“Susceptibility testing for Staphylococcus aureus in community patients. Just in case..?”

You have isolated a Staphylococcus aureus from a superficial wound swab from a community patient. What antibiotics do you need to test against?


If you are fortunate enough to have Automated Susceptibility testing (Vitek, Phoenix etc) you may test the isolate against a panel of 20 different Gram positive antibiotics at a not unsignificant cost. You will also be testing against a lot of antibiotics that are never going to be used to treat that patient.

If you perform anti-microbial susceptibility testing mainly by disc diffusion on agar plate, then you will likely test the Staphylococcus aureus against 6 or 7 antibiotics of your choosing. Again, in the vast majority of patients, some of these antibiotics will neither be reported nor used.

In the laboratory I work in, we cut the number of antibiotics routinely tested for a Staphylococcus aureus isolate from six to three, of which we routinely report two of them to the clinicians. In the three years that we have had this policy we have not had one negative comment from our laboratory users. (On occasion of course we do test more, ie MRSA isolate, patient with multiple allergies.)

It made me realise just how much susceptibility testing that we do on a “Just in Case” basis, not only for Staphylococcus aureus, but also for a multitude of other micr-organisms also.

It will be very interesting if MALDI-TOF technology becomes proficient at identifying MRSA in the future. If this happens, then (if no history of allergy on the request form), one could simply put out a report saying: “Staphylococcus aureus isolated, (not MRSA). Susceptibilities on request.”

A radical, but ultimately reasonable and cost-effective approach, which I am sure would be accepted by laboratory users…


4 thoughts on ““Susceptibility testing for Staphylococcus aureus in community patients. Just in case..?”

  1. Perhaps the request form should list the patient’s allergies, so that you can only test for certain things if the patient is allergic to betalactams?

    (how much time does it take to detect MRSA and then start testing anti-MRSA antibiotics, as opposed to testing say vanc/clinda/bactrim from the get go?)

    1. I think a compulsory question regarding allergies is an excellent thing to put on an electronic request form, so that the requestor must answer this question before progressing with the request.

      Testing for extra antibiotics usually takes an extra 12-24 hours depending on what platform you use. Probably not a significant delay in a non-invasive sample. Horses for courses.

      Excellent comment. Many thanks Hildy, Michael

      1. I’m guilty of both overtreating and overtesting – I often doubletreat my patients empirically (cefazolin and clindamycin, or flucloxacillin and vancomycin depending on whether I suspect a non S aureus source) and order mecA PCR on the sample, stopping the MRSA cover if the mecA is negative. My local lab would report the mecA PCR within 24-36 hours, which was a lot faster than waiting for a culture to grow and then testing it. This is in the context of say the rheumatoid with a swollen joint that I’ve just aspirated, where MSSA PCR (I’m not sure what this looks for – is there a probe for the coagulase gene?) would also give more rapid results.

        1. Thanks Hildy.

          Clearly in invasive infections the faster you get susceptibilities, the quicker you get optimal treatment started, and if you can afford PCR, all the better! I think it is the “Nuc” gene which is specific for Staphylococcus aureus. I am sure there are others.


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