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 Staphylococci Gram stain
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. See Blood Culturing.
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.