My laboratories still perform anti-microbial susceptibility testing by the “old fashioned” disc diffusion methodology. We have never been able to justify the costs of automated susceptibility testing (AST)systems such as Vitek or Phoenix, and the cost differences are not insignificant. (approx $1 v $8-10 per test). Multiply that by a few thousand tests and you see what I mean.

The clinicians seem to be happy with this as are the accreditation agencies. I appreciate that the turnaround time might be a bit quicker for AST systems, but is it worth the cost. I am not sure…

AST systems such as Vitek and Phoenix also give you an MIC approximation. However I think zone diameter also gives you a good indicator of how susceptible or how resistant an organism may be in-vitro, which is clearly important in some clinical cases.

I think the current AST systems promote susceptibility testing that is unnecessary, both in terms of the number of antibiotics that are tested against, and also the number of isolates that are set up for susceptibility testing. This adds to the cost further, and can also promote unnecessary use of antibiotics.

It is interesting that the TLA systems that are being developed at the moment are looking to incorporate both disc diffusion (plating, disc placing, reading & interpretation) and AST systems within their processes. This suggests to me that both approaches will be around for many years yet.

I don’t believe that our laboratories are inferior because we don’t have a Vitek or a Phoenix or something similar . On the contrary the use of disc diffusion makes us think hard about what isolates we submit for susceptibility testing, and we are very selective regarding the antibiotics we test against, As a result of this a smart, efficient and clinically acceptable approach to susceptibility testing has evolved within our laboratory network. 

As much as I am a fan of automation, I like the considered approach to susceptibility testing that disc diffusion offers, and would require some degree of convincing to move away from it….

Michael