Research on ADHD in older adults is scarce, and data from prevalence studies in this population have not been synthesised. Understanding prevalence estimates of ADHD in older adults can guide clinicians in adjusting clinical assessment procedures and treatment approaches for these individuals. This systematic review and meta-analysis aimed to investigate the prevalence of ADHD in adults aged ≥50 years via different assessment methods. Five electronic databases were searched up to 26 June 2020. Observational cohort and cross-sectional studies, with participants aged ≥50 diagnosed with any of the following criteria were included:

• Research diagnosis of ADHD based on the Diagnostic and Statistical Manual of Mental Disorders (DSM) III, IV, IV-TR or 5 criteria.
• Clinical diagnosis according to International Classification of Diseases ([ICD] 9 or 10) or DSM III, IV, IV-TR or 5 as reported in registers/medical files or self-reported medical history.
• Presence of medications recommended in the pharmacological treatment of ADHD as outlined in the National Institute for Health and Care Excellence (NICE) for ADHD (NICE, 2018), and/or non-pharmacological treatment for ADHD, as reported in registers/medical files or self-reported prescription.

Of the 9784 papers screened, 20 studies with 32 data sets were included in the meta-analysis. The studies were published between 2005 and 2019, and included data collected between 1997 and 2015. Three meta-analyses were performed to assess pooled prevalence estimates of ADHD in older adults for each assessment method. The estimated pooled prevalence was 2.18% (95% confidence interval [CI] 1.51–3.16) for ADHD research diagnosis using the random-effects model. The estimated pooled prevalence was 0.23% for clinical diagnosis (95% CI 0.12–0.43) and 0.09% for ADHD treatment (95% CI 0.06–0.15).

A significant difference in pooled prevalence estimates of ADHD in older adults between the studies based on research diagnosis, clinical diagnosis and treatment was shown in the subgroup analysis (Q (2) = 108.74, p < 0.0001). Statistically significant differences between the prevalence provided in studies using research diagnosis of ADHD versus studies using either clinical diagnosis or treatment were observed: Q (1) = 35.52, p < 0.0001 and Q (1) = 99.40, p < 0.0001, respectively. A statistically significant difference in the prevalence estimated in studies using clinically diagnosed ADHD versus treated ADHD was also seen in the subgroup analysis: Q (1) = 4.80, p < 0.0001.

This systematic review and meta-analysis has several limitations. An unequal female-to-male distribution was present in eight of the 20 studies included in the analysis and may have interjected bias into the analysis as sex-stratified ADHD prevalence estimates could not be provided for older adults. In addition, 13/20 studies did not have prevalence estimates in older adults stratified by sex. Mean age and standard deviation data were not available in many of the studies and a relatively small number of studies representing each assessment method were included, therefore, more detailed subgroup analyses could not be conducted. The use of retrospective self-report symptom recall are also limitations of this meta-analysis (Simon et al, 2009).

Findings of this meta-analysis suggest that there is a substantial gap between the prevalence of elevated ADHD symptoms in older adults (aged ≥50 years) assessed via validated scales in community samples and estimates based on ADHD clinical diagnosis and treatment. The analysis also found that less than half of individuals with a clinical diagnosis of ADHD were treated. Additional research addressing the effects of different assessment methods, sex and age on the prevalence of ADHD in individuals aged ≥50 years is needed.

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