Statistical analyses

Dementia cases in HARMONY and baseline examination of KP (1987–1989) and SNAC-K (2001–2004) were considered as prevalent cases. Incident cases were identified using follow-up examinations for KP (1991–1998) and SNAC-K (2004–2010), and the entire follow-up time for SATSA, OCTO-twin, and GENDER. Cases identified as prevalent were analyzed for ability to detect cases, and incident cases were separately analyzed for time of onset to detection.

The ability of the registers to detect dementia cases was determined by sensitivity, calculated as the number of demented individuals identified in the registers divided by the number of dementia cases identified in the population-based studies. The ability of the register to detect non-cases was determined by specificity, computed as the number of non-demented individuals identified in the registers divided by the number of non-demented individuals identified in the population-based studies. In addition, we have estimated the probability of positive register-based cases being confirmed by the population-based studies, the positive predictive value (PPV), given by the number of dementia cases identified in the population-based studies divided by the dementia cases recorded in the registers. Clopper-Pearson exact confidence limits for proportions [21] were used to construct 95% confidence intervals (CI) for the estimates.

For the NPR, the sensitivity was estimated for register-based diagnoses between 1964 and the last available follow-up from the register, which was in 2000 for KP, and in 2012 for SNAC-K and the National Swedish samples. The specificity and PPV for the NPR were estimated for register-based diagnoses between 1964 and the cognitive screening date for each participant. To determine true positive and false positive cases in an unbiased way, the date of cognitive screening was used as the date on which the register diagnosis was determined to be true or false. The sensitivity of the CDR was estimated among deaths occurring between the last cognitive screening and the last available follow-up from the register in 2008 for KP, 2011 for SNAC-K, and 2012 for the National Swedish samples. Specificity and PPV for the CDR were estimated among study participants who died within one year from the last cognitive screening in order to limit possible misclassification due to non-dementia cases developing dementia between screening and death.

Incident dementia cases were followed from the year of diagnosis of dementia until first hospitalization with a reported dementia diagnosis (first detection in NPR), last available date in NPR, or date of death, whichever came first. Using Laplace regression, we evaluated the time to detection of dementia cases in NPR. The time to detection for patients with a dementia diagnosis in the NPR that preceded the dementia diagnosis in the population-based studies was considered as zero (n = 69). Using the Laplace regression, we examined whether sex, age at diagnosis, and educational attainment of the dementia cases affected the time to detection. Age at diagnosis and educational attainment were evaluated as binary variables. The cut-off for the age at diagnosis was chosen as the median value (85 years), the cut-off for educational attainment was chosen as compulsory school versus any higher education.

Statistical analyses were performed using Stata 14.2 (StataCorp, College Station, Texas).