Diagnostic utility of letter fluency, category fluency, and fluency difference scores in Alzheimer's disease

Word generation tasks, specifically letter fluency and category fluency, are a standard part of most test protocols used by clinical neuropsychologists to measure aspects of language and executive retrieval functions. Given the reliance on these measures as clinical tools, it is important to understand the diagnostic implications of patients' performances. In Alzheimer's disease (AD), category fluency has generally been found to be disproportionately impaired, whereas letter fluency is usually more mildly impaired. It has been proposed that this performance pattern occurs because access to temporal-lobe semantic stores is necessary for category fluency but not letter fluency. In this study, the diagnostic utility of category fluency, letter fluency, and difference scores (letter fluency minus category fluency) in AD were examined. Forty AD patients and 221 normal control subjects took category fluency and letter fluency tests. Traditional t-test comparisons revealed that the groups differed significantly on the two test tasks and in terms of mean difference scores. However, using sensitivity and specificity to calculate predictive values, only category fluency and letter fluency (but not difference scores) were useful in predicting AD in individuals. Furthermore, category fluency was superior to letter fluency in this regard. Likelihood ratio tables are provided for use in calculating the odds of AD for specific category fluency and letter fluency scores generated by individual patients.