JAC Advance Access published online on April 21, 2004
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkh229
© 2004 by The British Society for Antimicrobial Chemotherapy
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1 National Public Health
Service for Wales, Abton House, Wedal Road, Cardiff CF14 3QX;
* To whom correspondence should be addressed. E-mail: Brendan.mason{at}nphs.wales.nhs.uk.
Objective: To investigate the effects
of laboratory testing policies, particularly selective testing,
rule-based reporting and isolate identification, on estimates of
community antimicrobial resistance. Materials and methods: Antibiotic resistance
estimates were analysed from an all-Wales dataset for approximately
300 000 community isolates of common pathogens. Results: Selective testing policies were often
associated with markedly increased resistance, particularly for
second-line testing. Site-specific testing tended to yield variant
resistance estimates for eye and ear isolates. Estimates from rule-based
reporting deviated markedly from test-result-based reporting. Urinary isolates
reported as Escherichia coli showed greater susceptibility
than those reported as undifferentiated urinary ‘coliforms’.
The proportion of isolates tested for an antibiotic by a laboratory
was a useful indicator of selective testing in this dataset. Selective
testing policies had invariably been applied where the proportion of
isolates of a species tested against an antibiotic was <90%.
As this proportion fell with increasingly selective policies, divergence
from pooled-all-Wales non-selective estimates tended to increase,
with a bias to increased resistance. Conclusions: Selective testing, rule-based reporting
and urinary coliform identification policies all had significant
effects upon resistance estimates. Triage based upon the proportion
of isolates tested seemed a useful tool in assigning analysis resources.
Where <20% of isolates were tested, selective
policies with inherent bias to increased resistance were common,
the low number of isolates gave high potential sampling errors,
and little confidence could be placed in the resistance estimate.
Where 20-90% of isolates were tested, detailed
analysis sometimes revealed resistance estimates that might be usefully
retrieved. Where
Revised March 7, 2004
Accepted March 9, 2004
Original article
Laboratory testing policies and their effects on
routine surveillance of community antimicrobial resistance
2 Department of Epidemiology, Statistics
and Public Health, University of Wales College of Medicine,
Cardiff CF14 4XW, UK
Abstract 
90% of isolates
were tested, there was no evidence of selective testing, and inter-laboratory
variation in estimates appeared to be safely ascribable to other
effects, e.g. methodology or real variation in resistance levels.
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