Effect of removal of duplicate isolates on cumulative susceptibility reports

The objective of our study is to assess the impact of different methods of duplicate isolate removal on cumulative susceptibility reports. Over a 1-year period, we studied the effect of 3 methods of duplicate isola te removal on the cumulative percentage susceptibility of 9 Gram-negative b acilli to 15 antimicrobials. Raw data from which no duplicate isolates were removed (NR) were generated by the Sensititre (R) breakpoint susceptibilit y testing system. D3 and D7 were methods of duplicate isolate removal defin ed as follows: same patient, bacterial species, irrespective of susceptibil ity within either three (D3) ol seven (D7) calendar days of the date of the : previous culture. The third method evaluated was an algorithm utilized by Cerner (R), a laboratory management program that defines: duplicate isolat es as follows: same patient, bacterial species, and NCCLS susceptibility ca tegory to an individual antimicrobial. Differences in percentage susceptibi lity between the three methods of duplicate isolate removal and NR were ass essed. The number of isolates studied ranged from 80 (E. aerogenes) to 681 (P. aer uginosa). Of the methods of duplicate isolate removal, the highest percenta ge susceptibility occurred most frequently with Cerner (R) followed by D7 a nd D3. Differences in percentage susceptibility between methods of removal and NR ranged from -11 to 25%, -5 to 8%, and -3 to 10%, with Cerner (R), D3 , and D7, respectively. The percentage susceptibility was at least 5% highe r than NR with a method of removal for 15 individual organism/antimicrobial combinations in which susceptibility was greater than or equal to 70% by a t least one of the methods. These occurred most frequently with Enterobacte r species and Cerner (R). Although there is no consensus on the ideal method of duplicate isolate rem oval, one should be cognizant that these manipulations may produce differen t cumulative susceptibility reports. (C) 2001 Elsevier Science Inc. All rig hts reserved.