Question 14

College answer

Syllabus topic/section:

2.1.3 Sepsis and infections / Sepsis and septic shock: L1

Discussion:  

This question on a core topic was well answered overall. Less successful answers veered towards exploring the presence of sepsis rather than the differentiation of true culture positive result from a contaminant. Answers in a tabular form could have provided clarity. Mention of particulars of CFUs, stating which organisms are more likely to be contaminants, comments on reproducibility of results and timing of cultures are features displaying the acceptable standard of knowledge required.

Discussion

A "discuss" question often implies the inspection of a topic for pros, cons, and some kinds of "controversy". In this scenario, however, this would not have been a sensible answer style. One may even have ordered it into a table structure, but the lazy author has decided to present it as an unordered list instead, as the use of rows would have resulted in weird behaviour for some of the sections (eg. where the possible contaminant organisms would have to be listed twice, as contaminants AND as possible infectious agents with patient-risk-related caveats).

Features suggestive of contamination:

• Poor collection technique (eg. from an old cannula, or from a CVC, or multiple attempts at venipuncture)
• Low pre-test probability (i.e.the patient does not look septic)
• The organism is a common contaminant:
  • Coagulase-negative staphylococci
  • Corynebacterium sp.
  • Bacillus species other than Bacillus anthracis
  • Propionibacterium acnes (these days, Cutibacterium acnes)
  • Micrococcus sp.
  • viridans group streptococci
  • Clostridium perfringens

Features suggestive of clinically relevant bacteremia:

• High pre-test probability of bacteremia
• High colony count (>100 CFU/ml)
• Fast culture time (<16 hrs)
• Reliable sampling technique:
  • Alcohol-based disinfectant
  • Disinfection of the tops of the blood culture bottles
  • Using a second clean needle to transfer blood
  • Taking cultures from a new skin puncture
  • Two sets of cultures
  • The use of trained phlebotomists
• The organism is typical, i.e. highly unlikely to be a contaminant:
  • Streptococcus pyogenes
  • Streptococcus agalactiae
  • Listeria monocytogenes
  • Neisseria meningitidis
  • Neisseria gonorrhoeae
  • Haemophilus influenzae
  • Bacteroides fragilis
  • Candida sp.
  • Cryptococcus neoformans
• Or,  the organism is from a list of common contaminants, but the patient has risk factors which suggest it should be taken seriously, eg:
  • Malignancy
  • Neutropenia, or another form of immunocompromise 
  • Intravascular devices or prostheses
  • Frequent skin breaches, eg. dependence on haemodialysis via fistula