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Diarrhoea and vomiting caused by gastroenteritis in children under 5 years

There was no systematic attempt to search grey literature (conferences, abstracts, theses and
unpublished trials). Hand searching of journals not indexed on the databases was not undertaken.
All searches were conducted between 21 September 2007 and 27 May 2008. Searches for
clinical questions were rerun from 12 to 14 August 2008, before the start of the consultation
period. This date period should be considered the starting point for searching for new evidence
for future updates to this guideline.
The detailed search strategies, including the methodological filters employed, are provided on
the accompanying CD-ROM, and on the NICE website.

1.7.2 Synthesis of clinical effectiveness evidence
Evidence relating to clinical effectiveness was reviewed using established guides 26–33 and
classified using the established hierarchical system shown in Table 1.1. This system reflects the
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susceptibility to bias that is inherent in particular study designs.
The type of clinical question dictates the highest level of evidence that may be sought. In assessing
the quality of the evidence, each study receives a quality rating coded as ‘++’, ‘+’ or ‘−’. For issues
of therapy or treatment, the highest possible evidence level (EL) is a well-conducted systematic
review or meta-analysis of randomised controlled trials (RCTs) (EL = 1++) or an individual RCT
(EL = 1+). Studies of poor quality are rated as ‘−’. Usually, studies rated as ‘−’ should not be used
as a basis for making a recommendation, but they can be used to inform recommendations. For
issues of clinical presentation, the highest possible level of evidence is a cohort study (EL = 2++).
For each clinical question, the highest available level of evidence was selected. Where appropriate,
for example if a systematic review, meta-analysis or RCT existed in relation to a question, studies
of a weaker design were not included. Where systematic reviews, meta-analyses and RCTs did
not exist, other appropriate experimental or observational studies were sought.
The system described above covers studies of treatment effectiveness. However, it is less
appropriate for studies reporting diagnostic tests of accuracy. In the absence of a validated
ranking system for these types of study, NICE has developed a hierarchy for evidence of accuracy
of diagnostic tests that takes into account the various factors likely to affect the validity of these
studies as seen in Table 1.2. 26

For economic evaluations, the search strategies adopted were designed to identify any relevant
economic studies. Abstracts of all papers identified were reviewed by the health economists and
were discarded if they did not relate to the economic question being considered in the guideline.
The relevant papers were retrieved and critically appraised. Potentially relevant references in the
bibliographies of the reviewed papers were also identified and reviewed. All papers reviewed were
assessed by the health economists against standard quality criteria for economic evaluation. 34

Table 1.1 Levels of evidence for intervention studies

Level Source of evidence
1++ High-quality meta-analyses, systematic reviews of randomised controlled trials (RCTs), or
RCTs with a very low risk of bias
1+ Well-conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias
1− Meta-analyses, systematic reviews of RCTs, or RCTs with a high risk of bias
2++ High-quality systematic reviews of case–control or cohort studies; high-quality case–control
or cohort studies with a very low risk of confounding, bias or chance and a high probability
that the relationship is causal
2+ Well-conducted case–control or cohort studies with a low risk of confounding, bias or
chance and a moderate probability that the relationship is causal
2− Case–control or cohort studies with a high risk of confounding, bias or chance and a
significant risk that the relationship is not causal
3 Non-analytical studies (for example, case reports, case series)
4 Expert opinion, formal consensus

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