The primary function of antimicrobial susceptibility testing
is to provide information to the clinician/ veterinarian on the choice of
appropriate chemotherapy whether it is for therapeutic or prophylactic purposes.
There has recently been confusion concerning bacterial
sensitivity testing in veterinary medicine. The purpose of this short paper
is to attempt to reduce that confusion and to explain what Axiom do in this
area and why.
Background
In human medicine there are many subgroups of the species Homo sapiens and
assumptions can be made about bacterial infection and associated treatment.
In veterinary medicine there are many species with numerous sub groups and
it is not possible to make similar assumptions.
The National Committee of Clinical Laboratory Standards
(NCCLS) is the standards committee for the USA covering mainly human,
but some veterinary antibiotic sensitivity testing, based on microbes recovered
in the US. They have different standards to the UK Health Protection Agency
(HPA) and British Society for Antimicrobial Chemotherapy (BSAC),
which are based solely on microbes found in the UK.
Minimal Inhibitory Concentrations
Minimal Inhibitory Concentrations (MIC) are the smallest concentration
of antibiotic that will inhibit the growth of a given bacteria.
An MIC offers a true sensitivity for each individual bacterium
against a given antibiotic and is considered the gold standard. Traditional
MIC evaluations are very time consuming and therefore not practicable for
routine use. Even in human medicine they are only performed in severe infections
e.g. meningitis, endocarditis, ITU patients or compromised neonates/geriatrics.
The majority of hospital pathology labs still report microbial sensitivity
using disc diffusion techniques.
MICs are not of use when giving oral antibiotics, as the
limiting factor will be the absorption from the gastrointestinal system,
or topical therapy where they are applicable. In the routine adult veterinary
clinical patient the only time that true MICs are likely to add any extra
information is in the very resistant Pseudomonas infections, which are currently
reported as resistant, but might truly be sensitive in vivo if a higher
concentration of antibiotic is achieved.
Breakpoints
In an effort to reduce the cost and time associated with true MICs, breakpoints
were introduced in human medicine.
Most human microbes, but not veterinary isolates have average
breakpoints concentrations available in widely published tables, gathered
from thousands of true MIC tests based on microbes isolated in a given,
well defined geography. These are the concentrations at which the average
isolate is considered resistant or sensitive.
In veterinary medicine there are no universally recognised
breakpoint for each antibiotic for all species.
Some veterinary laboratories use human data and make assumptions, but this
may be misleading. Even the NCCSL recognises “for those agents for
which veterinary specific interpretative data is not available, the use
of MIC breakpoints must be used with caution.”
Thus the routine use of breakpoints in veterinary medicine
is not recommended as data is frequently based on false premise and offers
no additional information from that of a properly controlled disc diffusion
test.
Disc Diffusion Methods
There are various disc diffusion methods some of which have considerable
faults.
The most basic method is the Stokes method. This involves simply putting
antibiotic discs onto a plate and reading “resistance”
or “sensitivity”. This is very crude has many factors
influencing the result and thus has severe limitations. It is the method
employed by most “practice” labs and some (smaller) commercial
facilities. These have been superseded by modified methods trying to remove
many of the influencing factors. In June 2003 the BSAC introduced a standard
disc diffusion method where zone diameters correlate inversely with MICs
(not breakpoints) from standard dilution tests.
This scientifically validated method is the one currently performed
at Axiom.
Sensitivity Testing at Axiom
All microbes have a sensitivity based on the most recent BSAC standard methodology.
We will not be reporting the MIC values routinely because they are irrelevant
to routine practice situations i.e. the use of oral or topical antibiotics.
They will be reported on an individual basis as clinical history dictates,
after consultation with one of the Axiom clinical pathologists.
Antibiotic sensitivities will be 'isolate' targeted instead
of the current practise of being 'site' targeted. |