Can someone who is lower functioning and cannot read be considered dyslexic? Where does IQ fall into defining if someone is dyslexic?

Dr. Pierson's Response: 

This is a good question and a tough one to answer. We do not need IQ to diagnose dyslexia. That said, inherent in the definition of dyslexia is that the individual's difficulties with learning to read (and spell) are "unexpected in relation to other cognitive abilities." I have the definition here [1].

So, these children, as preschoolers, are on the typical trajectory of learning, particularly where the spoken language is concerned. And then, when they hit school, all of a sudden, they begin to have problems in regard to learning to read, spell, and write (i.e,. written language).

In my private clinical practice, when we diagnose dyslexia, we get a measure of a student's understanding of vocabulary and spoken language comprehension at the sentence/ discourse level. Receptive vocabulary has been shown to be a good proxy for verbal intelligence. We then use those scores as a benchmark to compare the student's skills in phonological processing (phonological awareness, phonological memory, and rapid naming), reading (decoding, fluency, and comprehension), spelling, and writing. We are looking for a pattern of strengths (spoken language) and weaknesses (phonological processing and written language). A student with dyslexia will have scores in phonological processing and written language that is not in line with his or her receptive vocabulary/spoken language comprehension.

So, it depends what the profile of the student who is "low functioning" looks like. If the low functioning area is in spoken language comprehension, then that student would get a diagnosis of receptive (and expressive) language disorder and potentially one also of reading disorder, but that student would not get a diagnosis of dyslexia. There are many pieces of the puzzle to put together when making a diagnosis.

I've some information as to what a comprehensive assessment entails here [2]. And here [3] is a piece that I wrote about why getting a good diagnostic assessment early important.

I hope this is helpful. Again, there is not a "formula" that would answer your question. A good diagnostician will be able to tease out the underlying challenges and the student's profile. This is important when developing an intervention approach because the child with a language disorder will need intervention in both comprehension of spoken language AND the skills that underlie learning to read; whereas, the student with dyslexia does not need work in spoken language comprehension. In regard to learning to read and spell, both populations will benefit from structured literacy [4] intervention.