Do we really need a WISC to diagnose dyslexia?
This is a question that keeps coming up among speech pathologists, psychologists and families who have been told they need to see a psychologist for a "proper diagnosis." The assumption is usually that a cognitive assessment, specifically a WISC, is required to diagnose dyslexia. It is a reasonable assumption, but it is not one that is fully supported by the research, and it is worth understanding why.
Yes, dyslexia involves cognition
Dyslexia is a neurodevelopmental condition and there are underlying cognitive processes involved in learning to read. That part is not debated by anyone working in this space. Where things get blurred is the assumption that cognitive assessment means IQ testing. They are not the same thing, and this is a distinction that matters enormously for the children and families waiting on answers.
What cognitive skills actually matter for dyslexia?
Research consistently shows that dyslexia is linked to difficulties in phonological processing, rapid automatised naming, orthographic processing, and working memory. These are cognitive skills, and more specifically they are cognitive-linguistic processes, the mental systems that allow us to process, store and retrieve language. They reflect how the brain handles sounds, symbols, memory and speed of processing, and they are far more directly related to reading than general intelligence. Dyslexia arises from these specific cognitive-linguistic processes, not from general intelligence. These are also processes that sit comfortably within the training and expertise of speech pathologists, which is precisely why collaboration between professions leads to a more complete picture of the child.
What does a WISC actually measure?
A WISC provides information about general reasoning, verbal comprehension, visual-spatial skills, working memory and processing speed. That information can be valuable in many contexts, and psychologists are expertly placed to gather and interpret it. But a WISC is not designed to assess the core cognitive-linguistic processes underlying dyslexia. The research is clear that IQ tests alone lack the sensitivity required for accurate dyslexia diagnosis, and that many of the skills evaluated by intelligence tests are not as important for success in beginning reading as phonological skills are. (Mather N, Schneider D., 2023) Phonological processing is not even measured on many commonly used intelligence tests. So while a WISC can contribute useful information to a broader profile, it does not assess the key underlying processes that define dyslexia.
What is the role of cognitive assessment then?
Cognitive assessment still matters, but it needs to be targeted. The goal is to understand the child's profile, identify the underlying cause of their reading difficulty and guide intervention, not to complete a full cognitive battery for the sake of it. Research supports a shift away from broad, global testing toward assessment that is directly relevant to reading. Instead of asking "what is this child's IQ?", we should be asking "how is this child processing language and print?" This is a question that speech pathologists and psychologists are both well positioned to contribute to, each bringing different but complementary tools and perspectives.
What should assessment focus on?
Assessment should focus on the cognitive-linguistic processes that underpin reading: phonological processing, rapid naming, orthographic processing and working memory. These are cognitive skills directly linked to reading development and central to understanding dyslexia. A targeted, reading-relevant cognitive assessment is more appropriate than relying on broad measures of general intelligence. In complex presentations, where ADHD, developmental language disorder, anxiety or other conditions may also be present, a psychologist's broader assessment expertise becomes especially important. The strongest assessments bring both professions to the table.
Where does the Tests of Dyslexia (TOD-C) fit?
The TOD-C assesses phonological processing, orthographic processing, decoding and word recognition, and reading-related memory and processing. These align closely with the cognitive-linguistic processes identified in the research as central to dyslexia. So while it may not look like a traditional cognitive assessment, it is directly assessing the processes that matter most. It should be used alongside a thorough case history, parent and teacher report and clinical observation, and in complex cases alongside a psychologist's broader assessment, to build a complete and defensible clinical picture.
Is a WISC ever needed?
Yes, there are situations where a WISC is genuinely valuable and where collaboration with a psychologist is not just appropriate but essential. When there are concerns about broader intellectual functioning, when identifying twice-exceptional students, or when the profile is complex with attention, executive functioning or multiple learning areas involved, a psychologist's expertise is exactly what is needed. The point is not that a WISC is never useful. The point is that it is not required for every dyslexia diagnosis, and that insisting it is can delay identification and support for children who need help now.
This matters more than it might seem. Research has identified what is described as the "dyslexia paradox": a child is unlikely to demonstrate a discrepancy of sufficient magnitude for a WISC-based diagnosis in the first two years of school, which is precisely when identification and intervention would be most beneficial. In other words, requiring a cognitive assessment as the gateway to diagnosis can inadvertently cause children to miss the window when early support would make the greatest difference. (Mather N, Schneider D., 2023) The research also tells us that by using measures of phoneme awareness and letter knowledge alongside family history, it is possible to predict risk for dyslexia as early as the beginning of kindergarten. This is exactly the kind of targeted, reading-relevant assessment that speech pathologists are well placed to carry out.
Why do speech pathologists hesitate to diagnose dyslexia?
This is a hesitation that many speech pathologists will recognise in themselves. Dyslexia diagnosis has traditionally sat within psychology, and that history has influenced where the boundaries have been drawn. Many speech pathologists are genuinely concerned about overstepping, and that concern comes from a good place. Collaboration and mutual respect between professions are important. But there is a difference between respectful collaboration and unnecessarily stepping back from an area that sits squarely within our expertise. Speech pathologists are trained in language, phonological processing and literacy development. These are central to dyslexia, and the children and families we see deserve us to bring that expertise fully to bear.
It is also worth remembering that, as researcher David Kilpatrick has noted, "the practitioner's greatest assessment tool is a strong knowledge base regarding the nature of typical word-reading development and the sources of reading difficulties." Kilpatrick, David. (2018). Experienced speech pathologists have exactly that. They bring an intimate knowledge of the scientific principles underpinning not only the assessment of dyslexia but the intervention too. And perhaps most importantly, speech pathologists are highly skilled in function. We are trained to make intervention and support appropriate for the individual, taking into account their learning profile, their social communication, their community and their pathway into the workforce. That is a skill set that extends well beyond diagnosis and one that makes a profound difference to the outcomes children and young people experience over the long term. For more information on what a speech pathologist brings to the dyslexia assessment process, see my blog post: Can a Speech Pathologist Diagnose Dyslexia?
Some speech pathologists feel they can't rule out all co-existing conditions, and that's true. But no single professional is expected to assess everything. A good assessment includes a clear scope, a thorough history and appropriate referral when needed. Knowing when to refer to a psychologist is not a limitation, it is good clinical practice.
Others will say they aren't trained in this area, and for some that may be true. But that reflects training and experience, not scope of practice. If you are trained in literacy assessment, cognitive-linguistic processes and interpretation of profiles then you are equipped to assess dyslexia. If not, the answer is upskilling, not opting out.
This is about the child, not the profession
As someone who works with children with dyslexia every day, I know that what families need most is clarity. They need to understand what is going on for their child, why reading and spelling feel so hard, and what can be done about it. They do not need to wait longer than necessary for answers because of uncertainty about who is allowed to provide them.
Psychologists bring essential expertise in broader cognitive assessment, complex profiles and mental health. Speech pathologists bring expertise in language, reading development and the underlying systems that drive literacy. Both roles are important and both are needed. The goal is not for either profession to own the diagnosis. The goal is to understand the child properly and to give families the clarity and confidence they need to move forward.
Research is clear that the main purposes of a dyslexia evaluation are to determine whether the individual has dyslexia, to specify the nature and degree of the underlying difficulties, to identify their strengths, and to select appropriate accommodations and interventions. A diagnosis is only as useful as the recommendations that follow it, and it is equally important to note that cognitive assessment cannot and should not be used as the sole basis for that diagnosis. A comprehensive approach that draws on multiple sources of information, including family history, history of speech and language difficulties, behavioural observations and standardised assessment data, gives families the clearest and most useful picture of their child.
Assessment findings should directly inform an explicit, structured literacy intervention, and connecting that assessment to intervention is where true collaboration between speech pathologists and psychologists produces the best outcomes for children.
Dyslexia is not a disorder of general intelligence. It is a difficulty with specific cognitive-linguistic processes that support reading, and assessment should reflect that. A WISC has a role in some cases, but it is not the defining component of a dyslexia assessment. A targeted, reading-relevant cognitive assessment gives clearer answers and more useful direction for intervention. When both professions work together, each contributing what they do best, that is ultimately what families get.
