title: Recruitment and representativeness of blood donors in the INTERVAL randomised trial assessing varying inter-donation intervals.
abstract: The interpretation of trial results can be helped by understanding how generalisable they are to the target population for which inferences are intended. INTERVAL, a large pragmatic randomised trial of blood donors in England, is assessing the effectiveness and safety of reducing inter-donation intervals. The trial recruited mainly from the blood service's static centres, which collect only about 10 % of whole-blood donations. Hence, the extent to which the trial's participants are representative of the general blood donor population is uncertain. We compare these groups in detail.
  
  
  
  We present the CONSORT flowchart from participant invitation to randomisation in INTERVAL. We compare the characteristics of those eligible and consenting to participate in INTERVAL with the general donor population, using the national blood supply 'PULSE' database for the period of recruitment. We compare the characteristics of specific groups of trial participants recruited from different sources, as well as those who were randomised versus those not randomised.
  
  
  
  From a total of 540,459 invitations, 48,725 donors were eligible and consented to participate in INTERVAL. The proportion of such donors varied from 1-22 % depending on the source of recruitment. The characteristics of those consenting were similar to those of the general population of 1.3 million donors in terms of ethnicity, blood group distribution and recent deferral rates from blood donation due to low haemoglobin. However, INTERVAL participants included more men (50 % versus 44 %), were slightly older (mean age 43.1 versus 42.3 years), included fewer new donors (3 % versus 22 %) and had given more donations over the previous 2 years (mean 3.3 versus 2.2) than the general donor population. Of the consenting participants, 45,263 (93 %) donors were randomised. Compared to those not randomised, the randomised donors showed qualitatively similar differences to those described above.
  
  
  
  There was broad similarity of participants in INTERVAL with the general blood donor population of England, notwithstanding some differences in age, sex and donation history. Any heterogeneity of the trial's results according to these characteristics will need to be studied to ensure its generalisability to the general donor population.
  
  
  
  Current Controlled Trials ISRCTN24760606 . Registered on 25 January 2012.
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.
authors.forename: Carmel
authors.initials: C
authors.lastname: Moore
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.
authors.forename: Thomas
authors.initials: T
authors.lastname: Bolton
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.
authors.forename: Matthew
authors.initials: M
authors.lastname: Walker
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.
authors.forename: Stephen
authors.initials: S
authors.lastname: Kaptoge
authors.affiliation: NHS Blood and Transplant - Oxford Centre, Level 2, John Radcliffe Hospital, Headley Way, Oxford, OX3 9BQ, UK.
authors.forename: David
authors.initials: D
authors.lastname: Allen
authors.affiliation: NHS Blood and Transplant, Northway, Filton, Bristol, BS34 7QH, UK.
authors.forename: Michael
authors.initials: M
authors.lastname: Daynes
authors.affiliation: NHS Blood and Transplant, Longley Lane, Sheffield, S5 7JN, UK.
authors.forename: Susan
authors.initials: S
authors.lastname: Mehenny
authors.affiliation: Department of Haematology, University of Cambridge, Cambridge Biomedical Campus, Long Road, Cambridge, CB2 OPT, UK.|NHS Blood and Transplant, Cambridge Biomedical Campus, Long Road, Cambridge, CB2 OPT, UK.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Sambrook
authors.affiliation: NHS Blood and Transplant, Cambridge Biomedical Campus, Long Road, Cambridge, CB2 OPT, UK.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Watkins
authors.affiliation: NHS Blood and Transplant, Reeds Crescent, Watford, Herts, WD24 4QN, UK.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.
authors.forename: Gail
authors.initials: G
authors.lastname: Miflin
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.
authors.forename: Emanuele
authors.initials: E
authors.lastname: Di Angelantonio
authors.affiliation: Department of Haematology, University of Cambridge, Cambridge Biomedical Campus, Long Road, Cambridge, CB2 OPT, UK.|NHS Blood and Transplant, Cambridge Biomedical Campus, Long Road, Cambridge, CB2 OPT, UK.|Wellcome Trust Sanger Institute, Wellcome Genome Campus, Hinxton, Cambridge, CB10 1SA, UK.
authors.forename: Willem H
authors.initials: WH
authors.lastname: Ouwehand
authors.affiliation: NHS Blood and Transplant - Oxford Centre, Level 2, John Radcliffe Hospital, Headley Way, Oxford, OX3 9BQ, UK.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.|Radcliffe Department of Medicine, University of Oxford, John Radcliffe Hospital, Headley Way, Oxford, OX3 9DU, UK.
authors.forename: David J
authors.initials: DJ
authors.lastname: Roberts
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK.
authors.forename: John
authors.initials: J
authors.lastname: Danesh
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK. interval@medschl.cam.ac.uk.|NIHR Blood and Transplant Research Unit in Donor Health and Genomics, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 8RN, UK. interval@medschl.cam.ac.uk.|Department of Public Health and Primary Care, The INTERVAL Trial Coordinating Centre, University of Cambridge, Cambridge, CB1 8RN, UK. interval@medschl.cam.ac.uk.
authors.forename: Simon G
authors.initials: SG
authors.lastname: Thompson
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001771:Blood Banks / Q000706:statistics & numerical data* / Q000600:supply & distribution; D001782:Blood Donors / Q000706:statistics & numerical data* / Q000600:supply & distribution; D046148:Donor Selection / Q000706:statistics & numerical data*; D004739:England; D005260:Female; D006304:Health Status; D006801:Humans; D008297:Male; D008875:Middle Aged; D018579:Patient Selection; D013997:Time Factors; D055815:Young Adult
keywords: Blood donation; Blood donors; Generalisability; Randomised trial; Recruitment; Representativeness