SAB - Staphylococcus aureus bacteremia

Thank you to Dr. Panisko and Team 3 for presenting a case of Staphylococcus aureus bacteremia (SAB) at morning report yesterday.

SAB is a common and nteresting clinical problem that is encountered frequently on the general internal medicine ward.  It is very rare to see a general internist who does not enjoy managing SAB!

In Canada and the USA, the incidence of SAB ranges between 20 and 40 cases per 100 000 population.  Not surprisingly, a substantial portion are hospital acquired (40%).  Hospital acquired SAB is more commonly related to intravascular devices, as well as hospital acquired pneumonia. Complicating the entire picture is the rise of MRSA (both community acquired and healthcare associated) that makes the management of SAB somewhat more complicated.

Staph. aureus is one of the most virulent pathogens affecting humans, and the overall case fatality ranges between 20 - 30%.  Not only does it have a high mortality, but also because of it's virulence it has a high associated morbidity.  Staph. aureus can cause widely metastatic infections producing abscesses in the lungs, spine, kidneys as well as heart valves.  It is certainly an organism that one does not take lightly.

A collection of clinical pearls that I've gleaned over the years of working at UHN regarding Staph. aureus bacteremia.

1.) Staph. aureus in the blood cultures should always be taken seriously 

The presence of "gram positive cocci in clusters" should never be ignored or written off.  Given the extremely high mortality, when a clinician receives this result (usually by telephone call), repeat blood cultures and prompt initiation of therapy (typically empiric Vancomycin) should begin.

2.) Search for a source / signs of metastatic spread 

I believe one of the reasons general internists respect this clinical problem is because SAB forces the clinician to perform a thorough history and physical exam.  Looking for peripheral signs of endocarditis (embolic phenomenon), palpating for bony tenderness related to osteomyelitis of the spine, listening for a murmur, and examining the skin for signs of a soft tissue infection should all be performed.  Given the high mortality, a search for a source should ensue and any intravenous or intra-arterial lines should be removed.  Many times patients need imaging to identify occult abscesses.

3.) Consult infectious diseases (ID) - mandatory at UHN / Mt. Sinai

There have been a number of recent studies published that have shown mortality benefit for patients associated with prompt consultation with ID consultants.  Most of the mortality benefit is thought to be related to a number of clinical factors.  When ID is consulted, patients are more likely to have repeat blood culture sampling, more echocardiograms (both TTE and TEE), more effective and appropriate initial therapy, and typically longer courses of antibiotics.  The NNT for consulting ID and preventing death in SAB is 10.

4.) Staph. aureus in the urine should prompt a search for bacteremia 

 Dr. W. Gold has taught me many things about medicine, but this pearl has yielded a diagnosis of SAB on at least two occasions so far!  Occasionally, a urine culture that has been drawn as apart of a work up for "sepsis NYD" or "fever NYD" will yield the growth of Staph. aureus.  This does not typically represent a Staph. aureus UTI as this is a very rare phenomenon that is seen in patients with instrumentation or recent urological procedures.  When a clinician observes staph. aureus in the urine, this should prompt the collection of blood cultures as the cocci are very small and can be shedding from the blood, and filtered through the glomeruli into the urine.

5.) A transthoracic echocardiogram (TTE) is not sensitive enough to rule out endocarditis 

The sensitivity of TTE to detect endocarditis is thought to range between 65 - 80%.  This is not sufficient to rule out a vegetation in a patient with a high pre-test probability for endocarditis.  The presence of persistent bacteremia, non-resolving fevers, or other stigmata of endocarditis should make one ponder the need for a trans-esophageal echocardiogram (TEE).  Luckily, by involving an ID consultant this clinical dilemma can be discussed and appropriately managed.

6.) Antibiotic choice will vary depending on the patient, the presence of MRSA, and the ID consultant!

Some ID consultants (purists?) will recommend that all MSSA (methicillin sensitive staph. aureus) bacteremia be treated with high dose cloxacillin IV for 4-6 weeks, where as other ID consultants are not so dogmatic and will vary between cloxacillin or cefazolin.  There are studies that have been conducted recently by clinicians in the ID department at the University of Toronto to try and provide an answer to this question (Clox or Ancef?).  For some patients, receiving an IV infusion every 4-6 hours is not practical or feasible by their community care services.  Outpatient Vancomycin therapy for patients with MRSA SAB can also be anxiety provoking as the need for drug monitoring and potentially dose adjustments can complicate the matter.  Again, discussion with an ID consultant, community care, and close follow up are always recommended.

Reference:
Management of Staphylococcus aureus bacteremia and endocarditis: progresses and challenges. Winfried V. Kern. Current Opinion in Infectious Diseases. 2010. 23:346-358.