Laboratory testing policies and their effects on routine surveillance of community antimicrobial resistance

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