For this study, authentic patient cases representing victims of real MCIs have been collected and made into patient cards, utilized in disaster management courses [15,16,17,18]. The Medical Response to Major Incident (MRMI) is a module-based simulation course that uses authentic and scientifically validated patient cards (MACSIM®), derived from victims of MCIs [19]. The patients have a necessary medical history, critical input, and output data for prehospital and hospital assessments [15,16,17,18].

Five patient cards were selected by the representative for the MACSIM® system The study administrators had no involvement in this selection. Five patient cards were chosen because of the results of the previous study, in which the number of patients per healthcare provider defining an MCI was set to three [5]. It was assumed that five patients were enough to create an MCI, although no time pressure could be applied to the participants due to the study design. Figure 2 shows all MACSIM® patients, which were given to each participant. Participants received a normal MACSIM card initially to familiarize themselves with the characteristics of the card, and relevant signs before triaging the study´s five patients (Appendix A, Appendix B, Table 1).

The researchers actively engaged in a multinational research group, Research Alliance in Disaster and Emergency Medicine (RADEM) were contacted regarding possible participation. Additionally, researchers from countries previously engaged in the evaluation of TTT were invited. Purposefully, these commonly accepted subject-matter experts recruited new participants from their research networks. Although the current study aimed to compare the results of the traditionally used primary triage model in participating centers with TTT, the type of triage system used in each country was not an inclusion criterion.

Physicians, nurses, and paramedics actively engaged in primary triage in the fields of prehospital, emergency care, and or disaster medicine. Staff with no experience in triage procedures were excluded.

Representatives from eight countries (India, Italy, New Zealand, Poland, Saudi Arabia, Sri Lanka, Sweden, and Thailand), called subject-matter specialists, joined the study, and together recruited 164 participants from their countries. One of the participants did not conduct the second round, thus, was excluded. Another 5 participants were excluded due to incomplete data (Table 2).

Each subject-matter specialist recruiting national participants for the study was informed regarding the study and received information about MACSIM® cards, and how to interpret the cards [15,16,17,18,19]. Moreover, they were informed about the study design and the period of study (November 2022-March 2023). Participants were not familiar with TTT and received lectures, face-to-face information, and/or written information. They had opportunities to test the tool and ask questions from each country leader. Each participant had to triage five (5) patients (numbered 1 to 5) by their national or institutional primary triage system in the first round and the TTT tool in the second round (the same patient cards). The time between the two assessments varied between 2 and 21 days (Table 2). This deliberate spacing aimed to serve the purpose of a washout period, ensuring that the effects of prior triage experiences did not influence subsequent assessments. Furthermore, neither the participants nor the country leaders were informed about the triage outcomes/keys. The results of each round were compiled and transferred to an Excel file and sent to the lead author. Each participant received full information regarding the study and how to participate and act in each round of the study. The results obtained after two triage rounds were to be statistically analyzed by one of the authors, with experience in statistical analyses, using validated statistical methods.

Each country leader was asked to comment on the performance of the participants and the feasibility and ease of understanding and performing the triage, using TTT. Participants could comment on the tool and its feasibility, and any concerns experienced during the trial. These qualitative data were collected by each country leader and were sent to the lead author for analysis and further assessment.

The statistical analysis was pursued to observe similarities or differences between the two systems (i.e., own system vs. TTT). Thus, testing whether one mutual triage system (TTT) can be used as a universal system. Professional statisticians validated the process and results. The diagnostic accuracy of the triage systems in the first round and TTT in the second round were assessed using a 2 × 2 contingency table. Cases were dichotomized into two groups: Immediate/Red/P 1 and non-immediate. The sensitivity, specificity, accuracy, positive predictive value (PPV), negative predictive value (NPV), over-triage, under-triage, positive likelihood ratio (LR+), and negative likelihood ratio (LR-) were calculated as compared with the reference standard, i.e., the outcome and the corresponding pre-determined color of the MACSIM® cards unknown to the respondents. The over-triage was calculated as 1- PPV based on the Cribari matrix method whereas under-triage rates were calculated as 1-sensitivity [20]. Wilson's method was used to calculate the 95% confidence intervals (CI) for all estimates of diagnostic accuracy (Tables 3, 4).

Additionally, a statistical analysis to assess the diagnostic systems in terms of over-triage, under-triage, and accuracy was conducted, using a linear mixed effect model (lmer4). This assured accountability for the correlation between observations within these groups, including countries and patient cases as random effects to account for any variability between triage systems in terms of their diagnostic performance. Two models were fitted, one comparing TTT in the second round with all national/institutional systems in the first round. Secondly, a per-system comparison between TTT and the most occurrent triage systems. P-values < 0.05 were considered statistically significant for fixed effects i.e., prediction of outcome for TTT. All statistical analyses were performed using R software version 4.2.2 (R Foundation for Statistical Computing, Vienna, Austria).

The study follows Swedish law's ethical guidelines and principles and is exempted from ethical approval requirements. In Sweden, where the study is conducted, ethical approval is mandatory if the research includes sensitive data on the participants such as race, ethnical heritage, political views, religion, sexual habits, and health or physical interventions or employs a method that aims to affect the person physically or psychologically [21]. Written and verbal information, including the study’s design, purpose, and data use, were provided for each participant. However, some countries required their ethical approval: The University of Otago, Wellington, ref D22/350 issued New Zealand ethics approval. Thailand ethical approval was issued by Mahidol University, Bangkok, ref MURA 2022/753.