The Reliable Digit Span (RDS) is a well-validated embedded indicator of performance validity. An RDS score of ≤7 is commonly referenced as indicative of invalid performance; however, few studies have examined the classification accuracy of the RDS among individuals suspected for dementia. The current study evaluated performance of the RDS in a clinical sample of 934 non-litigating individuals presenting to an outpatient memory disorders clinic for assessment of dementia.
The RDS was calculated for each participant in the context of a comprehensive neuropsychological assessment completed as part of routine clinical care. Score distributions were examined to establish the base rate of below criterion performance for RDS cutoffs of ≤7, ≤6, and ≤5. One-way ANOVA was used to compare performance on a cognitive screening measure and informant reports of functional independence of those falling below and above cutoffs.
A cutoff score of ≤7 resulted in a high prevalence of below-criterion performance (29.7%), though an RDS of ≤6 was associated with fewer below-criterion scores (12.8%) and prevalence of an RDS of ≤5 was infrequent (4.3%). Those scoring below cutoffs performed worse on cognitive measures compared with those falling above cutoffs.
Using the RDS as a measure of performance validity among individuals presenting with a possibility of dementia increases the risk of misinterpreting genuine cognitive impairment as invalid performance when higher cutoffs are used; lower cutoffs may be useful when interpreted in conjunction with other measures of performance validity.