Are routine sensitivity test data suitable for the surveillance of resistance? Resistance rates amongst Escherichia coli from blood and CSF from 1991-1997, as assessed by routine and centralized testing

Surveillance of antibiotic resistance can be undertaken by compilation of r outine data or by central testing of isolates. Routine results can be obtai ned cheaply and in sufficient quantities for correlation with population an d prescribing denominators but there is concern about their quality. As one of a series of ongoing studies to assess this quality, we compared the pro portions of resistance amongst Escherichia coli from patients with bacterae mia or meningitis between 1991 and 1997 (i) as recorded in routine data rep orted to the PHLS and (ii) as found in tests performed at the PHLS Laborato ry of Enteric Pathogens (LEP). These two data sets both showed an overall u pward trend in the proportion of isolates resistant to ampicillin, trimetho prim, gentamicin and ciprofloxacin. The average annual percentage increase in resistance was estimated in separate logistic regression models, and 95% confidence intervals (CI) were determined. The annual percentage increases in the proportions of isolates reported resistant were similar in the two data sets for trimethoprim, gentamicin and ciprofloxacin but differed for a mpicillin. The upward trends were statistically significant except for gent amicin resistance in the LEP data set, where the 95% CI straddled zero. The proportions of resistant isolates for each antibiotic in the two data sets each year were in poorer agreement than the trends; however, the 95% CI of the difference of proportions resistant between the routine and LEP data s ets straddled zero in 4 or 5 of the 7 years studied. Some discrepancies mig ht be explained by geographical bias in the sampling or by differences in d efinitions of resistance. Thus (i) the proportion of resistant isolates tes ted at LEP almost always fell within the ranges bounded by the highest and lowest proportions for individual Regional Health Authorities, as recorded in the routine data, and (ii) the fact that LEP consistently recorded less gentamicin resistance but more ciprofloxacin resistance than the routine co uld be explained by breakpoint differences. We conclude that routine suscep tibility data for ampicillin, ciprofloxacin, gentamicin and trimethoprim ap pear sound for E. coil and might be suitable for correlation with other dat a, e.g. for prescribing.