Reconsidering the clinical utility of the Test of Premorbid Functioning (TOPF) as a measure of premorbid cognitive capability in patients with temporal lobe epilepsy

Estimating premorbid cognitive capability provides a baseline to accurately identify and quantify cognitive impairments incurred following neurological illness or injury, including in epilepsy. Irregular word reading tests like the Test of Premorbid Functioning (TOPF) are routinely used for this purpose, underpinned by the assumption that they rely on aspects of cognitive capability impervious to neurological insult. Given accumulating evidence in distinct neurological populations that the TOPF is sensitive to pathology, this study investigated the TOPF as an estimate of premorbid cognitive capability in 102 temporal lobe epilepsy (TLE) patients (M = 38.70 years, SD = 13.76, 56 female) and 168 controls (M = 25.60 years, SD = 11.98, 118 female). Patient TOPF scores were significantly poorer relative to controls (g = 1.27). Rates of impairment on each cognitive measure differed significantly depending on how impairment was classified, that is, either using TOPF performance (premorbid approach) or normative data (normative approach). Among controls, the premorbid approach yielded higher impairment rates than the normative approach across all cognitive tests. Among patients, the premorbid approach yielded lower impairment rates than the normative approach on some cognitive tests. Correlations between the TOPF and other neuropsychological measures were mostly weak, ranging from r = -0.04 to r = 0.36 in controls and r = -0.03 to r = 0.53 in patients. Findings suggest that the TOPF is susceptible to TLE pathology, which violates the assumptions that underpin the use of irregular word reading as an estimate of premorbid cognitive capability in this population.