Aim: The overall aim of this thesis was to explore the onset of sepsis and ambulance clinicians’ initial assessment of patients with sepsis.

Methods: The thesis consists of four studies where the onset and assessment are studied from different perspectives Studies I and II are qualitative descriptive studies. In study I, 24 individual and dyadic interviews with patients and/or family members were conducted. In study II, 14 individual and dyadic interviews with ambulance clinicians were conducted. Thematic analysis based on descriptive phenomenology was used in studies I and II. Studies III and IV are quantitative observational studies. In study III, 327 medical records of patients with sepsis who were cared for by ambulance clinicians were examined. In study IV, serum lactate and plasma glucose were measured prospectively in 1842 patients. Different statistical methods were used in studies III and IV to analyze the data.

Results: The onset of sepsis was experienced as an insidious illness, difficult to comprehend as something critical. As symptoms and signs worsened, it was still difficult for patients and family members to understand the seriousness of the illness. As the illness progressed, family members became more worried, and at some point realized that the patient was seriously ill. Several aspects were important for ambulance clinicians to assess and identify sepsis patients. Their assessment was influenced by previous experiences in their search for clues regarding the severity of patients’ conditions. Ambulance clinicians described that their assessment involved an ongoing search for information and a need to discuss with colleagues. Patients transported by ambulance who had more adverse outcomes following sepsis were more often found to have altered mental status, lower oxygen saturation, lower body temperatures, and higher serum glucose prior to entering the emergency department. Elevated plasma glucose and serum lactate were not found to be associated with an increased likelihood of sepsis when measured by ambulance clinicians.

Conclusion: There are challenges related to the onset and assessment of patients with sepsis, as no specific symptoms or signs can be used for early sepsis identification by ambulance clinicians. This places demands on ambulance clinicians’ clinical reasoning and ability to identify these patients. Listening to patients’ and family members' stories about the course of the disease, using previous experiences, discussing with colleagues, and observing the surrounding are important for the assessment and identification of  patients with sepsis.