Premorbid tests estimate cognitive ability prior to neurological condition onset or brain injury. Tests requiring oral pronunciation of visually presented irregular words, such as the National Adult Reading Test (NART), are commonly used due to robust evidence that word familiarity is well-preserved across a range of neurological conditions and correlates highly with intelligence. Our aim is to examine the prediction limits of NART variants to assess their ability to accurately estimate premorbid IQ.

We examine the prediction limits of 13 NART variants, calculate which IQ classification system categories are reachable in principle, and consider the proportion of the adult population in the target country falling outside the predictable range.

Many NART variants cannot reach higher or lower IQ categories due to floor/ceiling effects and inherent limitations of linear regression (used to convert scores to predicted IQ), restricting clinical accuracy in evaluating premorbid ability (and thus the magnitude of impairment). For some variants this represents a sizeable proportion of the target population.

Since both higher and lower IQ categories are unreachable in principle, we suggest that future NART variants consider polynomial or broken-stick fitting (or similar methods) and suggest that prediction limits should be routinely reported.

Tests requiring the pronunciation of irregular words are used to estimate premorbid cognitive ability in patients with clinical diagnoses, and prior cognitive ability in normal ageing. However, scores on these word-reading tests correlate with scores on the Mini-Mental State Examination (MMSE), a widely used screening test for possible cognitive pathology. This study aimed to test whether the word-reading tests’ correlations with MMSE scores in healthy older people are explained by childhood IQ or education.

Wechsler Test of Adult Reading (WTAR), National Adult Reading Test (NART), MMSE scores and information about education were obtained from 1024 70-year-olds, for whom childhood intelligence test scores were available.

WTAR and NART were positively correlated with the MMSE (r ≈ 0.40, p < 0.001). The shared variance of WTAR and NART with MMSE was significantly attenuated by ~70% after controlling for childhood intelligence test scores. Education explained little additional variance in the association between the reading tests and the MMSE.

MMSE, which is often used to index cognitive impairment, is associated with prior cognitive ability. MMSE score is related to scores on WTAR and NART largely due to their shared association with prior ability. Obtained MMSE scores should be interpreted in the context of prior ability (or WTAR/NART score as its proxy).