Pseudobacteraemia

Definition:

Pseudobacteraemia occurs when bacteria isolated from blood cultures have originated from outside the patient’s bloodstream¹.

It can also be defined as not falling within the realm of a "true bacteraemia". In one study², true bacteraemia was defined as patients in one of the following clinical settings:

1. Patients with the same species isolated from 2 or more sets of blood cultures.
2. Patients with the same species isolated in 1 of initial 2 sets of blood cultures and additional blood cultures, have systemic inflammation response syndrome (SIRS).
3. Patients with a species growing in 1 set of blood cultures, and without an obvious evidence of an infectious source, in the presence of systemic inflammation reaction syndromes, had at least one of the following:
   1. Shock, metabolic acidosis, or disseminated intravascular coagulation.
   2. Indwelling intravascular devices* for more than 48 h.
   3. Receipt of hemodialysis or peritoneal dialysis.

*Including central venous catheters, temporary or permanent pacemakers, or arterial catheters.

Recovered Organisms:

Most reports of pseudobacteraemia involve organisms which are rare or unlikely pathogens, probably because these are more easily identified as being likely contaminants. The majority of reports are of Gram-negative bacteria especially coliforms and pseudomonads. Most of these organisms are inherently resistant to many antibiotics and antiseptics and are able to survive well in the environment producing potential reservoirs of contamination. It is interesting to note that Enterobacter cloacae, Klebsiella oxytoca, Serratia marcescens and Aeromonas hydrophila, which have been implicated in cases of pseudobacteraemia after cross-contamination of citrate bottles, are all able to metabolize citrate as their sole carbon source. The reason for this maybe that Gram positives are frequenty associated with contamination, while all Gram negatives are treated as signifiacnt until proven otherwise.

Sources of Contamination:

• Venepuncture
  • Skin of phlebotomists
  • Mouths of phlebotomists
  • Contammated gloves usei by phlebotomists
  • Contaminated solutions for skin antisepsis
  • Contaminated blood collection bottles
  • Contaminated vial of thrombin used by phlebotomist
  • Contaminated paediatric mist tents
  • Contaminated holders for blood culture tubes
  • Blood gas analysers
• Laboratory
  • Laboratory personnel
  • Contaminated penicillinase
  • Contaminated blood culture media
  • Contaminated blood analysers
  • Contaminated solutions used to disinfect blood culture bottles
  • Contamination of microbiology laboratory during a period of building construction
  • Contaminated blood culture incubators

Pseudo-pseudobacteraemia:³

This occurs when negative blood cultures are in fact reported as positive due to computer error. In this case 14 blood cultures wer incorrectly reported as positive for N. gonorrhoeae.

Prevention:

• Increased awareness of the problem of nosocomial infection by education of venesectors, clinicians, microbiologists and technical staff.
• Any medical personnel who are likely to take blood cultures should be instructed in the correct technique by the infection control team.
• Infection control staff should evaluate procedures from time to time to ensure that correct protocols are being followed.
• Manufacturers of blood culture equipment should ensure that it is sterile.
• In the laboratory, standard operating procedures should be adhered to.
• Manufacturers’ guidelines should be followed, where necessary.
• Automated blood culture analysers should be kept away from vents/windows. The rear vents should be covered.
• Mock controls should be set up to assess sterility of automated laboratory equipment.
• Manufacturers of blood drawing equipment and blood culture equipment should work with microbiologists so that improved designs may reduce the risk of contamination.
• Any suspicion that a commercial product is involved should be notified to prevent further cases.

Investigation:

• There should be routine surveillance of nosocomial infection by the infection control team so that any change or epidemic can be detected.
• Liaison between the laboratory, infection control team and ward staff is essential.
• Any suspicion of contamination should be vigorously pursued.
• Frequent liaison and dialogue between clinicians, microbiologists, ward personnel, infection control team and the laboratory will facilitate an investigation.
• A review of procedures, i.e. all steps in taking and processing blood cultures should be performed. Information should be sought from those involved and enquiries should be done in person to establish precise details of how blood cultures were obtained and processed.
• An early case control study should be included in most cases to compare the exposures of suspected cases and controls. Control patients will usually be randomly selected patients with negative blood cultures during the same period and within the same location in the hospital.

Pseudobacteraemia should be suspected when:

• There is an increased isolation rate of one organism, especially a new or unusual pathogen.
• The patient’s signs or symptoms are not consistent with the organism isolated from blood cultures.
• Junior doctors, under these circumstances, should be advised to request simple investigations, e.g. C-reactive protein, erythrocyte sedimentation rate, white blood cell count, to establish the diagnosis.
• When the positive blood cultures are primary, i.e. the organism is not isolated from other plausible sites.
• High grade bacteraemias in the absence of any signs of endocarditis when most or all blood culture sets are positive.
• The laboratory should identify all blood culture isolates to species level, thus facilitating recognition of an outbreak or pseudo-outbreak.
• The laboratory should do complete susceptibility testing which may be extremely useful as a typing method.
• Blood culture isolates should be saved for at least 3-6 months so that they are available for retrospective analysis.

Pseudobacteraemia is largely preventable and should not be viewed as an inevitable consequence of medical progress. Laboratories should maintain careful surveillance of culture results, and unusual organisms or patterns of isolation should be continuously reviewed. Since many occurrences of pseudobacteraemia result from faulty aseptic technique, improved staff education will reduce the incidence of such episodes.

AMH.