Diagnosis of infection

• • clinical suspicion of infection
  • sample to laboratory
  • laboratory analysis
  • data interpretation
• High quality samples and documentation provide high quality results
• Explore symptoms

  • fever
  • pain
  • speed of onset of redness
  • unilateral redness
  • lymph nodes enlarged
• Explore previous medical history

  • any disease which causes immunodeficiency
  • recent surgery/trauma
  • previous MRSA infection
  • venous insufficiency
  • recent hospital admission/previous MRSA infection
  • recent athletes foot
• medication and allergy history

  • immunosuppressant drugs
  • immunisation history
  • allergies to antibiotics
• social history

  • smoker/alcohol excess
  • IV drug use
  • occupation
  • travel history
  • recreation e.g. river or sea water exposure
  • pets
  • live in nursing home
• family history

  • any other family members unwell
• High quality and relevant sample

  • microorganism or immune response
  • sufficient sample at correct timeline
  • contact lab if uncertain of the tests or sample types
• sample collected well

  • eliminate/minimise contamination
  • clear instructions if the patient is collecting the sample
  • sample prior to antimicrobials if possible
  • labelled with sufficient clinical information
  • consider lab opening times
• Microscopy can be used for bacteria, fungi, protozoa and metazoa but not viruses
• Microscopy

  • fast
  • presumptive result
  • easier when higher numbers
  • not detected doesn't mean not present
• Non selective agar

  • low numbers from expected sterile sites
  • will support the growth of a wide range of organisms
  • used for secondary culture
• Selective and differential agar

  • non-selective sites - skin, throat, faeces
  • looking for certain organism
  • often contain chemical indicator
• High salt is tolerated by staphylococcus
• Mannitol is fermented by S. aureus but not S. epidermis
• Confirmatory tests

  • require pure culture
  • ability to produce enzymes and metabolise sugars
  • commercial kits - species level
• Diffusion test

  • pure culture spread onto non-selective agar
  • antibiotics on filter paper
  • incubate and examine growth
• Dilution test - MIC

  • bacterial culture with different dilutions of antibiotics
  • highest dilution which inhibits bacterial growth
  • can be performed at same time as confirmatory tests
• Sterile sites - interpretation is easy (joints, bloods)
• Non-sterile sites - need an appreciation of what is normal flora e.g. commensal or causative
• Clinical details including timescales on microform will affect what the lab decides to test for and how the clinician interprets and advises on the result.