The assessment of premorbid ability: A critical review

Estimating premorbid intelligence level has become an established and central component of neuropsychological research and practice. Three main approaches are currently employed, estimate based on demographic variables, reading ability, or lexical decision-making. Each have relative strengths end weaknesses. Demographic-based regression equations have the advantage that the estimate is completely independent of the subject's current cognitive state, but such approaches only predict similar to 50% of the variance in measured intelligence. Also, in neurodevelopmental conditions, maximum educational and occupational attainment may be compromised by the disease process. For use in the individual case, the margin of error may also be unacceptably large. Estimates based on oral pronunciation of irregular words, typified by the National Adult Reading Test (NART), have become the most popular method. Pronunciation accuracy correlates highly with measured intelligence, and is relatively resistant to the effects of a wide range of clinical disorders. Combining demographic variables with NART scores appears to significantly increase the percentage of variance in measured intelligence compared with either demographics or NART alone. Unfortunately a snowball has started rolling, where the NART is now used almost routinely in clinical research and practice. Closer inspection reveals that NART performance may be compromised in a number of conditions including moderate to severe Alzheimer's disease, chronic schizophrenia, Korsakoff's syndrome, Huntington's disease and cranial irradiated glioma patients. It is essential that the insensitivity of the NART to the clinical condition under investigation is demonstrated prior to its routine use as a premorbid measure. Placing the NART words in context (i.e. within meaningful sentences) may provide a more accurate estimate of premorbid function. The final approach involves lexical decision-making, where subjects are presented with two items and are required to state which of the pair is a real word. This approach overcomes some of the problems associated with the pronunciation based tests, i.e. it can be used with patients with dyslexia or articulatory problems, and may provide a more valid estimate of premorbid ability in self-educated individuals who may be familiar with the word and its meaning, but be unsure regarding pronunciation. Spot-The-Word performance correlates highly with verbal intelligence, and in elderly patients with suspected cognitive decline, Spot-The-Word performance appears to be unaffected. However, further evidence is required regarding the relative insensitivity of Spot-The-Word to confirmed clinical conditions such as Alzheimer's disease.