IDFN On Call: GPCs in the Blood

By Nathaniel Nolan

It is 10:43 PM and you have just laid down in bed when the on-call phone rings. A nervous intern is on the other end. He just got culture results back for one of the day-time admits. Two sets of blood cultures are growing gram-positive cocci, in clusters. Molecular testing is currently on-going. The patient is currently on the medical ward, but appears ill. The intern asks for next steps and recommendations.

Background:

Bacteremia is a serious and life-threatening illness if left untreated. Staphylococcus aureus bacteremia carries a mortality rate of approximately 20-30%, and can be higher depending on patient population. It is common for the on-call ID fellow to receive middle of the night phone calls, particularly from house staff looking for backup and guidance. While most calls can be handled without needing to emergently go in to see the patient, key details should be gathered to ensure appropriate therapies are recommended.

When a call comes in regarding Gram-positive cocci, your mind may first go to Staphylococcus aureus. This is a common pathogen with high mortality. However, there are many other disease-causing gram-positive cocci, including the Enterococci and Streptococci. When the organism is reported to be in clusters, S aureus, the Coagulase-negative Staphylococci (including S lugdunensis, S epidermidis, etc.), Micrococcus sp. and Aerococcus sp. should be on your differential. Even though Enterococci and Streptococci are more often in pairs or chains, if enough bacteria are on the gram stain, these can appear clumped and be mislabeled as clusters.   

Gram Stain of gram-positive cocci in tetrads identified as Micrococcus luteus. Source

Data to acquire:

When investigating any bacteremia, one of the first questions one should ask is about the source. One of the most common sources of gram-positive bacteremia, specifically S aureus, is the presence of intravascular devices, such as central lines and other intravenous catheters. Time to source control is strongly associated with clinical outcomes in S aureus bacteremia. If possible, workup and management of bacteremia source should start at the time of bacteremia identification. At the least, teams should investigate all intravenous catheters overnight with recommendation of removing any that might be infected.    

It is also worth asking about the results of any rapid diagnostic tests. In the advent of molecular microbiology, there are an array of rapid diagnostic tests that can quickly glean genotypic identification and identify the presence of resistance genes. Some examples of these systems are the Verigene, GeneXpert, and BioFire systems. These typically have a turn around time of a few hours, depending on lab staffing. Data available from these systems, such as the presence of mecA, vanA, or vanB, can help with initial antibiotic selection.

Key Treatment Decisions:

Key treatment decisions for the patient typically include initial (or empiric) antibiotic therapy. If there is rapid diagnostic data, such as species or resistance gene, there may be more comfort in using specific antimicrobials. If little data is known, an anti-MRSA therapy should be used, with plans for de-escalation if able. Typical first line regimens include Vancomycin and Daptomycin, both dosed based on renal function and body weight. Ceftaroline, a cephalosporin, also has MRSA coverage but is not routinely recommended for empiric treatment given lack of data. Similarly, Linezolid covers MRSA but is typically not recommended as empiric therapy.      

Anticipating next steps:

If culture data is not conclusive for true bacteremia (i.e., one set of cultures of gram-positive cocci without an identification), it can be helpful to repeat cultures prior to initiation of therapy. If identification reveals coagulase-negative staphylococci in a patient who otherwise does not have a source, negative repeat cultures taken pre-antibiotic may support a conclusion of blood culture contamination (and early discontinuation of therapy).

Despite any guidance provided over the phone, patients with concern for (or confirmed) S aureus bacteremia should have an infectious disease consult. Studies have suggested that ID consultation improves care for patients with Staphylococcus bacteremia.

Though not emergent, patients with S aureus bacteremia will need at least a transthoracic echo and possible transesophageal echocardiogram. 

Uploaded on 7/28/22

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