulting in designations such as:

• 1A (Strong recommendation, high-quality evidence)
• 1B (Strong recommendation, moderate-quality evidence)
• 1C (Strong recommendation, low-quality evidence)
• 2A (Weak recommendation, high-quality evidence)
• 2B (Weak recommendation, moderate-quality evidence)
• 2C (Weak recommendation, low-quality evidence) 1

Oxford Centre for Evidence-Based Medicine Levels

• Level 1A: Systematic reviews of homogeneous RCTs 1
• Level 1B: Individual RCTs with narrow confidence interval and >80% follow-up 1
• Level 2A: Systematic reviews of homogeneous cohort studies 1
• Level 2B: Individual cohort studies or low-quality RCTs (less than 80% follow-up) 1
• Level 2C: "Outcomes research" studies 1
• Level 3A: Systematic reviews of homogeneous case-control studies 1
• Level 3B: Individual case-control studies 1
• Level 4: Case series, poor-quality cohort and case-control studies 1
• Level 5: Expert opinion without explicit critical appraisal 1

American Academy of Pediatrics (AAP) Grades

• Grade A: Consistent level 1 studies (meta-analyses, well-designed RCTs, high-quality diagnostic studies) 1
• Grade B: Consistent level 2 or 3 studies or extrapolations from level 1 studies (RCTs with methodological flaws, well-designed observational trials) 1
• Grade C: Level 4 studies or extrapolations from level 2 or 3 studies (poor-quality observational studies, case series) 1
• Grade D: Level 5 evidence or troublingly inconsistent or inconclusive studies of any level 1

Factors That Can Decrease Evidence Quality

• Serious limitations in study quality (risk of bias) 1, 2
• Important inconsistency between studies 1, 2
• Indirectness of evidence (differences in population, intervention, comparator, or outcome) 1, 2
• Imprecision or sparse data 1, 2
• High probability of publication bias 1, 2

Factors That Can Increase Evidence Quality

• Large magnitude of effect (relative risk >2 or <0.5) 1
• Dose-response gradient 1
• All plausible confounders would reduce the demonstrated effect 1

Strength of Recommendation Taxonomy (SORT)

• Level A: Recommendation based on consistent and good-quality patient-oriented evidence 3
• Level B: Recommendation based on inconsistent or limited-quality patient-oriented evidence 3
• Level C: Recommendation based on consensus, usual practice, opinion, disease-oriented evidence, or case series 3

Clinical Application

When evaluating medical evidence, it's important to:

• Consider both the quality of evidence and the strength of recommendations 1, 2
• Recognize that high-quality evidence doesn't automatically translate to strong recommendations 2, 4
• Understand that recommendations consider not only evidence quality but also risk-benefit balance, patient values and preferences, and resource considerations 1, 5

The hierarchical nature of these grading systems helps clinicians quickly assess the reliability of research findings and the confidence they can have in implementing recommendations in clinical practice 2, 4.