The Delphi Definition of Dyslexia (2025): What It Means for Dyslexia Diagnosis and Assessment in the UK

If you are considering a dyslexia assessment in the UK, you may have heard professionals referring to the Delphi Definition of Dyslexia (2025).

This updated dyslexia definition UK guidance now shapes how dyslexia is identified, diagnosed and supported by specialist assessors across the country.

But what does the new dyslexia definition 2025 actually say? And what does it mean in practical terms for families seeking clarity about reading and spelling difficulties?

As a specialist dyslexia assessor with over 30 years’ experience, I use the Delphi Definition (2025) as the framework for diagnostic decision-making in line with current professional standards and SASC guidance.

This article explains the definition clearly and outlines what it means for dyslexia diagnosis, specialist assessment and structured support.

What Is the Delphi Definition of Dyslexia?

The Delphi Definition (2025) describes dyslexia as:

A set of processing difficulties that affect the acquisition of reading and spelling.

The emphasis is on underlying cognitive processing differences, not simply low attainment.

The definition highlights:

• Phonological processing and dyslexia

• Phonological memory difficulties

• Phonological awareness

• Speed of processing differences

It also makes clear that dyslexia:

• Is persistent

• Occurs despite appropriate teaching

• Cannot be explained by intellectual ability, sensory impairment or lack of opportunity

Developed through structured expert consultation, the Delphi method brought together leading researchers and practitioners to reach professional consensus. This provides greater clarity around what is the dyslexia definition used in the UK today.

Why a New Dyslexia Definition Was Needed

For many years, definitions of dyslexia varied. Some relied on discrepancy models comparing IQ and attainment. Others focused solely on literacy scores.

This led to confusion in:

• Dyslexia diagnosis UK practice

• Thresholds for specialist dyslexia assessment

• Professional reporting standards

The new dyslexia definition 2025 moves away from outdated discrepancy thinking. Instead, it focuses on:

• Dyslexia cognitive processing profile

• Persistent literacy difficulties

• The relationship between phonological processing and reading development

For families, this means a diagnosis is based on a consistent framework rather than individual interpretation.

Phonological Processing and Dyslexia

Research consistently shows that phonological processing difficulties sit at the core of many dyslexia profiles.

Phonological processing refers to how the brain recognises, stores and manipulates the sounds within words.

Children and adults with dyslexia may find it difficult to:

• Break words into sounds

• Blend sounds into words

• Hold verbal information in short-term memory

• Retrieve sounds quickly and automatically

These differences affect the development of accurate and fluent word reading.

Difficulties in this area often explain:

• Dyslexia reading fluency difficulties

• Slow or effortful decoding

• Inconsistent spelling patterns

This is why phonological assessment forms a central part of a specialist dyslexia assessment process.

Reading and Spelling Under the Delphi Framework

The Delphi Definition makes clear that dyslexia affects:

• Single word reading accuracy

• Speeded decoding

• Reading fluency

• Spelling development

Reading comprehension may also be affected where slow decoding places pressure on working memory.

However, dyslexia is not defined by poor comprehension alone. It is rooted in word-level processing.

This distinction is important when differentiating dyslexia vs reading difficulty more generally.

Persistence and the Dyslexia Assessment Process

A key element of the Delphi Definition is persistence.

Dyslexic difficulties:

• Do not simply resolve with maturity

• Are not caused by lack of effort

• Remain evident despite appropriate teaching

When parents ask, “Does my child have dyslexia?” this question of persistence is central.

A specialist dyslexia assessment UK should examine:

• Phonological awareness

• Phonological memory

• Rapid automatised naming

• Working memory

• Single word reading

• Reading fluency

• Comprehension

• Spelling

• Cognitive strengths and reasoning

Assessment also considers:

• Developmental history

• Educational background

• Family history of specific learning difficulty (SpLD)

• Observations during testing

Diagnostic decisions are based on convergence of evidence, not a single score.

Information about what is involved in a specialist dyslexia assessment in the UK can be found on the assessment services page.

Strengths Within a Dyslexia Cognitive Processing Profile

The Delphi Definition does not describe dyslexia as low intelligence.

Many individuals demonstrate:

• Strong verbal reasoning

• Good conceptual understanding

• Creative thinking

• Practical problem-solving

Identifying these strengths is essential within a balanced dyslexia diagnosis.

A clear understanding of both difficulty and strength allows schools to plan appropriate structured literacy intervention and classroom adjustments.

What the Delphi Definition Means for Support

Once a dyslexia diagnosis has been made, support typically focuses on:

• Structured, cumulative literacy teaching

• Phonological skill development

• Building reading fluency

• Supporting spelling through explicit instruction

Ongoing online dyslexia tuition and specialist support may be appropriate where persistent literacy difficulties remain despite school intervention.

Support should reflect the individual’s cognitive processing profile rather than a generic literacy programme.

Considering a Dyslexia Assessment in the UK?

Parents often seek a specialist dyslexia assessor when they notice:

• Slow reading development

• Dyslexia reading fluency difficulties

• Inconsistent spelling

• Family history of dyslexia

• Continued literacy struggles despite support

The Delphi Definition (2025) provides a clear and research-based framework for understanding these patterns.

If you are considering a dyslexia assessment, seeking advice from a qualified specialist assessor is the appropriate next step. We have a number of qualified APC assessors available in Bromley, Orpington and Croydon plus we offer remote assessments. Call Sarah for advice on 07501 459284.  

Final Thoughts

The Delphi Definition of Dyslexia (2025) brings clarity to the question: What is the dyslexia definition used in the  UK now?

By focusing on phonological processing, automaticity and persistent literacy impact, it strengthens the consistency of dyslexia diagnosis across the UK.

For families, it provides understanding.For schools, it provides structure.For assessors, it provides a clear professional framework.

Most importantly, it shifts the conversation from “Is this really dyslexia?” to “What does this learner need in order to make progress?”