[Systematic reviews in practice. IX. Determining their place in treatment recommendations in practice guidelines].

In a systematic review (SR), the available evidence for a clinical problem is systematically and comprehensively collected from different studies, the likelihood of bias is assessed and the results are summarised in a reproducible manner. The results of an SR can be used in either individual patient care or the formulation of a practice guideline. Throughout the world, (national) organisations for guideline development use systems for classifying the validity of evidence according to the study design ('levels of evidence'), and this is also the case in the Netherlands. In formulating a treatment recommendation, consideration must be given to the various factors which determine the strength of the evidence: study design, consistency of the results (if more studies are available), quality of the individual studies, magnitude and precision of the reported effect and the clinical relevance of the outcome measures. If insufficient studies with the same outcome measure are available, problems can arise in an SR. In practice, problems can also arise with respect to estimating the cogency of an SR compared to another review or primary studies. For the same subject, there might be discordant reviews, differences between reviews and megatrials might exist or just one randomised clinical trial might be available. These problems can often be solved with the aid of a systematic analysis. This requires both methodological and clinical expertise.