Recording of antimicrobial resistance of urinary tract isolates--effect of repeat samples on resistance levels

The effect of including repeat urine samples on the trimethoprim resistance level of urinary tract isolates in outpatients and inpatients has been studied on the basis of consecutive urinary tract isolates collected in the Turku City Hospital in 1982 and from outpatients in the Turku area in 1984. In the Turku City Hospital trimethoprim resistance occurred in 49.1% of all strains studied and when repeat samples from the same patients were excluded only in 39.4% (P less than 0.001). In outpatients, the corresponding figures were 17.5% and 15.7%, respectively (P greater than 0.10). These findings call for care to exclude repeat urine samples especially in those collected from inpatients. The effect of catheter isolates on the resistance figures and division of samples according to the patients' age are also discussed.