Table 1– Description of levels of evidence
Evidence categorySources of evidenceDefinition
ARandomised controlled trialsRich body of dataEvidence is from end-points of well-designed randomised controlled trials that provide a consistent pattern of findings in the population for which the recommendation is made. Category A requires substantial numbers of studies involving substantial numbers of participants.
BRandomised controlled trialsLimited body of dataEvidence is from end-points of intervention studies that include only a limited number of patients, post hoc or subgroup analysis of randomised controlled trials, or meta-analysis of randomised controlled trials. In general, category B pertains when few randomised trials exist, they are small in size, they were undertaken in a population that differs from the target population of the recommendation, or the results are somewhat inconsistent.
CNonrandomised trialsObservational studiesEvidence is from outcomes of uncontrolled or nonrandomised trials or from observational studies.
DPanel consensus judgementThis category is used only in cases where the provision of some guidance was deemed valuable but the clinical literature addressing the subject was deemed insufficient to justify placement in one of the other categories. The panel consensus is based on clinical experience or knowledge that does not meet the above-listed criteria.