Table 1 Characteristics of the included studies (grouped according to the relevant retention category)
Trial | Study design | Host trial (number randomised) | Disease/condition | Participants | Setting | Retention strategies (number of participants) | Outcome (retention study) | Time point(s) used in the analysis |
|---|---|---|---|---|---|---|---|---|
1. Studies that involved a change in mode of data collection | ||||||||
Atherton 2010 [25] | Cohort study | Prevention of Pelvic Infection (POPI) trial (1329) | Chlamydia screening | Young female students | 12 universities and colleges across London | A postal follow-up questionnaire (299) Online questionnaire (1030) | Questionnaire response rate | 12 months after randomisation 4 weeks and 12 months after follow-up commenced |
Childs 2015 [26] | Cohort study | Prevention of Lower Back Pain in the Military (POLM) trial (4325) | Low back pain | Geographically dispersed soldiers in the US Army | A military training setting | Web-based survey (632) Telephone call center (571) Both the web-based and telephone survey (233) M2 database (2788) | Follow-up rate | Monthly follow-up surveys (12 weeks after training). Telephone contact with soldiers who had not responded to 3 monthly web-based surveys at the end of the first year |
Dormandy 2008 [27] | Before and after study | SHIFT (Screening for Haemoglobinopathies in the First trimester) (775) | Antenatal Sickle Cell and Thalassaemia (SCT) screening | People from minority ethnic groups and with high levels of material and social deprivation | UK primary care | Postal questionnaire completion only (61) Choice of telephone or postal completion (714) | Questionnaire response rate | 11 months after randomisation |
Johnson 2015 [28] | Before and after study | The hospital outpatient alcohol project (HOAP) (837) | Alcohol consumption | Hospital outpatients with hazardous or harmful drinking | Outpatient department of a large tertiary referral hospital in Newcastle (population 540,000), Australia | Postal questionnaire + link a web-based questionnaire (520/837) Telephone follow-up (317) | Questionnaire response rate | 6 months after randomisation 4 weeks later if questionnaire remained unreturned |
Lall 2012 [29] | Cohort study | Back Skills Training Trial (BeST) (701) | Back pain | Patients with subacute and chronic low back pain | UK primary care | Postal questionnaires Telephone interviews | Questionnaire response rate | 12 months after randomisation 6 weeks later if questionnaire remained unreturned |
Peterson 2012 [30] | Post hoc analysis method | Randomised clinical trial of elective coronary artery bypass grafting (248) | Coronary artery disease | Coronary artery bypass graft surgery patients | The New York Hospital–Cornell Medical Center | Routine follow-up approach (return to hospital for follow-up) (187) Home follow-up (61) | Follow-up rate and its impact on main trial outcome | 6-month follow-up |
2. Studies that used a different questionnaire format for follow-up | ||||||||
Bailey 2013 [31] | Before and after study | The Sexunzipped trial (2006) | Sexual well-being | Young people aged 16 to 20 years and resident in the UK | Online study | Online questionnaire (1208) A shortened version of the online questionnaire by post (798) | Retention of valid participants at 3-month follow-up | 3 months after randomisation |
3. Studies that used a different design strategy for follow-up | ||||||||
Ulmer 2008 [32] | Historically controlled study | Randomised controlled trial of a telephone-delivered behavioral intervention (153) | Hypertension | Participants with uncontrolled hypertension | New York Harbor Healthcare System | A 4-week simple run-in period before participation in the study | Drop-out rate | 12 months after randomisation |
4. Studies that used a change in mode of reminder delivery | ||||||||
Hansen 2014 [33] | Cohort study | ‘Preventive consultations for 20- to 40-year-old young adults’ randomised trial (495) | Change in health behavior | Young adults with multiple psychosocial and lifestyle problems | General practices in Denmark | Follow-up questionnaire and up to two reminders by mail (495) Telephone reminder to primary non-responders (179) | Questionnaire response rate | 1-year follow-up after randomisation |
Varner 2017 [34] | Nested cohort analysis | An RCT assessing minor traumatic brain injury (MTBI) discharge instructions (118) | Traumatic brain injury | Patients ages 18 to 64 years presenting to the ED with a chief complaint of a head injury or suspected concussion | Emergency department (ED) of an academic tertiary care hospital in Toronto, Ontario | Telephone contact (78) A reminder text message (40) | The proportional difference in loss to a follow-up between the two groups | First 4 months Final 3 months |
5. Studies that used incentives | ||||||||
Brealey 2007 [35] | Historically controlled study | DAMASK Trial (a pragmatic randomised trial to evaluate whether GPs should have direct access to MRI for patients with suspected internal derangement of the knee) (547) | Knee problems | Patients aged 18 to 55 with suspected internal derangement of the knee | General practices across North Wales, North East Scotland, and Yorkshire | No incentive (105) Unconditional direct payment of £5 to patients for the completion and return of questionnaires (442) | Completion rate of postal questionnaires | 12 months after randomisation |
Rodgers 2016 [36] | Prospective cohort study | An RCT evaluated the effectiveness of a brief social work intervention (479) | Instances of violence and heavy drinking among women | Abused women who were also heavy drinkers | Two US academic urban EDs | Cash incentives for participants enrolled during the first 8 months of the study (111) A reloadable bank card for all subsequent participants (358) | Participant completion rates of follow-up study activities and overall retention | 3-, 6- and 12-month follow-up after randomisation |
6. Studies that used multi-faceted strategies | ||||||||
Ezell 2013 [37] | Post hoc analysis method | RCT comparing asthma outcomes of students exposed to tailored asthma management versus those exposed to generic asthma management (422) | The burden of asthma | Students in grades 9 through 12 | Six Detroit public high schools | Incentives ($80 for completion of all program modules) (380). 4 retention strategies to locate missing participants (re-dials of non-working telephone numbers, Facebook, assistance from school) (125) | Attrition rate | 12-month follow-up |
Sellers 2015 [38] | Before and after study with no control group | The BAN trial was designed to evaluate the efficacy of 3 mother-to-child HIV transmission prevention strategies (2369) | HIV prevention | Pregnant women who tested HIV-positive and their infants | Four antenatal clinics Lilongwe, Malawi | Routine strategies (support groups, home visits) (1686) Intensive tracing efforts (638) | Attrition rate | 28 weeks after randomisation |