We have shown significantly different success rates in follow up of two studies of prematurely born infants. Comparison of participant characteristics between those who were and were not successfully followed up highlighted that there were no consistent predictive factors across the two studies which identified individuals at higher risk of loss to follow up. In the larger UKOS trial, non-white ethnicity, socioeconomic deprivation and maternal tobacco use were more prevalent in those who did not attend for follow up. In the virus study, lower birth weight, singleton birth and persistent oxygen requirement at hospital discharge were associated with greater follow up participation. In light of the lack of common factors associated with loss to follow up, we suggest that factors related to follow up study design may be more relevant.
We have demonstrated higher retention rates in a cohort with whom there had been regular contact to the follow-up stage [6]. Staff continuity and development of trust has been highlighted as helping to prevent attrition in a longitudinal study of lead-exposed children [11] and in a study of development in children of mothers with a history of substance misuse [12]. In both the studies we report the senior investigators remained the same, but junior researchers who undertook the measurements had changed from those in the original studies. The regular contact with the children and their families with cards and a newsletter in UKOS, we suggest may contribute to the differences in attrition between the two studies.
The factors associated with attrition differed between the two studies. The participants of UKOS were all born less than 29 weeks of gestational age and all had been ventilated from birth. It is then not surprising that the severity of their initial illness did not determine whether they consented to follow-up. Instead the significant risk factors for attrition were social factors, greater deprivation scores and intrauterine tobacco exposure. Previous studies have suggested that indicators of lower socioeconomic status are predictive of attrition in longitudinal studies commencing in infancy [13, 14]. Those data highlight individuals who may require more intense explanation regarding the importance of taking part in the follow-up. In addition, more were of non-white ethnicity which may suggest a need for more tailored support for those whose first language may not be English. In contrast, in the virus study determinants of initial illness severity were significantly related to attrition rate. Infants recruited into the virus study, although born prematurely, were significantly more mature than the UKOS cohort. It is likely those who had a higher birth weight, were not oxygen dependent at discharge or of a multiple birth had a very short time of routine clinical follow-up, which may have led to lower interest in engagement in the later research.
In the paediatric setting, various approaches have been suggested to optimise retention of study participants. Appointments must be flexible around family and work commitments [11, 15] and provision must be made for study participant’s siblings, either by allowing siblings to attend appointments or, if this is not practicable, providing formal childcare [11, 12]. Practical assistance in the form of arranging transport to attend studies (rather than simply reimbursing participants after attendance) and organising onward referrals to address any new incident health needs were further highlighted as being beneficial [11]. Both of our studies welcomed siblings to attend and offered appointments at weekends and during school holidays in addition to weekday appointments. Transport was arranged for all the UKOS participants as the majority lived outside London, whereas for the virus study reimbursement was given for travel, but not arranged, as participants lived within easy reach of the testing location. At the follow-up of both of our studies, the researchers assessed specific health needs and made onward referrals and recommendations as needed, but this was not explicitly advertised in the literature families received prior to recruitment.
The perception of the importance of the research may be a key determinant of choosing to participate [16]. In addition, in community-based clinical trials, it has been identified that participants’ understanding of the study importance was a key determinant of retention [2]. The UKOS blog gave parents updates on publications arising from the study, thereby likely emphasising the success of the research programme.
Obtaining relatives’ contact details as well as those of the participants or parents [17], making interim contact between study visits [18] and providing small financial incentives to update contact information [19, 20] have been suggested as effective methods to reduce attrition due to loss of contact. The annual birthday and seasonal greeting cards sent to the UKOS cohort asked parents to maintain up to date contact details. To undertake such activity carries a cost, thus it is important that additional funding for participant retention that extends beyond the end of the main funding source should be sought [19].
The use of study-specific (rather than institutional) logos may aid study retention [19, 21] by enforcing the identity of the study and may be a stronger reminder of previous participation than written descriptions. The UKOS newsletters regularly featured photographs of the study team, including senior investigators and previous members of the team as well as the study logo.
