Level of Evidence 1
A high degree of clinical certainty
Level A
Based on consistent Class I evidence (well-designed, prospective randomized controlled studies)
Level B
Single Class I study or consistent Class II evidence or strong Class II evidence especially when circumstances preclude randomized clinical trials
Randomized controlled trial (RCT) of the intervention of interest with masked or objective outcome assessment, in a representative population. Relevant baseline characteristics are presented and substantially equivalent among treatment groups or there is an appropriate statistical adjustment for differences.
Clinicians' trust level of evidence 1 recommendations, issued on preponderantly solid randomized clinical trials (RCTs), to guide best practice decision-making. However, sometimes physicians following one clinical practice guidelines (CPG) find themselves in a situation in which they do not follow another, issued on the same strong evidence base. The aim of Volovici et al. is to reflect on the consistency of recommendations in different guidelines (between-guideline consistency). They also consider within-guideline consistency (or durability), defined as the number of recommendations carried over from one edition to another in consecutive editions of the same CPG. For illustration purposes, they use two examples: hypertension guidelines and traumatic brain injury (TBI) guidelines. They conclude that just like research, CPGs also need to have between-guideline and within-guideline consistency (akin to the reproducibility of studies). Clinicians and researchers should take into account the lower consistency of guidelines that are not based on at least one strong RCT 1).
Evidence obtained from at least one properly designed randomized controlled trial.
Level 1a
Evidence from large randomized clinical trials (RCTs) or systematic reviews (including meta-analyses) of multiple randomized trials which collectively has at least as much data as one single well-defined trial.
Level 1b
Evidence from at least one “All or None” high quality cohort study; in which ALL patients died/failed with conventional therapy and some survived/succeeded with the new therapy (for example, chemotherapy for tuberculosis, meningitis, or defibrillation for ventricular fibrillation); or in which many died/failed with conventional therapy and NONE died/failed with the new therapy (for example, penicillin for pneumococcal infections).
Level 1d
Evidence from at least one RCT.
The guidelines by the American Heart Association Stroke Council for treatment of chronic hydrocephalus secondary to subarachnoid hemorrhage by the permanent Cerebrospinal fluid shunt is presented as a Class I recommendation but is based upon Level of evidence C 2)