20b. How blinding was achieved

What to write

If blinded, how blinding was achieved and description of the similarity of interventions

Examples

“Jamieson Laboratories Inc. provided 500-mg immediate release niacin in a white, oblong, bisect caplet. We independently confirmed caplet content using high performance liquid chromatography . . . The placebo was matched to the study drug for taste, color, and size, and contained microcrystalline cellulose, silicon dioxide, dicalcium phosphate, magnesium stearate, and stearic acid.”1

“Placebo tablets were identical to NAC [N‐acetylcysteine] tablets in color, shape, size, and odor. They were all kept in identical containers and were administered by an investigational drug pharmacist.”2

“The study treatment and placebo tablets and bottles were identical in physical appearance . . . The IWRS [interactive web response system] housed treatment codes and bottle numbers for study treatment. In case of an emergency, the investigator had the sole responsibility for determining if unmasking of a participant’s treatment assignment was warranted to provide appropriate medical care. Participant safety was always the first consideration in making such a determination. The IWRS was programmed with blind-breaking instructions to guide the investigator on how to obtain treatment assignment in the event of an emergency unmasking. The investigator was requested to contact the medical monitor promptly in case of any treatment unmasking. If a participant’s treatment assignment was unmasked, the sponsor was to be notified within 24 h after unmasking. The date and reason for the unmasking were recorded in the source documentation and electronic case report form, as applicable. Investigators broke the masking for four participants: one in ELEVATE UC 12 (on etrasimod) and three in ELEVATE UC 52 (on etrasimod).”3

Explanation

Blinding of participants, healthcare providers, data collectors, and outcome assessors in a trial requires adequate procedures to both achieve and maintain blinding.4,5 Just as we seek evidence of adequate allocation concealment to assure us that assignment was truly random, we seek evidence on the method of blinding.

If researchers contend that the trial investigators, participants, and assessors were blinded, then they should provide information about the mechanism used to establish blinding (eg, placebo identical to the experimental intervention, sham intervention, sham surgery).45 They should describe the similarity of treatment characteristics (eg, route of administration, appearance, smell, taste) and where relevant methods used to mask some characteristics of the treatments (eg, use of special flavours to mask a distinctive taste, opaque coverage to conceal intravenous treatments with different appearances, double-dummy procedures).4,5

Blinding can be difficult to maintain over time because of dosage adaptation over time or the occurrence of specific side effects. Specific procedures to maintain blinding can be implemented (eg, centralised assessment of side effects, centralised adapted dosage, or provision of sham results of complementary investigations).

Even if blinding of participants and healthcare providers is not possible, blinding data collectors and outcome assessors could still be implemented to limit ascertainment bias. This could be achieved, for example, through centralised assessment of complementary investigation (eg, anonymised radiography), physician mediated data (eg, video, photography, audiotape), and clinical events (eg, adjudication of clinical events from extract of the case report form).

Details of how blinding was achieved are important because slight, but discernible, differences between interventions can lead to large problems in bias. Notably, inadequate matching related to discernible differences in colour and taste seem particularly problematic.6 It is important that authors report any known compromises in blinding. For example, authors should report if it was necessary to unblind any participants at any point during the conduct of the trial. Moreover, authors should report the risk of unblinding but, unfortunately, such reporting is rare. In a random sample of 300 publications describing blinded randomised clinical trials indexed in PubMed, only 8% reported on risk of unblinding.7 It is also important to report any procedures, pretrial or concurrent, that are intended to reduce or evaluate risk of compromised blinding.68 Indeed, some pretrial assessments of unblinding may be helpful in reducing the risk of unblinding in the eventual randomised trial. Thus, authors of randomised trial articles should report procedures to avoid, document, and address cases of overt unblinding.6,7

Where appropriate, authors should also describe any procedures used for emergency unblinding (ie, disclosing the assigned intervention of a trial participant for specific reasons such as harms). They should indicate whether they used fixed code to indicate group assignment (eg, A=group 1; B=group 2) or a unique code for each participant. Use of a fixed code will increase the risk of unblinding because unblinding a participant could result in unblinding several or all trial participants.911

Some people have advocated testing for blinding by asking participants or healthcare providers at the end of a trial whether they think the participant received the experimental or control intervention.12 Because participants and healthcare providers will frequently know whether the participant has experienced the primary outcome, this makes it difficult to determine whether their responses reflect failure of blinding or accurate assumptions about the efficacy of the intervention.13 Thus, given the uncertainty this type of information provides, the usefulness of tests of blinding has been questioned.6,7,14 Testing for blinding was not an included item in CONSORT 2010, and still is not in CONSORT 2025. Nevertheless, if investigators decide to conduct tests of blinding, we encourage them to completely report their findings with appropriate limitations.8

Training

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References

1.
Mills E, Prousky J, Raskin G, et al. The safety of over-the-counter niacin. A randomized placebo-controlled trial [ISRCTN18054903]. BMC Clinical Pharmacology. 2003;3(1). doi:10.1186/1472-6904-3-4
2.
Padoei F, Mamsharifi P, Hazegh P, et al. The therapeutic effect of n‐acetylcysteine as an add‐on to methadone maintenance therapy medication in outpatients with substance use disorders: A randomized, double‐blind, placebo‐controlled clinical trial. Brain and Behavior. 2022;13(1). doi:10.1002/brb3.2823
3.
Sandborn WJ, Vermeire S, Peyrin-Biroulet L, et al. Etrasimod as induction and maintenance therapy for ulcerative colitis (ELEVATE): Two randomised, double-blind, placebo-controlled, phase 3 studies. The Lancet. 2023;401(10383):1159-1171. doi:10.1016/s0140-6736(23)00061-2
4.
Boutron I, Estellat C, Guittet L, et al. Methods of blinding in reports of randomized controlled trials assessing pharmacologic treatments: A systematic review. Vallance P, ed. PLoS Medicine. 2006;3(10):e425. doi:10.1371/journal.pmed.0030425
5.
Boutron I, Guittet L, Estellat C, Moher D, Hróbjartsson A, Ravaud P. Reporting methods of blinding in randomized trials assessing nonpharmacological treatments. Ford I, ed. PLoS Medicine. 2007;4(2):e61. doi:10.1371/journal.pmed.0040061
6.
Bello S, Moustgaard H, Hróbjartsson A. Unreported formal assessment of unblinding occurred in 4 of 10 randomized clinical trials, unreported loss of blinding in 1 of 10 trials. Journal of Clinical Epidemiology. 2017;81:42-50. doi:10.1016/j.jclinepi.2016.08.002
7.
Bello S, Moustgaard H, Hróbjartsson A. The risk of unblinding was infrequently and incompletely reported in 300 randomized clinical trial publications. Journal of Clinical Epidemiology. 2014;67(10):1059-1069. doi:10.1016/j.jclinepi.2014.05.007
8.
Hróbjartsson A, Boutron I. Blinding in randomized clinical trials: Imposed impartiality. Clinical Pharmacology & Therapeutics. 2011;90(5):732-736. doi:10.1038/clpt.2011.207
9.
Andrew E. A proposal for structured reporting of randomized controlled trials. JAMA: The Journal of the American Medical Association. 1994;272(24):1926. doi:10.1001/jama.1994.03520240054041
10.
Schulz k grimes DA . Essential concepts in clinical research: Randomized controlled trials and observational epidemiology. 2nd ed. Elsevier, 2019.
11.
Schulz KF, Grimes DA, Altman DG, Hayes RJ. Blinding and exclusions after allocation in randomised controlled trials: Survey of published parallel group trials in obstetrics and gynaecology. BMJ. 1996;312(7033):742-744. doi:10.1136/bmj.312.7033.742
12.
Fergusson D. Turning a blind eye: The success of blinding reported in a random sample of randomised, placebo controlled trials. BMJ. 2004;328(7437):432-430. doi:10.1136/bmj.37952.631667.ee
13.
Sackett DL. Turning a blind eye:why we don’t test for blindness at the end of our trials. BMJ. 2004;328(7448):1136.2. doi:10.1136/bmj.328.7448.1136-a
14.
Schulz KF, Altman DG, Moher D, Fergusson D. CONSORT 2010 changes and testing blindness in RCTs. The Lancet. 2010;375(9721):1144-1146. doi:10.1016/s0140-6736(10)60413-8

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Citation

For attribution, please cite this work as:
Hopewell S, Chan AW, Collins GS, et al. CONSORT 2025 statement: updated guideline for reporting randomised trials. BMJ. 2025;389:e081123. doi:10.1136/bmj-2024-081123

Reporting Guidelines are recommendations to help describe your work clearly

Your research will be used by people from different disciplines and backgrounds for decades to come. Reporting guidelines list the information you should describe so that everyone can understand, replicate, and synthesise your work.

Reporting guidelines do not prescribe how research should be designed or conducted. Rather, they help authors transparently describe what they did, why they did it, and what they found.

Reporting guidelines make writing research easier, and transparent research leads to better patient outcomes.

Easier writing

Following guidance makes writing easier and quicker.

Smoother publishing

Many journals require completed reporting checklists at submission.

Maximum impact

From nobel prizes to null results, articles have more impact when everyone can use them.

Who reads research?

You work will be read by different people, for different reasons, around the world, and for decades to come. Reporting guidelines help you consider all of your potential audiences. For example, your research may be read by researchers from different fields, by clinicians, patients, evidence synthesisers, peer reviewers, or editors. Your readers will need information to understand, to replicate, apply, appraise, synthesise, and use your work.

Cohort studies

A cohort study is an observational study in which a group of people with a particular exposure (e.g. a putative risk factor or protective factor) and a group of people without this exposure are followed over time. The outcomes of the people in the exposed group are compared to the outcomes of the people in the unexposed group to see if the exposure is associated with particular outcomes (e.g. getting cancer or length of life).

Source.

Case-control studies

A case-control study is a research method used in healthcare to investigate potential risk factors for a specific disease. It involves comparing individuals who have been diagnosed with the disease (cases) to those who have not (controls). By analysing the differences between the two groups, researchers can identify factors that may contribute to the development of the disease.

An example would be when researchers conducted a case-control study examining whether exposure to diesel exhaust particles increases the risk of respiratory disease in underground miners. Cases included miners diagnosed with respiratory disease, while controls were miners without respiratory disease. Participants' past occupational exposures to diesel exhaust particles were evaluated to compare exposure rates between cases and controls.

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Cross-sectional studies

A cross-sectional study (also sometimes called a "cross-sectional survey") serves as an observational tool, where researchers capture data from a cohort of participants at a singular point. This approach provides a 'snapshot'— a brief glimpse into the characteristics or outcomes prevalent within a designated population at that precise point in time. The primary aim here is not to track changes or developments over an extended period but to assess and quantify the current situation regarding specific variables or conditions. Such a methodology is instrumental in identifying patterns or correlations among various factors within the population, providing a basis for further, more detailed investigation.

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Systematic reviews

A systematic review is a comprehensive approach designed to identify, evaluate, and synthesise all available evidence relevant to a specific research question. In essence, it collects all possible studies related to a given topic and design, and reviews and analyses their results.

The process involves a highly sensitive search strategy to ensure that as much pertinent information as possible is gathered. Once collected, this evidence is often critically appraised to assess its quality and relevance, ensuring that conclusions drawn are based on robust data. Systematic reviews often involve defining inclusion and exclusion criteria, which help to focus the analysis on the most relevant studies, ultimately synthesising the findings into a coherent narrative or statistical synthesis. Some systematic reviews will include a [meta-analysis]{.defined data-bs-toggle="offcanvas" href="#glossaryItemmeta_analyses" aria-controls="offcanvasExample" role="button"}.

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Systematic review protocols

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Meta analyses of Observational Studies

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Randomised Trials

A randomised controlled trial (RCT) is a trial in which participants are randomly assigned to one of two or more groups: the experimental group or groups receive the intervention or interventions being tested; the comparison group (control group) receive usual care or no treatment or a placebo. The groups are then followed up to see if there are any differences between the results. This helps in assessing the effectiveness of the intervention.

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Randomised Trial Protocols

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Qualitative research

Research that aims to gather and analyse non-numerical (descriptive) data in order to gain an understanding of individuals' social reality, including understanding their attitudes, beliefs, and motivation. This type of research typically involves in-depth interviews, focus groups, or field observations in order to collect data that is rich in detail and context. Qualitative research is often used to explore complex phenomena or to gain insight into people's experiences and perspectives on a particular topic. It is particularly useful when researchers want to understand the meaning that people attach to their experiences or when they want to uncover the underlying reasons for people's behaviour. Qualitative methods include ethnography, grounded theory, discourse analysis, and interpretative phenomenological analysis.

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Case Reports

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Diagnostic Test Accuracy Studies

Diagnostic accuracy studies focus on estimating the ability of the test(s) to correctly identify people with a predefined target condition, or the condition of interest (sensitivity) as well as to clearly identify those without the condition (specificity).

Prediction Models

Prediction model research is used to test the accurarcy of a model or test in estimating an outcome value or risk. Most models estimate the probability of the presence of a particular health condition (diagnostic) or whether a particular outcome will occur in the future (prognostic). Prediction models are used to support clinical decision making, such as whether to refer patients for further testing, monitor disease deterioration or treatment effects, or initiate treatment or lifestyle changes. Examples of well known prediction models include EuroSCORE II for cardiac surgery, the Gail model for breast cancer, the Framingham risk score for cardiovascular disease, IMPACT for traumatic brain injury, and FRAX for osteoporotic and hip fractures.

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Animal Research

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Quality Improvement in Healthcare

Quality improvement research is about finding out how to improve and make changes in the most effective way. It is about systematically and rigourously exploring "what works" to improve quality in healthcare and the best ways to measure and disseminate this to ensure positive change. Most quality improvement effectiveness research is conducted in hospital settings, is focused on multiple quality improvement interventions, and uses process measures as outcomes. There is a great deal of variation in the research designs used to examine quality improvement effectiveness.

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Economic Evaluations in Healthcare

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Meta Analyses

A meta-analysis is a statistical technique that amalgamates data from multiple studies to yield a single estimate of the effect size. This approach enhances precision and offers a more comprehensive understanding by integrating quantitative findings. Central to a meta-analysis is the evaluation of heterogeneity, which examines variations in study outcomes to ensure that differences in populations, interventions, or methodologies do not skew results. Techniques such as meta-regression or subgroup analysis are frequently employed to explore how various factors might influence the outcomes. This method is particularly effective when aiming to quantify the effect size, odds ratio, or risk ratio, providing a clearer numerical estimate that can significantly inform clinical or policy decisions.

How Meta-analyses and Systematic Reviews Work Together

Systematic reviews and meta-analyses function together, each complementing the other to provide a more robust understanding of research evidence. A systematic review meticulously gathers and evaluates all pertinent studies, establishing a solid foundation of qualitative and quantitative data. Within this framework, if the collected data exhibit sufficient homogeneity, a meta-analysis can be performed. This statistical synthesis allows for the integration of quantitative results from individual studies, producing a unified estimate of effect size. Techniques such as meta-regression or subgroup analysis may further refine these findings, elucidating how different variables impact the overall outcome. By combining these methodologies, researchers can achieve both a comprehensive narrative synthesis and a precise quantitative measure, enhancing the reliability and applicability of their conclusions. This integrated approach ensures that the findings are not only well-rounded but also statistically robust, providing greater confidence in the evidence base.

Why Don't All Systematic Reviews Use a Meta-Analysis?

Systematic reviews do not always have meta-analyses, due to variations in the data. For a meta-analysis to be viable, the data from different studies must be sufficiently similar, or homogeneous, in terms of design, population, and interventions. When the data shows significant heterogeneity, meaning there are considerable differences among the studies, combining them could lead to skewed or misleading conclusions. Furthermore, the quality of the included studies is critical; if the studies are of low methodological quality, merging their results could obscure true effects rather than explain them.

Protocol

A plan or set of steps that defines how something will be done. Before carrying out a research study, for example, the research protocol sets out what question is to be answered and how information will be collected and analysed.

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