Table 3 Description of the intervention and usual care groups, according to the Template for Intervention Description and replication (TIDieR) criteria [23]

Item 1. Brief name: Provide the name or a phrase that describes the intervention

PLUS-algorithm

Usual ED patient care

Item 2. Why: Describe any rationale, theory or goal of the elements essential to the intervention

LRTIs are a common motivation for ED consultations and stand as the leading cause of inappropriate antibiotic prescription. LUS has an excellent sensitivity and specificity to identify pneumonia and PCT tends to be higher in bacterial and/or severe infections. None of these tools alone is sufficient to optimize antibiotic prescription, while a combined approach could improve diagnostic performance, ensure safety and better guide clinicians.

Usual care represents real life clinical management.

Item 3. What (materials): Describe any physical or informational materials used in the intervention, including those provided to participants or used in intervention delivery or in training of intervention providers

Participants will receive the informed consent form, with detailed information of the project objectives, procedure, risks and benefits.

Participants will receive the informed consent form, with detailed information of the project objectives, procedure, risks and benefits.

Item 4. What (procedures): Describe each of the procedures, activities or processes used in the intervention, including any enabling or support activities

Participants are managed by the physician in charge with the PLUS-algorithm. A pneumonia clinical prediction score (Van Vugt) is calculated automatically by the algorithm in the eCRF, physicians perform a LUS. In case of pneumonia on LUS and/or elevated clinical prediction score, PCT is measured. The algorithm recommends antibiotics in case of elevated PCT (≥ 0.25 μg/L). In patients with features of pneumonia on LUS and a low PCT, severity score (DS-CRB-65) is calculated. The algorithm recommends antibiotics in patients with a high severity score.

Participants are managed as usual. The physicians decide on antibiotic prescription as they usually do (usually based on a chest X-ray and blood tests)

Item 5. Who provided: For each category of intervention provider (for example, psychologist, nursing assistant), describe their expertise, background and any specific training given

Physicians in charge in the ED, have received a one-hour presentation on the scientific rationale behind the algorithm, a two-hour e-learning training on LUS, and a one-hour theoretical and two-hour practical training by an experienced instructor in LUS

Physician in charge in the ED have received a one-hour training on epidemiology and management of CAP in Switzerland based on Swiss guidelines [26] as well as explanations on the background of the study

Item 6. How: Describe the modes of delivery (such as face-to-face or by some other mechanism, such as internet or telephone) of the intervention and whether it was provided individually or in a group

Face-to-face individual meeting

Face-to-face individual meeting

Item 7. Where: Describe the types of location where the intervention occurred, including any necessary infrastructure or relevant features

ED

ED

Item 8. When and how much: Describe the number of times the intervention was delivered and over what period of time, including the number of sessions, their schedule, and their duration, intensity or dose

The intervention is delivered once during ED management.

PCT will be repeated after 6 to 24 hours in a subgroup of patient (all admitted patients who did not receive antibiotics because of a low PCT value and in those in whom the algorithm was overruled). In case of worsening disease within 28 days of enrolment in patients who did not receive antibiotics, reassessment will be done, and PCT will also be repeated.

Once during ED management

Item 9. Tailoring: If the intervention was planned to be personalized, titrated or adapted, then describe what, why, when and how

Not applicable

Not applicable

Item 10. Modifications: If the intervention was modified during the course of the study, describe the changes (what, why, when, how)

Not applicable

Not applicable

Item 11. How well (planned): If intervention adherence or fidelity was assessed, describe how and by whom; if any strategies were used to maintain or improve fidelity, describe them

The study monitoring plan includes the evaluation of study center compliance to the protocol and, non-adhesion to the algorithm will be escalated to the sponsor as a study deviation.

Non-adhesion to the algorithm AB recommendation also triggers automatic alerts to the sponsor for direct feedback to the physician in charge.

Not applicable

Item 12: How well (actual): If intervention adherence or fidelity was assessed, describe the extent to which the intervention was delivered as planned

We will describe if the intervention was delivered as planned. Patients with incomplete intervention on day 0 will only be analyzed in the intention-to-treat analysis.

Not applicable