Infection of Unknown Site (scroll down for treatment guidelines)

Empiric antibiotic therapy should be directed to the most likely source of infection in the patient. Use this guidance for initial short-term antibiotic management only if the likely source of infection remains unknown following clinical assessment.

Send at least 2 sets of blood cultures and other specimens as considered appropriate to microbiology.


Category A

Patients with mild to moderate infection may not need empiric antibiotic therapy pending further assessment and results of microbiological and other investigations


Categories B + C

Patients with moderate to life threatening infection.

Early treatment of unwell bacteraemic/septicaemic patients with an appropriate antibiotic improves outcome. Empiric antibiotic (or other anti-infective) therapy is therefore likely to be indicated for these patients. Note that other interventions (e.g. surgical, radiological, critical care) may be required in addition to antibiotic therapy for successful management of some infections - see the Sepsis Six


Consider the following factors regarding initial antibiotic treatment of these patients. If in doubt discuss with microbiologist

  • Previous microbiological results from the patient e.g.
    Does the patient have a past history of : MRSA or multi-resistant “coliforms” e.g. ESBL producing organisms


  • Risk factors for MRSA include:
    Previous MRSA colonisation /infection, care home resident, presence of an invasive medical device (e.g. intravascular or urinary catheter), chronic skin breaks ,hospitalisation in the previous year


  • Risk factors for ESBL producing organisms (see UTI section) include:
    Age >60years with serious underlying medical conditions,recent hospitalisation or broad spectrum antibiotic therapy (especially cephalosporins or quinolones),recurrent urinary tract infection, urinary catheterisation, ESBL producing organism previously isolated from the patient.


  • Community or hospital acquired infection?
    If an in-patient for >48 hours or has been an in-patient in the previous 7 days consider the infection likely to be hospital acquired

Treatment Guidelines 

Likely community acquired, no risk factors for MRSA or ESBL producing organisms


Community or Hospital acquired, past history of MRSA or intravascular catheter in place


Community or Hospital acquired, past history of infection with ESBL producing organism


Community or Hospital acquired, past history of infection with MRSA (or intravascular catheter in place) and ESBL producing organism