It is usually of interest to take into account OVA’s diversity (e.g., very old age, socially isolated, rural, ethnicity, languages, education), type of care (e.g., home care) in order to explore a wide range of experiences or needs [5, 8] It may be tempting to exclude hard-to-reach populations such as highly vulnerable and marginalized OVA (e.g., homeless, cognitively impaired). However, to foster representativeness, it is crucial that recruitment focus on the target population, eligibility criteria and research objectives [6] rather than easiness to reach.

Providing clear and understandable information about the study which is accessible to all OVA is another critical issue during the recruitment phase given their health and social vulnerabilities [6]. Recruitment requires an “age friendly” approach by recruiters, in which communication should be appropriately adapted to the needs of OVA [9] including the content, style, and approach [6]. In addition, caregivers and healthcare professionals, staff (e.g., nursing homes) might resist recruiting more vulnerable older patients [2].

Several solutions have been proposed in the literature [9–11]. For instance, messages should be age-appropriate and drafted with stakeholders [11], and need to be in short and explicit paper format [2, 10]. Multiple modes of communication should also be considered, such as newspapers, online social media, and mass media that have their own benefits and risks. For example, recruiting OVA through mass email lists will be insufficient to capture the segment of OVA that are less likely to participate or who do not have access to technology [10]. The recruitment strategy should also focus on local venues where older people usually go (e.g., primary care clinics, church) [5]. Peers may also be leveraged as influencers and can share their positive perceptions in participating in research [9]. Finally, recruitment of hard-to-reach populations is most efficient if done by face-to-face contact, word-of-mouth, families or by known and trusted healthcare professional [5, 10] in order to convey reassurance, trustworthiness and develop strong connections with targeted communities. It is thus key to involve and educate resource people (healthcare professionals, social workers, relatives, etc.) in the field in order to help facilitate and legitimize acceptance of the study [9, 11].

Informed consent is particularly difficult for OVA as it strongly influences social relationships and can feel threatening for individuals who may fear losing health services if they refuse to participate [12]. In addition, ensuring informed consent for very vulnerable OVA such as those with cognitive disorders is particularly challenging [13]. Blanket exclusion of those with cognitive impairment is to be avoided [6]. Instead, decisions regarding the capacity to consent should be based on an individual basis [6].

The literature proposes several potential solutions to address challenges which are specific to OVA [13]. First and foremost, first encounters by researchers should ideally happen in a safe place and relaxed atmosphere such as the patient’s home [11]. When possible, verbal consent should be favored over signed consent which tends to generate more concerns among OVA [12]. In addition to ensuring information is conveyed in an appropriate and plain language, care should be taken to speak loudly and slowly where patients have auditory limitations [2]. Time should be taken to explain and repeat the purpose of the study, how the study will be conducted and how results will be kept safe and confidential [2]. Informed consent for OVA with cognitive impairment is particularly challenging [14, 15]. It is important to note that informed consent can often still be obtained for persons at mild and moderate stages of dementia while, consent by a legal representative may be considered for more advanced stages to ensure that the decision to participate or not is based on the person’s wishes and preferences. Using tools to assess the capacity to consent is therefore an essential component in recruiting OVA for research [14, 15].

Retention is usually an important challenge for longitudinal studies with OVA who are prone to fatigue, cognitive impairment, disabilities, illness or sensory limitations [2, 16]. Research teams should plan a variety of strategies to facilitate recruitment and retention. Designing studies that maximises the benefit/burden ratio can help with recruitment and retention [16] such as providing opportunity for socialization, access to test results [2, 16] and minimizing travel [16, 17]. Finally, regular “keep in touch” calls and mailings (i.e., newsletters and greeting cards) have been shown to increase retention [17].