Empiric therapy of bacteremia

Bacteremia is a serious finding requiring urgent assessment. It is either a result of direct bacterial infection of endovascular tissue or, much more commonly, a reflection of uncontrolled bacterial infection at a specific body site. It is not a diagnosis but often leads quickly to or confirms one. For more information concerning clinical aspects of sepsis see Severe Infections and Sepsis Syndrome.

This document is designed to help with the initial response to a first report of bacteremia from the microbiology laboratory.

When a blood culture bottle is “flagged” as positive by the automated blood culturing instrument, the technologist performs a Gram stain and phones the findings to the ward or ordering physician.

There are a limited number of possibilities. The organism will be Gram negative or Gram positive. It will be a coccus or a bacillus. For some types, extra information concerning specifics of shape and cellular arrangement will also be given.

Gram negative bacilli

These may be described as “resembling coliforms”, “resembling fusiforms” or “thin”

Resembling coliforms

The qualification “resembling coliforms” is added if the technologist is confident that the organism viewed is likely to be a member of the family enterobacteriaciae see Gram negative bacilli. E.coli (the prototypic enterobacteriaciae) is one of the most common organisms found in blood cultures.

Other coliforms include Klebsiella spp., Serratia spp., Proteus spp., Citrobacter spp., and Enterobacter sp.. The last four genera are sometimes referred to as the “SPICE” group and are distinguished by higher levels of resistance to antibiotics, esp. Beta-Lactam antibiotics. These resistant organisms are more often seen in complicated hospital-associated infections esp. in ICUs.

Sources of coliform bacteremia

The most likely sources of coliform bacteremia discovered in a patient that has presented with an acute condition to hospital are:

  • The urinary tract i.e. pyelonephritis or prostatitis as cystitis is not usually associated with bacteremia.

  • The biliary tract i.e. ascending cholangitis generally secondary to obstruction by stones or tumors. Simple cholecystitis is less likely to be associated with bacteremia.

  • The colon Any process that results in leaking of colon contents provokes abscess formation that can be a source of bacteremia. Common conditions include appendiceal abscess and diverticulitis.

Often, by the time the blood culture is positive, the source is clear by symptoms, signs and other lab tests. Urinalysis and urine cultures may be positive. Abdominal surgery may have already been performed or is planned.

Likely effectiveness of empirical therapy

This is determined by assessing likely source, prior microbiology, local susceptibility patterns and response to therapy if already initiated.

Likely urinary source

And patient is already on one of:

  • Ceftriaxone
  • Piperacillin tazobactam

These are likely effective empirical choices except if the patient has recent evidence of resistant organisms (e.g. SPICE group) for which a carbapenem is indicated.

Therapy not initiated:

Ceftriaxone is an appropriate recommendation if resistant organisms have not been identified in the past.

Therapy with ceftriaxone or piperacillin tazobactam is ineffective:

Imipenem should be recommended. Surgical care may be required. (e.g. perinephric abscess, obstructed ureter)

Likely Biliary or Colon source

And patient already on:

  • Piperacillin tazobactam
  • Ceftriaxone

These are effective agents. Piperacillin tazobactam is often advocated because of excellent biliary levels and activity against some enterococci and anaerobes commonly found in GI infections. Ceftriaxone also achieves good biliary levels and is often given with metronidazole. If the patient is on ceftriaxone alone advise addition of metronidazole.

Therapy not initiated:

Piperacillin tazobactam is appropriate.

Therapy with ceftriaxone or piperacillin tazobactam is ineffective:

Imipenem should be recommended. Surgical care may be required.

See Therapy Recommendations Cholecystitis-Cholangitis and Therapy Recommendations Diverticulitis

Gram positive cocci

These will be described as “in clusters”, “in chains” “in pairs”, or “in pairs and chains”.

It is usually quite easy for the technologist to identify the arrangement of Gram positive cocci and the report “Gram positive cocci in clusters” strongly suggests a species of Staphylococcus will be identified in the culture.
See Notes-Staphylococci

The initial evaluation of this finding is a determination of the likelihood that the organism is S. aureus in which case it is a true pathogen or one of the Coagulase negative staphylococci (CNS) in which case it is quite likely to be a skin contaminant.

Lab differentiation of S. aureus and CNS

A tube coagulase test will be performed on a centrifuge-concentrated “pellet” of the microorganisms in the bottle and is usually positive within 2 hours if the organism is S. aureus. The report will be updated to read “S. aureus isolated – presumptive identification”.

If the tube coagulase test is negative for 6 hours and initial growth on the plate is consistent with CNS (often, but not always) a report “Coagulase negative Staphylococcus species isolated – presumptive identification” will be issued.

The pelleted material is also inoculated onto special media that rapidly identifies MRSA. This chromogenic agar may be referred to as a “chrome plate” by laboratory staff. It is commonly positive within 6 hours and if the isolate is coagulase positive (i.e. is S. aureus) the report will be updated to “Staphylococcus aureus MRSA Isolated – presumptive identification”.

Findings suggestive of CNS being a contaminant

The isolate is found in only one culture
This is one of the primary reasons that two different venipunctures are required when performing blood cultures.

If CNS is found in only one culture of multiple the final report will reflect the likelihood of it being a contaminant. “Routine susceptibilities not performed. Culture results may represent contaminants or true pathogens. If clinical relevance is determined consult Microbiology for further workup. Specimen is held for 7 days.”

The clinical scenario does not fit
The majority of true CNS bacteremias are associated with indwelling central venous catheters that have been in place at least several days. Patients with permanent intravascular foreign material are also at risk e.g. prosthetic heart valves, vascular grafts.

Appropriate empiric therapy

For anyone who is acutely unwell and has gram positive cocci in clusters in blood culture, it is prudent to give a dose of vancomycin while waiting for further information. This will cover for the possibility of MRSA and resistant CNS.

Other Therapy Guidelines

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