*2.2. Participants*

The study was based at 14 hospitals in England and Northern Ireland (Supplementary Study Sites). We obtained from each hospital an anonymised list of all individuals who received kidney transplants between 1/4/13 and 31/3/17, stratified by LDKT/DDKT status. Individuals < 18 years at time of transplantation, or who lacked mental capacity according to the Mental Capacity Act 2005 were excluded. We calculated the study sample size using a variable not analysed here: the patient activation variable [24]. The study was designed to detect a 7-point di fference in a continuous measure of patient activation (analysis of this variable not presented here) between LDKT cases and DDKT controls with 90% power, assuming a 5% significance level. The calculation indicated that 170 patients would be needed, and that, therefore, a total of 944 would be needed to allow analyses stratified by Index of Multiple Deprivation rank quintile and allow for 10% missing data. This sample size allows for the detection of a far smaller di fference (0.16 Standard Deviation) for a dichotomous exposure or between 6–8% for a categorical outcome [24]. We performed stratified random sampling to select, on average, 110 LDKTs and 110 DDKTs from each site, weighted by the number of transplants performed at each study site. Sex and 5-year age group strata matched sampling was used to try to ensure a similar sample distribution by age and sex.

Between October 2017-November 2018, collaborators at study sites mailed paper questionnaires to participants. Questionnaires were accompanied by an invitation letter, a return postage-paid envelope, and a patient information sheet. A website-address was provided for participants who preferred to complete the questionnaire online. Non-responders were sent a second questionnaire after 4–6 weeks. We extracted anonymised data from returned paper questionnaires at the University of Bristol, and uploaded these onto a secure REDCap database [25].

#### *2.3. Questionnaire Content*

We have previously reported the development of the questionnaire alongside the findings of a single centre pilot study [24]. Participants were asked to indicate the number of living relatives ≥18 years from a list (spouse/partner, parents, sisters/brothers, children, aunts/uncles, first cousins) as a proxy for their potential living-donor pool. Friends and colleagues were not included, as they contribute very small numbers to the donor pool: between 2006–2017 only 8% of UK living donors were in this category (unpublished data provided by NHS Blood and Transplant to co-author P.B). We asked participants how many relatives had (i) o ffered to donate, (ii) been asked to donate by the respondent, and (iii) started donor assessment. Participants were asked for the reasons why any of the listed relatives could not donate; individuals were asked to tick all options that applied and were allowed to select multiple reasons from the following list, derived from previous qualitative research into barriers to donation [26]: Age—too old or too young to donate; Health—not healthy enough to donate; Weight—too over or underweight to donate; Location—they live too far away to be able to donate; Financial/Cost—the financial impact of donation would be too much; Job—not able to take the time o ff work to donate; Blood group—no<sup>t</sup> the right blood group to donate; No-one to care for them after donation. A box entitled "Other—please give details" was provided for free-text entries. Individuals who the respondent considered suitable for donation but who did not donate because another person did were not considered as "not able" to donate. The responses indicated the patient-reported, and therefore the patient-identified, reasons for non-donation.

#### *2.4. Main Exposure and Other Demographics*

We collected data on self-reported ethnicity, religion, age, sex, and marital status. Participants could select "Would rather not answer" for all demographic questions. Participants indicated their ethnicity according to the UK's Office for National Statistics (ONS) 2011 census categories [27]: White; Asian/Asian British (Indian, Pakistani, Bangladeshi, Chinese); Black/African/Caribbean/Black British; Mixed/Multiple (White and Black Caribbean, White and Black African, Any other Mixed/Multiple ethnic background); Other (Arab, Any other ethnic group). For the religion variable, participants were asked to select one option from the following: No religion; Christian; Muslim; Jewish; Hindu; Sikh; Buddhist; Other. Age was a categorical variable in 10-year age groups.

#### *2.5. Statistical Analysis*

We used descriptive statistics to summarise the characteristics of transplant recipients and their reported reasons for non-donation from family members. Black, Asian and minority ethnic group participants comprised "Asian/Asian-British", "Black/African/Caribbean/Black British", "Mixed/Multiple ethnic groups", and "Other Ethnic group". We derived a binary variable of Black, Asian and minority ethnicity (code = 1) versus White ethnicity (code = 0) as our primary exposure. The Chi2 test was used to compare the characteristics of White and Black, Asian and minority ethnic group participants, and the reasons given for non-donation. We used multivariable logistic regression to describe the association between the reporting of each reason for non-donation with respondent self-reported ethnicity. We used two models: (i) unadjusted and (ii) adjusted for potential confounders. We specified, a priori, potential confounders including sex and age. We considered socioeconomic position as a mediator on the causal pathway between ethnicity and living donation, rather than a potential confounder: we did not adjust for it in our model as this would result in potential over-adjustment and attenuation of the e ffect of ethnicity. We used robust standard errors to account for clustering within kidney centres. We tested for interactions between ethnicity and age and sex. We identified missing data and described patterns of missingness. We performed both a complete case analysis and a sensitivity analysis using multiple imputation using chained equations to derive 40 imputed datasets per group, for the exposure variable and potential confounders and then combined using Rubin's rules. All statistical analyses were undertaken using Stata 15 [28].

#### *2.6. Qualitative Analysis*

Individuals were able to provide free-text qualitative data responses to the question "Thinking about those people you think could not donate a kidney to you, what are the reasons for this?". All free-text responses from Black, Asian and minority ethnic group respondents were analysed, so no sampling was required. The written free-text responses were typed onto the REDCap database [25]. Free-text responses and participant demographics were then downloaded from REDCap onto an Excel spreadsheet file. NVivo qualitative software was used to facilitate analysis. Data were analysed using inductive thematic analysis [29], as described by Braun and Clarke [30]. After familiarization with the data, sections of text within the responses were coded by assigning descriptive labels. Codes were collated on the basis of shared properties to create initial potential themes, which were then refined. Themes were revisited and finalised during the preparation of the report for publication. Coding and thematic analysis were undertaken independently by both K.W. and P.B. Coding discrepancies were resolved by discussion to enhance rigour and reliability. All themes were reported using a minimum of three illustrative quotes. After completing analysis for Black, Asian and minority group respondents (*n* = 56), a matching number of White participants (*n* = 56) were purposively sampled aiming for diversity in terms of age, sex and socioeconomic status, and qualitative responses analysed for comparison.

The Strengthening The Reporting of OBservational studies in Epidemiology (STROBE) and COnsolidated criteria for REporting Qualitative studies (COREQ) guidelines were used to prepare the manuscript [31,32].

#### *2.7. Ethical Approval and Consent*

We received NHS Research Ethics Committee (REC) (REC reference 17/LO/1602) and Health Research Authority (HRA) approval. A consent form formed the first page of the questionnaire. The study was funded by a Kidney Research UK Project Grant (RP\_028\_20170302). The clinical and

research activities being reported are consistent with the Principles of the Declaration of Istanbul as outlined in the "Declaration of Istanbul on Organ Trafficking and Transplant Tourism".
