INTERVIEWS AND DEVELOPMENT OF THE QUESTIONNAIRE
Intensive care survivors with a length of stay longer than 72 hours, attending the post-ICU clinic of a 16-bed mixed ICU in a university hospital at least six months after discharge, were eligible for the study. Using a purposive, maximum variation sampling approach, potential interviewees representative for different ages, admission diagnoses and time from ICU discharge were invited to participate. 14 They were selected by one of the researchers (JM) with regards to willingness and ability to share information during the visit. Sample size was based on data saturation. Between February and May in 2015, interviews were conducted either in the post-ICU clinic or in their home, based on the interviewee’s own choice. Using a semi-structured technique, we explored their present situation, current symptoms or difficulties, quality-of-life, and social function. Interviews were started with open-ended questions, but as they progressed, details about findings were sought for. Once the interviewee could think of nothing further general themes from previous interviews were discussed to potentially evoke memories and details, thus making the interviews inductive. There was no time limitation for the interviews. Additionally, issues from literature, other questionnaires and scales were discussed (Table S1).
As interviews were conducted, they were sorted qualitatively:15 Notes were taken during interviews in addition to verbatim transcription of the recorded interviews. Long quotations were shortened while preserving the core meaning of the issues and care was taken to maintain the wording used by the interviewee. Quotes from notes and transcripts with similar themes were grouped into categories ("domains") and sub-categories ("issues”). Duplicate issues were removed, and the remaining issues were formulated as questions.
 E.g. Quote: “My normal social life doesn’t really work anymore, I get too tired” → Domain: Fatigue→ Issue: Fatigue affecting social life → Question: “Have fatigue affected your social life, in the past month?
Response scales were created to match each question as closely as possible using incidence, prevalence, intensity and agreement when applicable (Table 1).
The time frame used in most response scales was "the last month". Care was taken not to overlap between alternatives and to include "Not applicable" if needed.
Composite questions about domain-specific quality of life and domain-specific future concerns were added at the end of each domain. Empty space and a request for missing issues or other comments were provided after each domain. Questions regarding background characteristics and comorbidities were added at the end of the questionnaire.
All questions were tested with cognitive interviews on additional ICU survivors to ensure that the questions were conceptually clear, easily understood, perceived as relevant as well as comprehensive.16 The additional interviewees were chosen with the same criteria as the initial interviewees, and interviews were recorded as well.
APPLICATION OF THE QUESTIONNAIRE
Eligible patients were adult ICU survivors admitted between February 2013 and December 2015 to one of three mixed ICUs in Sahlgrenska University Hospital, Gothenburg, Sweden (in total 31 ICU beds), and with a minimum ICU length of stay of 72 hours. They all had been discharged from the ICU between six months and three years prior to the study. Exclusion criteria were primary neurological/neurosurgical reason for admission, limited understanding of Swedish as judged by study personnel, no Swedish personal identity number, no Swedish address or phone number, or a secret Swedish personal identity. The non-ICU treated subjects were obtained from the Swedish Population Register, matched for age and sex with respect to ICU survivors returning a completed questionnaire. For the version of the questionnaire addressing the non-ICU treated subjects we removed all questions specific to a previous ICU stay (e.g. Have you had difficulties describing your ICU experiences?) and added one question checking for previous intensive care.
All eligible participants received an initial letter with information about the study, and within a week they received a phone call asking for participation. The questionnaire was sent together with a pre-paid return envelope, and reminder phone calls were made if the questionnaire was not returned within two weeks. The questionnaire was sent to the ICU survivors between April 2016 and October 2017, and to the non-ICU treated subjects between March 2017 and December 2017.
Univariate descriptive statistics are presented as frequencies and percentages for all categorical variables. Continuous variables were screened for normality using Shapiro-Wilks (p > 0.05) and box-plots. For non-normally distributed continuous variables median and range or median and interquartile range (IQR) are reported. Bivariable comparisons were made between ICU survivors and non-ICU treated subjects for all ordered categorical variables and continuous variables in order to identify differences between the groups by applying the Mann-Whitney-U-test. These results are presented as means and mean rank sums and the associated p-value calculated in the Mann-Whitney-U-test. In addition, all the bivariable comparisons for ordered categorical variables were analyzed with Fisher's exact test as a robustness check. Dichotomous variables were also assessed with Fisher's exact test. All tests were two-tailed, and significance level was set to 0.05.
Questionnaires were scanned with Remark Office OMR (Remark Office OMR 10, Gravic Inc, Malvern, USA). Statistical analyses were performed using the IBM SPSS v26 package (IBM SPSS v26 Statistics, IBM, Armonk, USA).