Dyslexia is a specific learning disability that affects an individual’s ability to acquire reading skills, despite having standard intelligence and adequate instruction. This challenge is rooted in the brain’s difficulty with phonological processing—the manipulation and understanding of the sound structure of language. Because no single symptom or score confirms the diagnosis, a full evaluation involves an integrated battery of tests administered by a qualified professional. The diagnostic process systematically moves from gathering background information to measuring intellectual capacity, quantifying academic deficits, and identifying specific underlying cognitive markers.
The Foundational Step: Initial Screening and Background Review
The diagnostic process begins with a thorough initial screening and review of the individual’s history before any formal standardized tests are administered. Professionals collect extensive developmental, medical, and educational records to understand the full context of the learning difficulties. This background review includes a detailed clinical interview to establish a timeline of language milestones and academic struggles.
A mandatory preliminary step is ruling out other factors that could explain the reading difficulty, such as sensory impairments. Vision and hearing screenings are conducted to ensure the individual can properly see the text and hear the sounds of language. Excluding these sensory issues confirms the learning difficulty is not due to an uncorrected physical problem, allowing the clinician to focus on cognitive and academic performance. Once sensory acuity is confirmed, the formal battery of cognitive and academic testing can begin.
Assessing General Cognitive Ability
The comprehensive evaluation involves assessing the individual’s general cognitive ability using standardized intelligence tests. Common examples include the Wechsler Intelligence Scale for Children (WISC) or the Woodcock-Johnson Tests of Cognitive Abilities (WJ COG). The primary goal of these tests is not to diagnose dyslexia directly, but to establish the individual’s overall intellectual potential and verbal reasoning skills.
Historically, diagnosis relied on the “discrepancy model,” requiring a significant gap between a high IQ score and low reading achievement. While modern criteria, such as those in the DSM-5, no longer strictly mandate this discrepancy, the assessment remains important. Establishing average or above-average intellectual potential provides context for the academic struggles. This confirms that the reading difficulties are unexpected given the person’s capacity to learn, pointing toward a neurological origin rather than a global cognitive impairment.
Measuring Academic Achievement Deficits
The next part of the evaluation focuses on quantifying the academic deficit by measuring performance across several areas of literacy. Standardized academic achievement batteries are administered to compare the individual’s skills against age- and grade-level peers. Widely used tools include the Woodcock-Johnson Test of Achievement (WJ ACH) and the Wechsler Individual Achievement Test (WIAT).
These batteries contain specific subtests designed to pinpoint the nature and severity of reading and writing problems. Key subtests measure basic word recognition (the ability to accurately identify single words) and reading fluency (the speed and accuracy of reading connected text). Performance is also assessed in reading comprehension and dictated spelling, which are frequently areas of weakness for individuals with dyslexia. The resulting low standard scores objectively document the core academic impairment required for a diagnosis.
Identifying Specific Processing Weaknesses
The most specialized part of the assessment involves identifying the specific cognitive processing weaknesses that underlie the observed reading deficits. This section confirms the neurobiological basis of dyslexia, which is typically a deficit in the phonological system. Tests target the cognitive markers that explain the academic problems.
The Comprehensive Test of Phonological Processing (CTOPP) is frequently used to measure these foundational abilities. This assessment examines phonological awareness—the ability to recognize and manipulate the sound units of spoken language. Tasks include rhyming, blending sounds into words, and segmenting words into individual sounds. The CTOPP also measures phonological memory by asking the person to repeat non-words or sequences of digits, which requires holding sound information briefly.
Another important area is Rapid Automatized Naming (RAN), which measures the speed with which an individual can retrieve and name a series of familiar items (e.g., colors, objects, or letters). Slow RAN is a common finding in dyslexia and is closely tied to difficulties with reading fluency. The final diagnosis integrates findings from all three domains: average intellectual potential, significantly low academic achievement in reading and spelling, and specific deficits in phonological processing abilities.