5. Summing it all up: Synthesis of clinical research

A systematic review is a thorough, comprehensive, and explicit way of interrogating the medical literature. It typically involves several steps, including:

1.     asking an answerable question (often the most difficult step),

2.     identifying one or more databases to search,

3.     developing an explicit search strategy,

4.     selecting titles, abstracts, and manuscripts based on explicit inclusion and exclusion criteria, and

5.     abstracting data in a standardised format. 

Systematic reviews are key inputs to the HTA process and driven by reproducible methods, which provide a structured, comprehensive overview of the published scientific evidence. Decision-makers may still differ – potentially quite broadly – in how they use the results of the systematic review or what weight they give to results in relation to other factors. However, the systematic nature of these reviews gives the decision makers the best chance of ensuring that the role scientific information may play in their decision is maximised.

Systematic reviews can still result in a biased estimate of clinical impact if information from important studies is not identified, or if information regarding important outcomes from published studies is not reported in a review. Also, the quality of the information must be considered before authors draw conclusions. Identifying that 15 out of 20 studies reported positive outcomes is not helpful if those studies were prone to error by design. As such, systematic reviews should be critically assessed.

When interpreting a systematic review or meta-analysis, the reader or an HTA body cannot always rely on available evidence and sometimes have to make a 'leap of faith': a smaller leap for systematic reviews that have reported appropriate approaches to the items in the checklist, and a bigger leap for those with less or no detail.