Back Translation in Clinical Trials: WHO Guidelines and ICH E6 Requirements Explained

Key Takeaways

• Back translation is a mandatory step in the linguistic validation of patient-facing clinical trial documents, including informed consent forms, patient information sheets, patient diaries, and patient-reported outcome instruments.
• WHO guidelines on translation and adaptation specify that back translation must be performed by an independent translator who is a native speaker of the source language and has had no access to the original document.
• ICH E6(R2) Good Clinical Practice reinforces the importance of translation accuracy for all patient-facing trial materials, with regulatory authorities scrutinising informed consent documentation in particular.
• Back translation alone is not sufficient to validate a clinical translation. The process must include a formal comparison of the original and back-translated versions, documented reconciliation of any discrepancies, and review by a clinical or subject matter expert.
• Sponsors and CROs who do not follow a documented back translation workflow risk rejection of site activation packs, ethics committee queries, and potential data integrity issues if patient-facing materials are later found to contain translation errors.

Clinical trial sponsors and CROs working across multiple countries face a consistent challenge: every patient-facing document must be available in the local language of each trial site, and every translation must be demonstrably accurate. A consent form that a patient misunderstands, a patient diary with an ambiguous symptom descriptor, or a quality of life questionnaire that changes meaning in translation are not just quality concerns. They are regulatory risks that can affect trial integrity, ethics committee approval, and patient safety.

Back translation is the primary method the industry uses to verify that a clinical translation accurately reflects the original document. Understanding what the guidelines actually require, and what a compliant back translation workflow looks like, is essential for sponsors, CROs, and translation agencies working in this space.

What the WHO guidelines say

The WHO's guidelines on translation and adaptation of instruments set out the standard approach to back translation for clinical and research contexts. The core requirements are:

Independence of the back translator. The back translation must be performed by a translator who has not seen the original source document and was not involved in the forward translation. This independence is what makes the comparison meaningful. A translator who has read the original, even once, cannot approach the back translation without being influenced by it.

Native speaker of the source language. The back translator must be a native speaker of the source language (typically English for international trials), not the target language. Their task is to render the target language text naturally into their native tongue, which produces the most reliable test of whether the forward translation has captured the original meaning.

Structured comparison. Once both versions are available, the original and the back translation are compared systematically, section by section. The comparison is not a search for word-for-word identity. Languages differ in structure, idiom, and conceptual organisation, so some divergence is expected and normal. The comparison looks for substantive differences in meaning, intent, or emphasis.

Documented reconciliation. Any discrepancies identified must be formally assessed and resolved. Where a discrepancy indicates a genuine error in the forward translation, the forward translation is corrected and the relevant passage back-translated again. The reconciliation process and its outcomes must be documented as part of the trial master file.

ICH E6(R2) and translation accuracy

ICH E6(R2), the Good Clinical Practice guideline that governs the conduct of clinical trials in the EU, UK, US, Japan, and most major regulatory markets, does not prescribe a specific back translation methodology. However, it is unambiguous about the importance of translation accuracy for patient-facing documents.

Informed consent documentation receives particular scrutiny. Regulatory authorities reviewing trial submissions expect evidence that consent forms were translated by qualified translators, that the translation accurately reflects the original, and that any patient in the trial was able to give genuinely informed consent in their own language. A poorly translated consent form is one of the most common triggers for ethics committee queries and regulatory inspection findings.

Beyond informed consent, ICH E6(R2) requires that all information provided to trial subjects, including patient information sheets, diary cards, and questionnaire instruments, is presented in a language the subject can understand. Where trial data relies on patient-reported outcomes, the accuracy of the questionnaire translation directly affects the validity of the data collected.

What a compliant back translation package contains

A properly documented back translation package for clinical trial use typically includes:

• The original source document
• The certified forward translation
• The back translation, with translator credentials and a declaration of independence from the forward translation
• A structured comparison table or report identifying all discrepancies between the original and the back translation
• A reconciliation commentary explaining how each discrepancy was assessed and resolved
• Revised forward translation where corrections were made following reconciliation
• Translator CVs or qualification summaries for both the forward and back translators

This documentation is retained in the trial master file and may be reviewed by ethics committees, regulatory authorities, or during GCP inspections.

Common mistakes in clinical back translation

Using the same translator for forward and back translation. Even when this is done in good faith to save time or cost, it invalidates the independence principle and produces a back translation that reflects what the translator meant to write rather than what they actually wrote.

Treating identical wording as the only acceptable outcome. Some sponsors reject a back translation because it does not reproduce the original word for word. Back translations that differ in phrasing while preserving meaning accurately are not necessarily evidence of an error. The comparison should assess meaning equivalence, not linguistic identity.

Skipping reconciliation documentation. Identifying discrepancies and then correcting the forward translation without documenting the reconciliation process leaves a gap in the trial master file. The record of what was found, assessed, and resolved is as important as the correction itself.

Using machine translation for either stage. Machine translation is not appropriate for clinical trial documents at either the forward or back translation stage. The error patterns produced by MT engines in complex medical and regulatory text are not reliably detectable through back translation alone.

Get clinical back translation support

Global LTS provides ISO 17100:2015 certified back translation for clinical trial documentation across 120+ languages. Our clinical translation team has direct experience with WHO and ICH E6 back translation requirements and produces fully documented back translation packages suitable for ethics committee and regulatory submission.

To discuss your clinical trial translation requirements, visit our back translation services page or our medical translation services page.