**1. Introduction**

Kidney transplantation is considered to be the optimal form of renal replacement therapy and has a positive impact on quality of life, survival rates of the recipients, and overall is considered cost-e ffective [1]. Due to organ shortage and longer waiting time, death on the waiting list is a serious concern and criteria for suitable organs have been extended. There are several advantages of live donor transplantation compared with deceased donor transplantation including lower risk of rejection, reduced waiting time for transplantation, and improved allograft and overall survival [2]. The frequency of live kidney donation is stable in the United States (US), while increasing in the Eurotransplant area and in the United Kingdom (UK) over the last 15 years. Despite these e fforts there are currently 94,621 patients on the kidney waiting list in the US according to the United Network for Organ Sharing (UNOS), 10,791 (at the end of 2018) potential recipients in the Eurotransplant area and as of March 2019, approximately 5000 patients were waiting for a kidney transplant in the UK. Analysis of di fferent surveys among the public revealed barriers towards live kidney transplantation [3], and strategies to overcome these barriers are necessary to increase the number of transplants.

Google TrendsTM generates data on spatial and temporal patterns according to specified keywords. A study comparing the reliability of Google TrendsTM in two settings, more common diseases with low media coverage and less common diseases with higher media coverage, found that Google TrendsTM seems to be influenced by media presence rather than by true epidemiological burden of one disease [4]. Several studies using Google TrendsTM data have been conducted recently. One of these investigated the influence of meteorological variables on relative search volumes for pain and found that selected local weather conditions were associated with online search volumes for specific musculoskeletal pain symptoms [5]. Analysis of Google TrendsTM search volume queries not only holds grea<sup>t</sup> promise in medicine, but also in other areas of research. Analysis of northern Europeans' (Finland, Germany, Norway, Ireland, and the UK) web searching behavior on Mediterranean tourist destinations revealed a relationship between thermal conditions and the searching behavior, and the authors observed no time lag between the prevalence of thermal conditions and searching of the keywords [6].

In transplant medicine, public awareness is key to promote discussion around organ donation, both live and deceased. In the current study, we investigate the public interest in kidney transplantation using data on Internet search queries extracted from the Google TrendsTM tool.

#### **2. Materials and Methods**

#### *2.1. Retrieving Transplantation Numbers for UNOS, ONT, and Eurotransplant*

Data were retrieved by accessing the respective websites of the transplant organizations ((https: //unos.org) for the UNOS, (http://www.ont.es) for the Organización Nacional de Trasplantes (ONT), (https://www.eurotransplant.org) for the Eurotransplant countries, and (https://www.nhsbt.nhs.uk) for the UK.

Information about live and deceased donor kidney transplantation over a period of 15 years (2004–2018) for the following countries was extracted from the web pages as stated above: United States of America (UNOS), Spain (ONT), Austria, Belgium, Croatia, Germany, Hungary, Slovenia, and the Netherlands (belonging to the Eurotransplant countries), and the UK (NHS Transplant Register).

#### *2.2. Retrieving Google TrendsTM Data on Kidney Transplantation*

The Google TrendsTM tool (https://trends.google.com/trends/) was used to retrieve data on Internet user search activities in the context of kidney transplantation. Google TrendsTM is a freely accessible tool

that enables researchers to study trends and patterns of Google search queries [7]. It was implemented in 2004 and data on Internet search queries are available since then on a monthly basis. Google TrendsTM expresses the absolute number of searches relative to the total number of searches over the defined period of interest. The retrieved Google TrendsTM index ranges from 0 to 100, with 100 being the highest relative search term activity for the specified search query in any given month [7]. Thus, a search index of 50 indicates that the search activity for kidney transplantation was 50% of that seen at the time when search activity was most intense [7].

Worldwide Google TrendsTM indices were retrieved between January 2004 and December 2018 using the search term "kidney transplantation". We retrieved Google TrendsTM indices for the US, Spain, the following European countries being part of the Eurotransplant network, namely Austria, Belgium, Croatia, Germany, Hungary, Slovenia, and the Netherlands, and the UK. No Google TrendsTM indices could be retrieved for Luxembourg. Whereas the worldwide search was performed in English, the individual searches in the respective countries were performed in the o fficial languages (see Table S1).

#### *2.3. Data Analysis*

Annual average Google TrendsTM indices were calculated based on the monthly data downloaded from the Google TrendsTM webpage. Time-lag correlations between transplant numbers and Google TrendsTM indices were calculated using the ccf function of the tseries R package using a time lag between −3 and +3. The ggplot2 R package was used to generate all graphics. R version 3.4.1 was used for all analyses.

None of the queries in the Google database for this study can be associated to a particular individual. The database retains no information about the identity, Internet protocol address or specific physical location of any user. Furthermore, any original web search logs older than nine months are anonymized in accordance with Google's privacy policy (www.google.com/privacypolicy.html).
