This is the first study evaluating the validity of self-reported number of teeth among a larger group of older adults (≥ 70 years) including data on cognitive function. We found that older Norwegian adults reported their number of teeth reasonably accurately compared to tooth counts from clinical examinations. The mean difference between self-reported and clinical number of teeth was low (− 0.22 teeth), and more than 70% of the participants reported their number of teeth within an error of two teeth. Correlation between self-reports and clinical examinations was high for the total sample. However, a lower correlation was found among participants with dementia, participants having ≥ 20 teeth, and participants with teeth restored or replaced by fixed prosthodontics. Moreover, almost all the older edentulous adults correctly reported having no teeth.
The reasonably good agreement between self-reported and clinical number of teeth found in study sample 1 is in line with recent studies on the validity of self-reported number of teeth [13, 14, 16, 23]. However, some limitations of the previous studies included that they had a smaller number of older adults than our study [13, 14, 23], were drawn from selected samples [14, 16, 23], or did not explore participants’ characteristics that could affect the accuracy of self-reported number of teeth [13, 16].
Consistently with previous studies [21, 23, 31], we found a tendency toward self-reported number of teeth being underreported compared to tooth counts from clinical examinations. In our study, the mean difference between the two measures was − 0.22 teeth. This low deviation between the two tooth count measures indicates that self-reported number of teeth can be a valid measure when used at a group level in epidemiological studies. This was further supported by our finding that more than 70% of the participants was within a self-report error of two teeth. Nonetheless, the 95% upper and lower limits of agreement in our study were wide and ranged from − 6.66 to 6.22 teeth, a finding that corresponds with Ueno’s study from 2018 [16]. The wide range in the upper and lower limits of agreement implies that caution should be taken when using self-reported number of teeth at an individual level.
In the present study, we also explored factors that might affect the validity of self-reported number of teeth among older people. In addition to sociodemographic factors, we addressed self-perceived general and oral health, cognitive function, number of teeth, and the complexity of the dental status assessed by the number of teeth restored or replaced by fixed prosthodontics. We found that impaired cognitive function, having ≥ 20 teeth, and a more complex dental status markedly decreased the accuracy of self-reported number of teeth.
Decline in the general counting ability is a known cognitive hallmark of Alzheimer’s disease [25]. However, to our knowledge, this is the first study to explore the impact of cognitive function on self-reported tooth count. In study sample 1, the correlation gradually decreased with impaired cognitive function with the lowest correlation in the group with dementia. These results emphasize that self-reported number of teeth in populations where dementia is prevalent should be interpreted with caution.
In study sample 2, about 80% of the participants had mild cognitive impairment or dementia. Despite the high prevalence of cognitive impairment, almost all participants correctly reported being dentulous or edentulous. This finding indicates that self-reports of having teeth or not is less influenced by cognitive impairment than self-reports of the exact number of teeth. Based on this we argue that self-reported edentulousness can be used as a valid oral health parameter even in older populations with a high prevalence of dementia.
Like previous studies [21, 23], we found that the accuracy of self-reported number of teeth was affected by increasing number of teeth recorded from the clinical examinations. Consistently with both Ueno et al. [21] and Matsui et al. [23], we found the self-reported number of teeth to be less accurate among those with ≥ 20 teeth compared to those with ≤ 19 teeth. We believe that the explanation for this is straightforward: a smaller number of teeth is easier to count and hence to self-report correctly.
In addition, we found that the self-report error increased with an increasing number of teeth restored or replaced by fixed prosthodontics (i.e., crowns, implants, bridge abutments and pontics). This is in line with previous studies, which have shown that a higher number of dental restorations and replacements reduces the accuracy of self-reported tooth counts [21,22,23,24]. Buhlin et al. observed that the difference between self-reported and clinical number of teeth was reduced when pontics were counted as natural teeth [24]. Similarly, Ueno et al. 2010 found that participants with fixed prosthetic pontics were more likely to incorrectly report their number of teeth than those with no pontics [21]. Furthermore, Matsui et al. reported the greatest discrepancy between self-reported and clinical tooth counts among those with many prosthetic teeth [23].
Root remnants is another factor that may affect the accuracy of self-reported tooth counts. Participants may not be aware of their root remnants or whether to include them in their self-reported number of teeth. We found that the underestimation of self-reported number of teeth increased when root remnants were included in the clinical number of teeth. This indicates that many root remnants were not counted in the self-reports. In a study by Axelsson, this was addressed by emphasizing not to count residual roots and fractured teeth in the questionnaire [32]. The findings of increased inaccuracy in self-reported number of teeth related to fixed prosthodontics and residual roots implies that future studies should include guidance on how to count present natural teeth.
Some limitations should be noted. Previous findings from the HUNT Study have shown that non-participants had somewhat lower socioeconomic status and slightly higher prevalence of chronic diseases and higher mortality than those who participated [33]. This may also apply to our study. Furthermore, in study sample 1, the oral health examinations were performed at field stations, which may have limited the participation of frail older adults. Lastly, as the HUNT Study was conducted in Norway, the findings may not be generalizable to populations with a different demographic and socioeconomic distribution.
The major strengths of our study were the large sample size of older adults and the population-based design. The sample was drawn from the general population in a Norwegian county with both small towns and rural areas. Additionally, the inclusion of a wide range of covariates, including cognitive function, enabled us to explore factors influencing the accuracy of self-reported number of teeth among older adults.