Developing consolidated guidelines for reporting and evaluation of studies using transcranial electrical stimulation (CoRE-tES): protocol for an international Delphi study and expert consensus process

Abstract

Introduction

In recent decades, transcranial electrical stimulation (tES) has become a widely used non-invasive method for modulating brain function in clinical and non-clinical populations. However, existing tES trials exhibit substantial methodological heterogeneity, often limiting the reproducibility and interpretability of findings. There currently exists a paucity of consensus-driven, standardised recommendations outlining the key factors that should be reported and/or controlled in tES studies. Accordingly, this project aims to develop Consolidated Guidelines for Reporting and Evaluation of studies using tES (CoRE-tES), a tool designed to assess the methodological quality and reporting of laboratory-based and home-based tES studies. These guidelines will support improved quality, consistency, replication and transparency in research involving tES modalities, including transcranial direct current stimulation, transcranial alternating current stimulation and transcranial random noise stimulation.

Methods and analysis

CoRE-tES will be developed and disseminated over five stages. Stage 1 will comprise a review of recent tES literature to assess methodological and reporting quality. Stage 2 will employ a Delphi process to seek agreement among international tES experts on a list of items for inclusion in CoRE-tES. In stage 3, a consensus meeting will be held to synthesise and prioritise the agreed items to form CoRE-tES. Stage 4 will involve production of the final CoRE-tES checklist and an accompanying evaluation and elaboration document. In stage 5, CoRE-tES will be disseminated via journal publication, conferences, professional meetings and social media campaigns.

Ethics and dissemination

Ethics approval has been obtained from the Western Sydney University Human Research Ethics Committee (approval number H16803). Findings will be disseminated through scientific conferences and peer-reviewed journal publications, and CoRE-tES will be indexed on the Enhancing the QUAlity and Transparency Of health Research Network website.

STRENGTHS AND LIMITATIONS OF THIS STUDY.

• A robust, standardised reporting guideline will be developed via a Delphi process and consensus meeting involving international experts in transcranial electrical stimulation.

• The steering committee includes world leaders in transcranial electrical stimulation research, with extensive experience and knowledge across neurophysiology, clinical trial design and research methodology.

• Although there will be broad international representation across panellists, the sample is limited to experts that can communicate in English.

Introduction

Transcranial electrical stimulation (tES) encompasses non-invasive brain stimulation techniques that modulate cortical excitability by delivering low-intensity electric currents to the scalp.¹,³ Common tES techniques include transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS) and transcranial random noise stimulation (tRNS). Over the past two decades, tES has gained considerable traction in both experimental and clinical research settings, with applications spanning neurological, psychiatric, cognitive and musculoskeletal domains.^{4 5} Such tES modalities have demonstrated effectiveness across a broad range of populations, including people with major depressive disorder,⁶ cognitive impairments,⁷ multiple sclerosis,⁸ stroke⁹ and both experimental and chronic pain.¹⁰,¹³ More recently, there has been growing interest in the use of tES in home-based contexts,^{14 15} particularly in response to increasing demand for remote, accessible and scalable interventions.¹⁶,¹⁹

Despite its promise, methodological heterogeneity in tES trials has limited the reproducibility and interpretability of findings.^{20 21} Several systematic and narrative reviews have highlighted variability in the efficacy of tES, which could be attributable in part to the inconsistencies observed in trial conduct and reporting, particularly across stimulation parameters, adverse event monitoring and blinding procedures.²²,²⁷ This problem is amplified in home-based studies, where methodological rigour is further challenged by the need for remote supervision, participant-led setup and digital adherence strategies.²⁸

While position papers and technical guides provide valuable recommendations regarding tES,^{420 25 29},³³ these documents do not represent consensus-driven, standardised tools for assessing methodological and reporting quality. Previous tES studies have typically adapted existing methodological and reporting checklists developed for transcranial magnetic stimulation^{34 35} or concurrent tES and functional MRI.³⁶ This is a notable limitation, as tES has distinct mechanisms of effect, stimulation parameters and safety challenges not reflected in other non-invasive brain stimulation methodologies.³⁷ In addition, previous recommendations do not consider recent developments such as home-based protocols with varied levels of supervision.³⁶ Other commonly used tools like the Consolidated Standards of Reporting Trials (CONSORT)³⁸ and Template for Intervention Description and Replication (TIDieR)³⁹ scales, while useful, are also not tailored to the unique methodological considerations inherent to tES trials. These gaps underscore the need for a dedicated, consensus-based methodological quality guideline that can be used to appraise both traditional laboratory-based tES trials and those employing remote, home-based delivery models.

Thus, the primary objective of this project is to develop Consolidated Guidelines for Reporting and Evaluation of studies using tES (CoRE-tES). This tool will identify and define key domains relevant to study design, execution and reporting; establish consensus on critical features through a structured expert Delphi process and provide a practical framework for the critical appraisal of tES research.

Methods and analysis

Scope

CoRE-tES is intended to assess both the methodological and reporting quality of studies using tES as an intervention in human participants. The work applies to randomised controlled trials, feasibility and pilot trials and both crossover and parallel-group designs. The CoRE-tES checklist will be relevant to interventions delivered in either laboratory-based or home-based (remotely monitored) formats and to both clinical (eg, chronic pain, depression, stroke) and non-clinical (eg, cognitive enhancement, experimental pain) populations. CoRE-tES is designed to complement, rather than replace, established risk of bias assessments and existing reporting checklists such as CONSORT or TIDieR, by integrating tES-specific considerations across methodological and reporting dimensions. CoRE-tES has been listed in the Enhancing the QUAlity and Transparency Of health Research (EQUATOR) Network registry for reporting guidelines under development.⁴⁰

Design

CoRE-tES will be developed in accordance with the EQUATOR Network recommendations for developers of health research reporting guidelines⁴¹ across five overlapping stages (figure 1). Stage 1 and 2 are estimated to be completed by January 2026, prior to a consensus meeting scheduled in April 2026. The guideline will be disseminated in late 2026.

Figure 1. Development process for CoRE-tES. CoRE-tES, Consolidated Guidelines for Reporting and Evaluation of studies using transcranial Electrical Stimulation.

Patient and public involvement

Patients and/or the public were not involved in the design of this research.

Steering committee

The steering committee comprises the project leads (RC and AS) and CoRE-tES advisory members (SS, APL, MN, UZ, MB, JPL, AB, SB, LC and LR), selected for their expertise in non-invasive brain stimulation. Collectively, the committee provides extensive knowledge and experience across tES methods, neurophysiology, clinical trial design and research methodology. The committee was established to identify issues with current tES reporting, collate a preliminary list of items and select participants for inclusion in the Delphi study. The CoRE-tES steering committee was responsible for developing the project protocol and will oversee the implementation of all phases of the guideline development.

Stage 1: Systematic review and generation of preliminary checklist items

The aim of this stage is to review the methodological and reporting quality of current tES literature, providing insight into factors that are commonly controlled and/or reported in tES studies. This will highlight areas of heterogeneity or inconsistency. This systematic review will be conducted and written in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analyses⁴² and A Measurement Tool to Assess Systematic Reviews⁴³ statements.

Search and selection of articles

A systematic search of PubMed, Embase, Medline, Web of Science and CINAHL will be conducted to identify studies published in the previous 5 years that have used tES. A 5-year timeframe has been selected to focus on recent reporting practices. The specific search strategy will include the following terms: “transcranial electric” OR “transcranial electrical” OR “tES” OR “transcranial direct current” OR “tDCS” OR “transcranial alternating current” OR “tACS” OR “transcranial random noise” OR “tRNS” OR “non-invasive electrical stimulation”. Health condition, journal or outcome(s) will not be used to limit the search. The search may also be supplemented by additional articles identified by the steering committee, drawing on their subject matter expertise and previous work in the field (via manual searching).

Randomised and non-randomised controlled trials involving the use of any form of tES in human participants will be eligible for inclusion. Studies will be excluded if they do not involve tES, are review articles, are observational studies, are not published in English, or if the full text is unavailable.

All retrieved studies will be exported to Covidence citation management software (Veritas Health Innovation, VIC, Australia). Following automatic duplicate removal, a random sample of up to 50 studies will be selected. This sample is based on previous reporting and methodological reviews and is a common approach for identifying a representative sample to evaluate study design and reporting practices.^{44 45} Two authors (AS and RC) will independently screen articles by title and abstract until 50 studies have been included. This will be followed by full-text screening by the same authors for adherence to the eligibility criteria. Any conflicts regarding the eligibility of articles will be resolved through discussion with the steering committee.

Data extraction

All data will be extracted using a bespoke data extraction template in Microsoft Excel. Two authors (AS and RC) will pilot the data extraction template using the first five studies. Once the data extraction template is finalised, the same two authors will extract data from 10% of the included studies to verify inter-rater agreement, with discrepancies resolved via discussion. One author (AS) will then independently extract all remaining data. Data extraction will include factors related to study characteristics (eg, publication, design and sample), tES parameters (eg, electrode type/material, electrode size, electrode montage/location, current intensity, stimulation duration) and information relating to reporting quality (eg, randomisation method, allocation concealment, blinding, sample size calculation, trial registration, statistical methods). Existing methodological and reporting guidelines specific to non-invasive brain stimulation (transcranial magnetic stimulation,³⁵ tES combined with functional MRI³⁶) and established checklists for intervention reporting (CONSORT statement,³⁸ TIDieR checklist³⁹) will guide item extraction. The EQUATOR Network library of reporting guidelines will be screened to identify any other pertinent items relating to non-invasive brain stimulation. Additional fields may also be included in the data extraction template (prior to piloting) based on the expertise of steering committee members in relation to evaluation of tES studies. A provisional list of items to be extracted is presented in online supplemental file 1.

Data synthesis

Descriptive statistics will be used to summarise the extracted data. Categorical variables and conformity with existing methodological and reporting guidelines^{35 36 38 39} will be presented as frequencies and percentages. Continuous variables will be summarised using the mean and SD for normally distributed data, while those with a non-normal distribution will be summarised using the median and IQR. Summary tables will be constructed to facilitate comparison across studies and to identify common gaps in reporting and methodological rigour. All analyses will be conducted using Microsoft Excel, with results presented in both tabular and narrative formats to ensure clarity and transparency.

Selection of preliminary Delphi items

The systematic review findings will provide insight into key factors that impact methodological and reporting quality across recent tES literature, which will inform the preliminary list of potential items for inclusion in CoRE-tES. In addition, contributions from the steering committee will be collected via a short questionnaire asking each member to list methodological and reporting issues they consider likely to influence the safety and efficacy of tES. The project leads (AS and RC) will also develop a description of each potential item.

A complete list of extracted items and item descriptions will be circulated to the steering committee members, who will be given a minimum of 2 weeks to review the list and provide preliminary feedback. A virtual meeting will then be held to discuss duplicate, overlapping or poorly worded items and descriptions, suggest revisions and categorise items in relevant subthemes. Based on these discussions, the project leads (AS and RC) will consolidate the initial guideline items and format them into the decided subthemes, in alignment with reporting guidelines for other non-invasive brain stimulation technologies.^{35 36} The resulting list of items will form the basis of the checklist to be refined via the Delphi study (stage 2).

Stage 2: Delphi study

The Delphi study aims to achieve expert consensus on a set of items to support the critical appraisal and systematic reporting of studies using laboratory-based or home-based tES. This process involves consulting a panel of experts to: (1) evaluate a preliminary list of reporting items, (2) suggest additional items, (3) refine the checklist and (4) identify those considered most essential for inclusion—informing the subsequent consensus meeting.

Ethics

Ethics approval has been obtained from the Western Sydney University Human Research Ethics Committee (approval number H16803).

Design

The Delphi technique is a widely accepted method to achieve consensus on a given question or topic among experts within a field.⁴⁶,⁴⁸ This study will use an e-Delphi approach, involving a series of structured online questionnaires (or ‘rounds’) in which participants (panellists) independently and anonymously rate proposed items and may suggest additional ones. After each round, analytical summary feedback will be provided to each panellist to encourage reconsideration of responses in subsequent rounds. This process will be repeated for three rounds or until consensus is reached.^{47 49}

Development of the Delphi survey

A preliminary list of items will be collated as described in stage 1. Following agreement on the initial survey contents and format by the steering committee, the online survey will be pilot tested by four respondents prior to study commencement. Pilot members will be selected by the steering committee to assess clarity of the survey instructions and phrasing of each item, as well as to ensure appropriate functioning of the electronic survey links.⁴⁶ Following pilot testing and any necessary revisions, the survey will be disseminated to a full ‘panel’ of expert participants.

Sample and recruitment

Panellists will be comprised of experts in the field of tES, identified primarily through the systematic review (stage 1). The first and last author of each study included in the systematic review will be invited to participate in the Delphi study. Further panellists may be identified through nomination by members of the steering committee and consultation with experts, and names will be cross-checked against major research databases (Web of Science, PubMed and CINAHL) to ensure each nominee has published at least one peer-reviewed article employing tES. Recruitment will be iterative, with the final list of potential panellists decided through consensus among the CoRE-tES steering committee. Sample sizes in Delphi surveys are highly dependent on the field of study and homogeneity of the sample. In health-related studies that require particular knowledge of a condition or intervention, 8–15 panellists are commonly reported,⁵⁰ but recent evidence suggests that 20–30 panellists may improve replicability.⁵¹ Thus, a sample of 30–40 panellists will be recruited, allowing for 30% attrition. Members of the panel will be recognised as contributors in the acknowledgements section of the guideline (with their permission) but participation in and of itself will not qualify a panellist for authorship.

Procedure

The Delphi study will be coordinated by an independent data collection company (Clinvivo, Kent, UK, https://www.clinvivo.com) to maintain blinding and reduce bias.⁵² Each iteration of the Delphi survey will be distributed to panellists electronically and each round will be open for 3 weeks. Panellists who have not completed the survey will be sent reminder emails every 7 days until they complete the round or until the round closes. The importance of completing all rounds will be emphasised, and only panellists who completed the previous round will be invited to participate in subsequent rounds.⁵³ All responses will be deidentified by Clinvivo to maintain anonymity between each panellist and the CoRE-tES steering committee.

Round 1

Round 1 of the survey will include a section explaining the purpose of the project and a panellist demographic questionnaire. Panellists will then be requested to indicate their opinion on the importance of controlling each item when conducting a tES study, where control indicates that a factor is regulated by methodological consistency (eg, using the same equipment), research design (eg, randomisation) or statistical analysis (eg, as a covariate factor).³⁵ Each item will have a corresponding 9-point Likert scale (1, ‘not important’, to 9, ‘critically important’) in ascending order.^{44 54} Each item will also have an additional Likert scale regarding the importance of reporting the item adequately and clearly in journal publications (1, ‘never report’, to 9, ‘always report’). For each subgroup of items, panellists will be given the opportunity to provide open-text feedback, including suggestions for wording revisions and recommendations for relevant literature to support the inclusion or exclusion of items.^{55 56} Panellists will also be invited to propose additional items for consideration in future rounds of the study.

Round 2

A second-round questionnaire will be sent to all panellists who complete the first round, including newly nominated items and adjustments to the wording of items from round 1. A summary of round 1 group responses for each item will be attached (mean scores and their SD, median scores and interpercentile ranges (IPRs) (30th and 70th), histograms and ‘Research And Development/University of California Los Angeles (RAND/UCLA) labels of importance and agreement level’ (see analysis below)), alongside the panellist’s own score.^{44 54} All items that reach consensus for provisional inclusion and exclusion will be highlighted, as well as items for which wording was modified. Panellists will be asked to re-score the importance of controlling and reporting each item, with consideration of the aggregated panel results. Criteria for cut-off scores for inclusion (median score ≥7) and exclusion (median score ≤3) of items in the guideline will be given.

Round 3

Panellists who complete the second round will be sent a summary of Round 2 group responses (mean scores and their SD, median scores and IPRs (30th and 70th), histograms and RAND/UCLA labels of importance and agreement level), together with the panellist’s own score for each item. All items that reach consensus for inclusion or exclusion will be highlighted, and panellists will not be asked to re-score them. For each of the remaining items (median score 4–6 or where disagreement exists), panellists will be asked to rate as ‘include’ or ‘exclude’ for consideration in the guideline.^{44 56 57} If all items reach consensus after round 2, then Round 3 will not proceed.

Analysis

Demographic information will be reported descriptively. Thematic analysis will be used to identify common themes and issues raised in the qualitative open-text comments from round 1, which will inform adjustments to the wording of items and the addition of new items for consideration in the subsequent round.

Responses will be analysed after each Delphi round using a modified version of the RAND/UCLA appropriateness method⁵⁴ where ‘importance’ is rated rather than ‘appropriateness’, as described by Cashin et al.⁴⁴ This method provides an index of importance and agreement through consideration of the median panel score and dispersion of each panel score.⁵⁴ Thus, the median score, IPR (30th and 70th) and the IPR adjusted for symmetry (IPRAS) will be calculated for each item. For a given item, IPR >IPRAS will indicate the presence of disagreement.⁵⁴ Consensus on items to be included in the guideline, based on responses from round 2, will be determined according to the RAND/UCLA definitions⁵⁴:

• Include: panel median of 7–9, without disagreement.

• Uncertain: panel median of 4–6, or any median with disagreement.

• Exclude: panel median of 1–3, without disagreement.

For the analysis of round 3 responses, consensus regarding the inclusion of items in the guideline will be categorised as follows:

• Include: ≥70% of panel members indicated the item should be included.

• Exclude: ≥70% of panel members indicated the item should be excluded.

Any item receiving 50–69% of votes for inclusion or exclusion in round 3 will be considered ‘borderline’. These items may be designated as optional components of the CoRE-tES statement or included as discussion points in the accompanying elaboration document, based on deliberations during the consensus meeting.⁵⁸

Missing data will be managed by conducting analyses on a per-round basis. Analyses will include participants who complete that round, and item-level analyses will be based on all available responses. Retention and response rates will be reported for each round, and descriptive comparisons will be made between participants who complete all rounds and those who withdraw to explore potential attrition bias. No data imputation or corrections for missing data are required for the Delphi format.

All data will be entered in Microsoft Excel and the Statistical Package for the Social Sciences (V.23; IBM Corp, Armonk, New York, USA).

Outcome of Delphi study

The Delphi study will produce a list of items on which consensus has been reached for consideration at the consensus meeting (stage 3).

Stage 3: Consensus meeting

Although the Delphi process will be used to establish consensus on individual items for inclusion in the CoRE-tES statement, an online consensus meeting with the steering committee and select panellists will be convened to refine and validate the outcomes of the Delphi. The purpose of this meeting is to ensure that the final guideline is conceptually coherent, clearly articulated and practically applicable. The meeting will also involve the development of accompanying explanations for each item and the elaboration document and will follow the methods outlined for developers of health research reporting guidelines.⁴¹ The consensus meeting will include approximately 15 participants, comprising members of the steering committee and a purposive sample of additional attendees to ensure diverse expertise and perspectives. In anticipation of a small proportion being unavailable to attend, up to 20 individuals may be invited. Meeting attendees will be selected to ensure that the collective expertise encompasses a broad range of relevant sub-specialties, including pain researchers, neuroscientists, psychiatrists, implementation scientists, and basic scientists. This meeting size is consistent with recommendations from established consensus development frameworks,⁴¹ which suggest that smaller, multidisciplinary groups facilitate more effective discussion and decision-making while maintaining balanced representation across stakeholder expertise.

Procedure

Meeting attendees will participate in a 1-day, virtual consensus meeting. Prior to the meeting, panellists will receive a summary of findings from the Delphi process. The meeting will include presentations on the evidence regarding the reporting quality of tES studies and the results from the Delphi study. A structured discussion, facilitated by a member of the CoRE-tES steering committee, will be held to: (1) review the Delphi results and confirm that the degree of agreement for each item reflects true consensus; (2) discuss ‘borderline’ items and items for which free-text comments indicate uncertainty or inconsistent interpretation and (3) refine the grouping and sequencing of items to improve clarity, eliminate redundancy and enhance usability. Meeting attendees will be invited to provide input throughout each stage of the discussion, and in cases of disagreement, a decision will be reached via anonymous voting. The meeting will conclude with a discussion on the content and development of related documents (eg, the CoRE-tES statement and accompanying explanation and elaboration (E&E) document), as well as strategies for dissemination and implementation. A written summary of the meeting outcomes will be circulated to meeting attendees for feedback and approval following the meeting.

Stage 4: Development of the draft statement/ checklist and E&E document

The E&E document will provide the background, rationale and justification for each item in the final guideline, including an example of adequate control and/or clear reporting (as relevant to each item).⁴¹ This aims to assist researchers and authors to meet the CoRE-tES standards by highlighting the importance of each item and relevant reporting issues.

A draft guideline and accompanying E&E document will be generated and circulated to the expert consensus meeting attendees to ensure clear, accurate and appropriate wording and content. The writing group will then prepare a near-final draft and a versioned change log describing all meeting-driven edits. These will be sent to all Delphi panellists for their information, with a 10–14 day window to submit brief comments limited to factual errors or clear inconsistencies with earlier voting. The steering committee will review and act on feedback. Where substantially similar concerns are raised by multiple panellists, the affected item(s) will be taken to a short, targeted discussion by the steering committee. Isolated or idiosyncratic comments will be handled editorially.

Stage 5: Guideline dissemination

The final stage aims to maximise the reach, availability and utilisation of CoRE-tES. The steering committee will develop a dissemination strategy, informed by discussions in the consensus meeting. This will include publication in one or more high-reach journals, a social media campaign and presentations at relevant brain stimulation conferences and professional meetings. In addition, CoRE-tES and the accompanying E&E document will be indexed on the EQUATOR Network website and Penelope.ai equator-wizard⁵⁹ and made accessible on an open CoRE-tES web domain. Finally, the steering committee will disseminate the CoRE-tES guideline and associated resources to laboratories and organisations that routinely use tES, including all Delphi panellists.

Publication plan

• Publication 1: study protocol.

• Publication 2: systematic review.

• Publication 3: Delphi study.

• Publication 4 and 5: simultaneous publications of CoRE-tES and the E&E paper.

Discussion

There is growing interest in tES as an intervention for a broad range of applications in clinical and non-clinical populations.⁶⁹,^{11 60} However, there is inconsistent methodological quality and incomplete reporting across existing tES studies, particularly when used as a home-based intervention.²²,²⁶²⁸ While a checklist has been developed for transcranial magnetic stimulation,³⁵ there is no corresponding guideline regarding factors that should be controlled and/or reported in tES studies, despite their distinct mechanisms, application and study design considerations.³⁷

The CoRE-tES project will produce a guideline for studies involving tES, with a subsection pertaining to home-based tES protocols, outlining key items that should be controlled and reported. To maximise the utility and adoption of the guideline, its development will follow rigorous, comprehensive and widely endorsed methodological approaches.⁴¹ A structured dissemination strategy will be employed to facilitate effective implementation and uptake of CoRE-tES. To support accessibility and practical use, the guideline will be made openly available alongside a range of supplementary resources.

Ethics and dissemination

Ethics approval was obtained from the Western Sydney University Human Research Ethics Committee (approval number: H16803). The dissemination strategy is outlined in Stage 5 - Guideline Dissemination. Briefly, the guideline will be disseminated via scientific journal publication, social media, presentation at relevant conferences and professional meetings and direct dissemination to laboratories and organisations that utilise tES.