The aim of this diagnostic accuracy study is to develop a clinical prediction rule based on signs, symptoms, patient characteristics and blood tests, to be used in ambulatory care to help physicians safely rule out a serious infection in an older patient.

It will be performed in general practices and emergency care departments across Flanders (Belgium).

This is a cross-sectional diagnostic cohort study in older adults presenting to ambulatory care, including a follow-up period of 30 days. This study will be used to construct an algorithm consisting of clinical features and blood test results to identify serious infections.

The study will run in ambulatory care in Flanders. Recruitment is expected to last 2.5 years, with each patient entering the study only once. Older adults will be approached for possible participation in the study by practice staff. There is only one study visit.

Those who agree to participate via written informed consent will have demographics and clinical features recorded. A blood sample will be taken in all participants on study entry. Follow-up information for all participants will be collected using electronic medical record notes (EMD) and patient diaries.

Treatment and other management decisions will be left to the treating physicians' discretion.

Inclusion Criteria:

• Aged 65 years or older
• Presenting to a general practice with an acute illness with a maximum duration of 10 days since onset
• Suspicion of infectious disease (this may include suspicion of an acute exacerbation of COPD)
• Patient or proxy of the patient is willing and able to give informed consent for participation.

Exclusion Criteria:

The participant may not enter the study if any of the following apply:

• Too clinically unstable to waste time on study procedures
• Indwelling catheter in situ
• Immunocompromised or currently receiving medication for systemic immune-suppression, including corticosteroids
• Being admitted to hospital for longer than 24 hours less than 7 days before study entry, or having undergone surgery in the previous 30 days
• Living in a nursing home (both short term and long term stay) (only in general practice cohort)
• Patient was already included in the study

All acutely ill adults aged 65 years or older presenting to ambulatory care in whom a physician suspects an infection will be eligible. Ambulatory care is defined as in-hours general practice and emergency care.