Health Technology Assessment 2004; Vol 8: number 25
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P Whiting,1* AWS Rutjes,2 J Dinnes,3 JB Reitsma,2 PMM Bossuyt2 and J Kleijnen1
1 Centre for Reviews and Dissemination, University of York, UK
2 Department of Clinical Epidemiology and Biostatistics, Academic Medical Centre, University of Amsterdam, The Netherlands
3 Wessex Institute for Health Research and Development, University of Southampton, UK
* Corresponding author
The assessment of the quality of studies included in a systematic review is as important for reviews of studies of diagnostic accuracy as it is for any other type of review. There is currently a lack of a validated tool for the assessment of such studies.
This project aims to develop a quality assessment tool which will be used in systematic reviews to assess the quality of primary studies of diagnostic accuracy.
Three systematic reviews were conducted to provide an evidence base for the development of the quality assessment tool. The methodological literature on diagnostic test assessment was reviewed to identify potential sources of bias. Systematic reviews of diagnostic tests that used any form of quality assessment were examined to identify how quality was incorporated. Lastly, a review of existing quality assessment tools was conducted to ascertain what methods exist for assessing the quality of diagnostic studies, and on what evidence they are based. Literature searches were used to identify studies for each of the reviews. Systematic inclusion criteria were applied; studies were selected for relevance and inclusion by one reviewer and checked by a second. Data for each of the reviews were extracted into an Access database by one reviewer and checked by a second. All discrepancies were resolved by discussion or through consultation with a third reviewer when agreement could not be reached. A narrative synthesis is presented for each of the reviews.
A Delphi procedure was used to develop the quality assessment tool. The information provided by the reviews was incorporated into this. A panel of nine experts in the area of diagnostic accuracy studies took part in the Delphi procedure. In the first round members were asked to indicate which of the items on the initial list of items (provided by the results of the reviews) should be included in the tool. Items for which there were high levels of agreement were selected for inclusion/exclusion in the tool; items for which there was disagreement were rated again as part of the next round. Panel members were also asked to make comments and to suggest rephrasings of the items or additional items if appropriate. During subsequent rounds the results of previous rounds were fed back to panel members and they were asked to rerate the items based on the results of the previous rounds. The procedure was continued until agreement was reached on which items were to be included in the quality assessment tools. Panel members were also asked to provide feedback on various other items such as the proposed scoring method, whether they endorsed the procedure, whether they had used the evidence provided to them, and whether they would like to see the development of additional topic and design specific items.
The Delphi procedure produced the quality assessment tool, named QUADAS. A background document was produced which gives details on what is meant by each item included in the tool and how each of the items should be scored.
Work to validate the tool will continue beyond the scope of this project. The validation process will include the piloting of the tool on a small sample of published studies, assessment of the consistency and reliability of the tool, piloting the tool in a number of diagnostic reviews, and using a regression analysis to investigate associations between study characteristics and estimates of diagnostic accuracy in primary studies, as combined in existing systematic reviews.
The reviews produced a list of 28 possible items for inclusion in the quality assessment tool. The first review found that the sources of bias supported by the most empirical evidence were variation by clinical and demographic subgroups, disease prevalence/severity, partial verification bias, clinical review bias and observer/instrument variation. There was also some evidence of bias for the effects of distorted selection of participants, absent or inappropriate reference standard, differential verification bias and review bias. The evidence for the effects of other sources of bias was insufficient to draw conclusions regarding the effects, if any, of these biases. The third review found that only one item, the avoidance of review bias, was included in more than 75% of tools. A further four items were each included in 5075% of tools: spectrum composition, population recruitment, absent or inappropriate reference standard and verification bias. Other items were included in less than 50% of tools.
The second review found that the quality assessment tool needs to have the potential to be discussed narratively, reported in a tabular summary, used as recommendations for future research, used to conduct sensitivity or regression analyses and used as criteria for inclusion in the review or a primary analysis. The resulting implication for the development of the tool is that some distinction needs to be made between high- and low-quality studies. It was decided that component analysis is the best approach to incorporate quality into systematic reviews of diagnostic studies. The quality tool was developed taking this into consideration.
The Delphi procedure consisted of four rounds, after which agreement was reached on the items to be included in QUADAS. The final tool included 14 items:
This project produced an evidence-based quality assessment tool to be used in systematic reviews of diagnostic accuracy studies. Through the various stages of the project the current lack of such a tool and the need for a systematically developed validated tool were demonstrated. Further work to validate the tool continues beyond the scope of this project. The further development of the tool by the addition of design- and topic-specific criteria is proposed.
Whiting P, Rutjes AWS, Dinnes J, Reitsma JB, Bossuyt PMM, Kleijnen J. Development and validation of methods for assessing the quality of diagnostic accuracy studies. Health Technol Assess 2004;8(25).
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