Clinical practice guidelines can improve healthcare processes and patient outcomes, but are often of low quality. Guideline appraisal tools aim to help potential guideline users in assessing guideline quality. We conducted a systematic review of publications describing guideline appraisal tools in order to identify and compare existing tools.
Among others we searched MEDLINE, EMBASE and the Cochrane Database of Systematic Reviews from 1995 to May 2011 for relevant primary and secondary publications. We also handsearched the reference lists of relevant publications.
On the basis of the available literature we firstly generated 34 items to be used in the comparison of appraisal tools and grouped them into thirteen quality dimensions. We then extracted formal characteristics as well as questions and statements of the appraisal tools and assigned them to the items.
We identified 40 different appraisal tools. They covered between three and thirteen of the thirteen possible quality dimensions and between three and 29 of the possible 34 items. The main focus of the appraisal tools were the quality dimensions “evaluation of evidence” (mentioned in 35 tools; 88%), “presentation of guideline content” (34 tools; 85%), “transferability” (33 tools; 83%), “independence” (32 tools; 80%), “scope” (30 tools; 75%), and “information retrieval” (29 tools; 73%). The quality dimensions “consideration of different perspectives” and “dissemination, implementation and evaluation of the guideline” were covered by only twenty (50%) and eighteen tools (45%) respectively.
Most guideline appraisal tools assess whether the literature search and the evaluation, synthesis and presentation of the evidence in guidelines follow the principles of evidence-based medicine. Although conflicts of interest and norms and values of guideline developers, as well as patient involvement, affect the trustworthiness of guidelines, they are currently insufficiently considered. Greater focus should be placed on these issues in the further development of guideline appraisal tools.
Citation: Siering U, Eikermann M, Hausner E, Hoffmann-Eßer W, Neugebauer EA (2013) Appraisal Tools for Clinical Practice Guidelines: A Systematic Review. PLoS ONE 8(12): e82915. https://doi.org/10.1371/journal.pone.0082915
Editor: Yu-Kang Tu, National Taiwan University, Taiwan
Received: July 5, 2013; Accepted: October 29, 2013; Published: December 9, 2013
Copyright: © 2013 Siering et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: No current external funding sources for this study.
Competing interests: The authors have declared that no competing interests exist.
Clinical practice guidelines (hereafter referred to as “guidelines”) are defined by the Institute of Medicine as “statements that include recommendations intended to optimize patient care that are informed by a systematic review of evidence and an assessment of the benefits and harms of alternative care options” . Beyond that, guidelines are used for a variety of purposes, for example, as a means to measure and improve the quality of care, to resolve malpractice claims, to contribute to the development of clinical decision aids or to support policy makers in the allocation of healthcare resources .
There is evidence to suggest that, when rigorously developed, guidelines have the power to translate the complexity of scientific research findings and other evidence into recommendations for healthcare action [2-5].
Several studies have shown that guidelines can improve healthcare processes and patient outcomes. Grimshaw, Eccles, and Tetroe 2004 conducted a systematic review of the effectiveness and costs of various guideline development, dissemination and implementation strategies. The majority (86.6%) of the 235 studies included in their review reported improvements in health care [6,7]. Two other systematic reviews reported similar results [8,9]. However, all of the authors noted that the studies included were of low methodological quality.
The AGREE Collaboration defines guideline quality as “the confidence that the potential biases of guideline development have been addressed adequately and that the recommendations are both internally and externally valid, and are feasible for practice” . This definition has been widely adopted in the scientific literature [11,12].
Potential deficits of guidelines include:
- conflicting recommendations [18-26],
- insufficient consideration of relevant patient characteristics (e.g., multimorbidity or ethnic differences) [27-30],
- low quality of the evidence underlying the recommendations [31-35],
- lack of transparency of methods applied by guideline developers, especially concerning the derivation of recommendations and the determination of their strength ,
- inadequate management of potential conflicts of interest [36-41].
Several groups, such as the Guidelines International Network , the Institute of Medicine , the World Health Organization , the National Institute for Health and Clinical Excellence , the Scottish Intercollegiate Guidelines Network , many medical societies [46-51], as well as individual experts in the field [12,52-55], have proposed manuals defining standards for guideline developers in order to increase guideline quality. Overall, these manuals address the following key elements in the development process: establishment of a multidisciplinary guideline development group, consumer involvement, identification of clinical questions or problems, conduct of systematic searches and appraisal of the evidence retrieved, procedures for drafting recommendations, external consultation, and ongoing reviewing and updating .
Parallel to the production of manuals for the development of high-quality guidelines, tools for their appraisal have been developed. These tools aim to help potential guideline users to assess guideline quality. The AGREE II Instrument – the guideline appraisal tool used most often internationally – contains questions covering the areas (1) scope and purpose, (2) stakeholder involvement, (3) rigour of development, (4) clarity of presentation, (5) applicability, and (6) editorial independence .
Graham 2000 identified and compared guideline appraisal tools in a systematic review , which was updated by Vlayen in 2005 . Vlayen identified 24 different tools containing questions that could be grouped into ten quality dimensions with 50 different items. Four of the 24 tools covered all of the guideline dimensions, but only four were validated and none assessed the evidence base of the clinical content of the guidelines. The authors stated that “the results of the search for evidence, the correct use of inclusion and exclusion criteria, and the critical appraisal of the retrieved evidence are not validated. Therefore, a major conclusion of this review is that in order to evaluate the quality of the clinical content and more specifically the evidence base of a clinical practice guideline, verification of the completeness and the quality of the literature search and its analysis has to be added to the process of validation by an appraisal instrument.”
The aims of this systematic review were to identify and compare existing guideline appraisal tools to see if the landscape of tools had changed. This comparison can then be used to support decision-making by clinicians, patients and policy makers concerning the selection of the most appropriate tool, as well as to identify potential for improvement.
We searched for relevant primary and secondary publications (systematic and narrative reviews) in MEDLINE, EMBASE, the Cochrane Database of Systematic Reviews (Cochrane Reviews), the Database of Abstracts of Reviews of Effects (Other Reviews), the Health Technology Assessment Database (Technology Assessments), the NHS Economic Evaluation Database, and the Cochrane Methodology Register. The systematic search was limited to publications in German and English published after 1994. The search in all databases was performed in May 2011. The search strategy included, among others, the search terms “guideline”, “appraisal”, “guideline adherence”, “quality”, “evidence based” and “evaluation”. The full search strategy, which was developed by an information specialist (EH), is attached to this publication as online File S1. In addition, we scrutinized the reference lists of the relevant primary and secondary publications retrieved in the above search to identify further publications.
We included articles with the following characteristics:
- Publication described the most recent version of an appraisal tool for clinical guidelines
- Availability of a full-text document (e.g., journal article or internet file).
Articles were excluded that only described the content of guidelines, the guideline development process or the application of an appraisal tool already identified in another publication.
Two reviewers (US, WHE) independently screened titles and abstracts of the retrieved citations to identify potentially eligible primary and secondary publications. The full texts were obtained and independently evaluated by the same two reviewers. Disagreements were resolved by consensus.
Since the primary aim of this review was to identify existing guideline appraisal tools and to describe and compare their formal and content characteristics, no risk of bias assessment was conducted for the publications included.
The content analysis was a two-stage process. The first stage involved the generation of items to be used in the comparison of appraisal tools by compilation of a list of all questions and statements from each of the tools included. These were grouped into common questions and statements and assigned to an item label. The items were then assigned to broader common categories, named quality dimensions, which were largely derived from Cluzeau et al. 1999 , Graham 2000  and Vlayen 2005 .
The individual steps of the content analysis procedures were always conducted by one person (US) and checked by another (WHE). Disagreements were resolved by consensus.
We identified 34 individual items and assigned them to thirteen quality dimensions (see Table 1 for detailed definitions).
|Quality dimensions / Item label||Definition|
|1. Information retrieval|
|Health questions and outcomes||Description of clinical health questions and relevant outcomes of the guideline|
|Literature search||Search for literature and other evidence|
|Literature selection||Criteria used to include and exclude literature and other evidence|
|2. Evaluation of evidence|
|Grading of evidence||Grading of the evidence, which may or may not include a statement about the strength of evidence (LoE)|
|Consistency between evidence and recommendations||Studies results are reported correctly in the guideline and support the recommendations|
|3. Consideration of different perspectives|
|Norms and values||Discussion of influence of norms and values on guideline development|
|Expert knowledge||Evaluation of expert opinion and clinical experience|
|Patient perspectives||Consideration of views and preferences of the target population in the guideline development process|
|4. Formulation of recommendations|
|Formulation of recommendations||Methods used in formulating recommendations which may or may not include a statement about the strength of recommendations (GoR)|
|Comparability||Patients, interventions and settings in the studies were comparable to those targeted by the recommendations|
|Costs||Consideration of resource implications of applying the recommendations|
|Barriers and facilitators||Description of barriers and facilitators to guideline application (compatibility of guideline with local norms and values; professional’s training, skill, and experience; availability of drugs or technology; local adaptation or modification of the guideline)|
|6. Presentation of guideline content|
|Benefits and harms||Presentation of health benefits, side effects, and harms of the recommended action|
|Link to evidence||Explicit link between the recommendations and the supporting evidence|
|Options for management||Presentation of alternative options for management of the condition or health issues|
|Exceptions||Description of situations in which guidelines may not apply|
|Patient preferences||Consideration of patient preferences in the application of guideline recommendations|
|Independent Review||External peer review before publication|
|Pilot test||Pilot test of the guideline prior to release|
|Rationale and objective||Description of the rationale or reason for guideline development and description of the goal or objective of the guideline|
|Guideline topic||Topic, or health problem, or technology dealt with|
|Practice setting||Practice setting for which the guideline is intended|
|Patient population||Patient population for whom the guideline is intended|
|Provider population||Group of health care providers for whom the guideline is intended|
|Guideline development group||Individuals and/or disciplines, or occupations represented in the guideline development group and their function in the group|
|Guideline development organization and funding||Organization or group who developed the guideline and sources of funding|
|Conflicts of interest||Consideration of (potential) conflicts of interest related to the individuals developing the guideline|
|11. Clarity and presentation|
|Clarity||Clear wording of the guideline and the recommendations|
|Presentation||Easily identifiable recommendations (e. g., summarized in a box, bold text, underlined). Graphical description of the stages and decisions in clinical care (clinical algorithm).|
|Scheduled review||Procedure for updating the guideline|
|13. Dissemination, Implementation, Evaluation|
|Dissemination||Distribution of the guideline to intended users|
|Implementation||Strategies to implement the guideline|
|Evaluation||Evaluation of the guideline and the adherence to the guideline once it has been implemented|
For the second stage of the analysis, we (US, WHE) extracted the following information from each publication:
- (1) Formal characteristics of the appraisal tool.
These included language, the use of existing appraisal tools for tool development, number of items and domains, possible answers, number of appraisers, calculation of domain scores and overall assessment, information on the development and validation of the appraisal tool, as well as publication in a journal.
- (2) Questions and statements of the appraisal tools.
One reviewer (US) then assigned the questions and statements to the items identified during the first stage of the content analysis. A second reviewer (WHE) confirmed this step by once again checking the questions of each appraisal tool and the items to which they had been assigned. Disagreements were resolved by consensus. The numbers of quality dimensions and items covered by each appraisal tool were then compared.
The review was not registered in advance, nor has a review protocol been published.
Selection of publications
We retrieved 5164 references from bibliographic databases and screened 446 full texts. In addition, we retrieved 62 further publications from the reference lists of the relevant primary and secondary publications. We identified a total of 42 eligible publications describing 40 different guideline appraisal tools (Figure 1). Excluded publications are listed in online File S2. Relevant secondary publications are listed in online File S3.
Description of Appraisal Tools
Table 2 shows the main formal characteristics of the 40 appraisal tools considered. 38 were published in English and two in German. 26 named at least one other publication that had influenced their developmentand ten named the AGREE Instrument ; other publications mentioned included those by Hayward 1995, Wilson 1995 and Field 1992 [61-63].
|Appraisal tool||Language||Based on||Additional information on development of appraisal tool||Generic appraisal toola||Subject of assessment||Number of questions (Domains)||Explanation of questions||Rating scale / Multiple choice / Additional comments||Number of appraiser||Domain scores / Overall assessment||Validation||Publication in journal|
|ADAPTE 2009 ||EN||n. s.||yes||yes||GL / Rec.||43 (3)||SE||no / yes / no||n. s.||no / no||yes||no|
|AGREE II 2009 ||EN||||yes||yes||GL||23 (6)||SE||yes / no / yes||2, better 4||yes / yes||yes||no|
|APWCA 2010 ||EN||n. s.||no||yes||GL||11 (-)||no||no / no / no||n. s.||no / no||no||yes|
|APA 2002||EN||n. s.||no||yes||GL||47 (21)||SE||no / no / no||n. s.||no / no||no||yes|
|Baxter 2003 ||EN||[61,91,113]||no||yes||GL||12 (4)||SE||no / no / no||n. s.||no / no||no||yes|
|BÄK 1997 ||GE||n. s.||no||yes||GL||12 (-)||SE||no / no / no||n. s.||no / no||no||yes|
|Calder 1997 ||EN||[60-62,116,117]||no||yes||GL||26 (5)||no||no / yes / no||2||no / no||no||yes|
|Chong 2009 ||EN||[10,13]||no||yes||GL||11 (2)||no||yesb / yesc / no||2||no / no||yes||yes|
|Chou 2008[84,85]||EN||[10,14,118]||no||yes||GL||26 (5)||SE||no / no / no||n. s.||no / no||no||yes|
|Cluzeau 1999 ||EN||||no||yes||GL||37 (3)||no||no / yes / no||n. s.||yes / yes||yes||yes|
|Cook 1998 ||EN||[61,62]||no||yes||GL||9 (3)||SE||no / no / no||n. s.||no / no||no||yes|
|DELBI 2008 ||GE||||yes||yes||GL||34 (8)||CE||yes / no / no||2, better 4||yes / no||no||yes|
|Fields 2000 ||EN||[61,62]||no||yes||GL||8 (-)||no||no / no / no||n. s.||no / no||no||yes|
|Foy 2002 ||EN||[120,121]||yes||yes||Rec.||13 (-)||no||yes / no / no||n. s.||no / no||no||yes|
|Fretheim 2002 ||EN||[10,122]||no||yes||GL||8 (-)||CE||no / no / no||2||no / no||no||yes|
|GLIA 2011 ||EN||[10,13,60,63,120,121,123,124]||yes||yes||GL / Rec.||30 (9)||no||no / yes / yes||2||no / no||yes||no|
|Grilli 2000 ||EN||n. s.||no||yes||GL||3 (-)||CE||no / yes / no||2||no / no||yes||yes|
|Guyatt 2002 ||EN||n. s.||no||yes||GL||4 (-)||SE||no / no / no||n. s.||no / no||no||yes|
|Hargrove 2008 ||EN||[10,88,125-128]||no||yes||GL||18 (3)||SE||no / yes / yes||3||no / no||yes||yes|
|Hart 2002 ||EN||[10,13,14]||no||yes||GL||9 (-)||no||yes / no / no||n. s.||no / no||no||yes|
|Hasenfeld 2003 ||EN||||no||yes||GL||30 (-)||no||no / yes / no||2||no / no||no||yes|
|Hayward 1995 [61,62]||EN||[129,130]||no||yes||Rec.||10 (3)||SE||no / no / no||n. s.||no / no||no||yes|
|Hindley 2005 ||EN||||yes||yes||GL||18 (12)||no||yes / no / no||at least 2||no / yes||yes||yes|
|Kulig 2003 ||EN||||yes||yes||GL||13 (3)||no||yes / no / no||2||yes / yes||yes||yes|
|Liddle 1996 ||EN||k. A||yes||yes||GL / Rec.||14 (3)||SE for 1 question||for some questions / for 1 question / yes||n. s.||no / no||yes||no|
|Linskey 2010 ||EN||n. s.||no||yes||GL||9 (-)||no||no / no / no||n. s.||no / no||no||yes|
|Marshall 2000 ||EN||[129,132,133]||no||yes||GL||9 (-)||SE||no / no / no||n. s.||no / no||no||yes|
|Mottur-Pilson 1995 ||EN||||yes||yes||GL||18 (-)||no||no / yes / yes||n. s.||no / yes||no||yes|
|Nonino 2004 ||EN||[10,14]||no||yes||GL||6 (-)||no||no / no / no||n. s.||no / no||no||yes|
|Pentheroudakis 2008 ||EN||n. s.||no||(no)d||GL||24 (4)||SE||no / no / no||n. s.||no / no||no||yes|
|Sanderlin 2007 ||EN||n. s.||no||yes||GL||5 (-)||SE||no / no / no||n. s.||no / no||no||yes|
|Sanders 2000 ||EN||||no||yes||GL||15 (3)||no||yes / no / no||n. s.||yes / yes||no||yes|
|Savoie 2000 ||EN||[113,136]||no||(no)d||GL||51 (15)||no||no / yes / yes||2||no / no||no||yes|
|Shaneyfelt 1999 ||EN||||yes||yes||GL||25 (3)||no||no / yes / no||2||no / no||yes||yes|
|Shiffman 2003 ||EN||[63,131,137]||yes||yes||GL / Rec.||18 (-)||no||no / no / no||n. s.||no / no||no||yes|
|Veale 1999 ||EN||n. s.||no||yes||GL||7 (-)||no||no / no/ no||n. s.||no / no||no||yes|
|Ward 1996 ||EN||||no||yes||GL||18 (8)||no||no / yes / no||n. s.||no / no||no||yes|
|Warriner 2011 ||EN||n. s.||no||yes||GL||11 (9)||SE||no / no/ no||n. s.||no / no||no||yes|
|WHO 2003 ||EN||n. s.||no||yes||GL||25 (8)||no||no / no / no||n. s.||no / no||no||no|
|Woolf 1995 ||EN||n. s.||no||yes||GL||10 (-)||no||no / no/ no||n. s.||no / no||no||yes|
Eleven appraisal tools provided additional information on their development process. The number of questions in the tools ranged from three to 51. 23 tools grouped their questions into domains. The number of domains ranged from two to 21. Eighteen tools contained at least some explanation of their questions.
Twenty tools used no specified scoring system, and twelve used a multiple choice answer, mostly a “yes/no” score, with or without the options ‘not sure’ or ‘not applicable’. Nine tools applied some form of scaling system. Six tools explicitly requested additional comments from guideline appraisers.
Thirteen appraisal tools recommended that guidelines should be appraised independently by at least two reviewers.
The calculation of a quality score for the domains of an appraisal tool and a qualitative or quantitative overall assessment of the guideline were suggested by five and six tools respectively. Only eleven tools had been subject to any sort of validation studies and only six of these [13,60,64-67] had been validated more thoroughly. All but five appraisal tools were published in peer-reviewed journals.
The tools varied considerably in terms of the number of quality dimensions covered. Ten (25%) covered at least twelve quality dimensions with at least one item; eleven (28%) covered only six or fewer quality dimensions.
The appraisal tools also differed in the extent to which each quality dimension was covered. Of the 34 possible items the number covered by each tool varied between three and 29 (Figure 2).
The quality dimensions “evaluation of evidence” (mentioned in 35 tools; 88%) and “information retrieval” (29 tools; 73%) were a main focus of the appraisal tools. However, the tools rarely assessed whether the study results were reported correctly in the guidelines and supported the recommendations (item “consistency” mentioned in six tools; 15%).
Another focus was the quality dimension “transferability” (33 tools; 83%) with the items “costs” (25 tools; 63%) and “barriers and facilitators” (23 tools; 58%). However, the tools rarely assessed whether patients, interventions and settings in the studies underlying the recommendations were comparable to those targeted by the recommendations (item “comparability” mentioned in eight tools; 20%).
Further quality dimensions covered by at least 70% of the appraisal tools were the dimensions “presentation of guideline content” (34 tools; 85%), “independence” (32 tools; 80%), “scope” (30 tools; 75%), “updating” (30 tools; 75%), and “formulation of recommendations” (28 tools; 70%). The item “composition of the guideline development group” in the quality dimension “independence” was covered frequently (32 tools; 80%), whereas few appraisal tools mentioned the item “consideration of (potential) conflicts of interest" related to the guideline development group (eleven tools; 28%).
The following two quality dimensions were covered by 50% or less of the appraisal tools: firstly, “consideration of different perspectives” (20 tools; 50%) with the items “patient perspectives” (thirteen tools; 33%), “norms and values” (nine tools; 23%), and “expert knowledge” (six tools; 15%), and secondly, “dissemination, implementation and evaluation of the guideline” (eighteen tools; 45%) (Figure 3).
A table with the complete content characteristics of the guideline appraisal tools is attached as online File S4.
The aim of this systematic review was to identify and compare existing guideline appraisal tools. We identified 40 different tools. Among those were 24 new tools not included in the systematic reviews by Graham 2000  and Vlayen 2005 , as well as an additional three updated tools.
Most appraisal tools assess whether the literature search, the evaluation and synthesis of the evidence, and the reporting of the evidence in the guidelines are in accordance with the principles of evidence-based medicine. However, the guideline development process comprises more than the systematic compilation of the evidence on a relevant clinical question. Burgers et al 2002 stated that guideline development is a technical as well as social process . The choice and interpretation of the evidence identified and the formulation of recommendations is affected by norms and values of the guideline development group [53,69-74]. Zuiderent-Jerak et al 2012 suggest that guidelines should reflect all knowledge, not just clinical trials . However, few appraisal tools assess whether the formulation of recommendations is supported by a formal consensus process or whether the norms and values of the guideline development group are clearly stated.
Current standards for guideline development [1,42] point out that patients should be full members of the guideline development group. However, many of the appraisal tools fail to capture consumer involvement, i.e. do not assess whether patients’ views were considered in the guideline development group.
Conflicts of interest may influence decisions in the health care system [76,77], also concerning the development of guidelines [36-38], and new and more stringent policies have been called for [42,55,78-80]. It is therefore surprising that only few appraisal tools assess whether conflicts of interest of members of the guideline development group have been recorded and addressed.
Selection of an appraisal tool
Most of the appraisal tools included can be assigned to one of three groups:
- Tools with general questions and with no or only a few appraisal criteria to decide whether the requirements of the questions are fulfilled [61,62,81-96].
- Tools with specific questions or appraisal criteria to decide whether the requirements of the questions are fulfilled [2,13,14,43,65-67,97-106].
- A small group of tools with specific questions and / or appraisal criteria with an additional qualitative appraisal [57,60,64,107,108]).
Differing results of guideline appraisals are more likely in cases where the questions of an appraisal tool are imprecise or specific criteria for answering the questions are lacking. This problem is particularly evident in the tools in the first group. For this reason the appraisal tools in the first group cannot be recommended for regular use.
It is also important to underline that appraisal tools in the first and second group mainly focus on methodological issues surrounding guideline development and reporting. However, they do not evaluate the quality of the clinical content itself [58,109]. For example, guideline appraisal tools in the first and second group assess whether the search strategy was reported in the guidelines, but they do not assess whether the search strategy was developed correctly or whether it was suited to identify evidence to answer the clinical question of the guideline.
While rigorous development and explicit reporting of the guideline development process are necessary, they do not guarantee appropriate recommendations or better health outcomes for patients, as the methodological rigour and quality of the clinical content of a clinical practice guideline are not necessarily correlated [58,110-112].
Only the five tools of the third group are designed to solve this problem, at least to some degree. While their main focus is still the appraisal of methodological aspects of guideline development and reporting, they nevertheless require judgments on whether relevant quality aspects have been adequately implemented. For example, they assess not only whether the search strategy was reported but also require a qualitative statement on whether the strategy was appropriate [57,60,64,107,108], whether the evidence identified was appropriately summarized in the recommendations [60,64,107,108] or whether an appropriate formal process was used to arrive at the recommendations [57,60].
Appraisal tools differ in the number of items and quality dimensions covered. If the aim is to conduct a comprehensive guideline appraisal, the AGREE II tool  or the German-language DELBI tool  may represent the best choice. Both tools cover all thirteen quality dimensions. The AGREE II tool has also been thoroughly evaluated.
However, an appraisal tool containing many quality dimensions may not necessarily represent the best choice in all cases. If the primary goal is to learn more about the applicability of a guideline, the GLIA tool  may be more suitable. This thoroughly evaluated tool appraises aspects that influence the applicability of a guideline. If the goal is to gain more information on the quality of the clinical content of a guideline, the ADAPTE tool  may be more suitable. This tool primarily includes questions that can be assigned to the quality dimensions “information retrieval” and “evaluation of evidence”. It has also been thoroughly evaluated, but demands considerable skill on the part of the guideline appraiser. Moreover, additional information not available in the guideline may be needed to answer the questions in this appraisal tool.
Depending on the problem being addressed, a tool containing only a few, but appropriate questions could be adequate. Furthermore, it may sometimes be advisable to omit some domains or items of an extensive appraisal tool.
Information S4 provides details of the items and quality dimensions covered by the different appraisal tools.
Strengths and weaknesses of the review
Our review provides a comprehensive overview of guideline appraisal tools. It nevertheless has a number of limitations.
A systematic search for appraisal tools is difficult, as there is no appropriate MESH or other term for appraisal tools. Because of the large number of appraisal tools used it is possible that not all appraisal tools were identified. Due to the comprehensive search strategy chosen, which included screening the reference lists of relevant primary and secondary publications, it is nevertheless unlikely that important and commonly used tools were not identified.
The systematic search for appraisal tools was limited to tools published after 1994. In the late 1980s and early 1990s, the development of clinical practice guidelines became more common. With the definition of clinical practice guidelines by Field and Lohr in 1990 , a shared understanding of guidelines and guideline quality emerged that influenced the development of guidelines, as well as the development of appraisal tools. Authors of appraisal tools published before 1995 were probably not able to consider these developments.
We used the questions and statements contained in the appraisal tools, as well as the publications by Cluzeau 1999 , Graham 2000  and Vlayen 2008 , to identify items and quality dimensions. According to this approach, the result of this review is a comparative description of the appraisal tools. There is no “gold standard” for the evaluation of appraisal tools. It is therefore possible that quality dimensions and items exist that were not identified, as they were not part of the publications and appraisal tools analysed, but may nevertheless be relevant for the appraisal of guideline quality. Furthermore, it was not always possible to clearly assign the questions or items of the appraisal tools to only one quality dimension. A further limitation of our review is that no external experts were consulted in the validation of the appraisal framework.
Unanswered Questions and Future Research
The appraisal tools analysed cover several different aspects of guideline quality. All tools allow for the grading of guideline quality. However, it is uncertain whether all items and quality dimensions contribute equally to the quality of a guideline . Further empirical studies are needed to answer the question as to which items and quality dimensions are essential for the assessment of guideline quality; for example, whether the external review of guidelines really improves their quality, whether conflicts of interest really lead to inappropriate recommendations or whether the explicit consideration of patient preferences really improves the patient-centeredness of a guideline.
In 2005 Vlayen stated “that in order to evaluate the quality of the clinical content and more specifically the evidence base of a clinical practice guideline, verification of the completeness and the quality of the literature search and its analysis has to be added to the process of validation by an appraisal instrument” . Some appraisal tools have started to deal with this problem but have not solved it so far.
The appraisal of the quality of the clinical content of guidelines is time-consuming, requires highly qualified personnel and may need additional information not available in the guidelines themselves. For example, an information specialist may be needed for appraisal of the appropriateness of a search strategy, it may be necessary to repeat a literature search to verify the completeness of the search results or the analysis of the literature identified has to be repeated to prove its correctness.
Some working groups have started to deal with the appraisal of the clinical content of a guideline [114,115], but it remains unclear whether the assessment of the evidence base can be included in guideline appraisal tools in their current form. Further research will have to clarify whether and how overall appraisal of the clinical content of a guideline can be included in guideline appraisal tools with a reasonable use of resources.
Appraisal tools differ in the number of items and quality dimensions covered and some tools cover some quality dimensions better than others. The most comprehensively validated appraisal tool is the AGREE II instrument, but the final choice of the appropriate tool depends on the research question. Nevertheless, appraisal tools containing unspecific questions and / or lacking criteria for answering the questions should not be applied. When choosing an appraisal tool it is important to keep in mind that their main focus is the appraisal of methodological aspects of guideline development and not the evaluation of the evidence base underlying a clinical practice guideline; further research should clarify whether and how an overall appraisal of the clinical content of a guideline can be performed.
Although conflicts of interest and norms and values of guideline developers, as well as patient involvement, affect the trustworthiness of guidelines, they are currently insufficiently assessed in guideline appraisal tools. They should thus be considered essential items in the further development of such tools.
Excluded studies (ordered by reasons for exclusion).
We thank Corinna Ernsting for supporting the screening of the search results, Verena Wekemann for checking the format of the citations and Natalie McGauran for medical writing support.
Conceived and designed the experiments: US ME EAN. Performed the experiments: US EH WH. Analyzed the data: US WH. Contributed reagents/materials/analysis tools: ME. Wrote the manuscript: US ME WH EH EAN.
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