Importance of Susceptibility Rate of 'the First' Isolate: Evidence of Real-World Data

For proper antimicrobial therapy, cumulative antibiograms should be representative of geographic region and be accurate. Clinical and Laboratory Standards Institute (CLSI) guidelines recommend that only the first isolates (FI) of a species per patient are used when reporting cumulative antibiograms. However, >50% of hospitals in the United States report antibiograms of all isolates. We compared antibiograms from the FI with those from total isolates (TI). Antimicrobial data of all isolates identified in the Microbiology unit of Ilsan Paik Hospital in 2019 were retrospectively acquired from the hospital information system. The susceptibility rates to antimicrobials of , , , , , , and were analyzed by FI and TI, respectively. Isolate counts and susceptibility rates of each species for the reported antimicrobials were compared. The numbers of isolates by FI/TI were as follows: 1824/2692 , 480/1611 , and 662/1306 , and 407/953 for gram-negative bacteria and 649/1364 , 211/313 , and 323/394 for gram-positive bacteria. All antimicrobial agents showed higher susceptibility rates when calculated as FI than as TI in gram-negative bacteria except colistin: 3.7% for , 14.5% for , 8.3% for , and 7.9% for . In , 8/11 antimicrobial agents revealed higher susceptibility rates for FI than for TI. and showed lower susceptibility rates for 7/10 antimicrobial agents for FI than for TI. The oxacillin susceptibility rates of were 36.6%/30.2% with FI/TI and vancomycin susceptibility rates for were 54.1% and 49.5%, respectively. When comparing cumulative antibiograms by FI with TI using real-world data, there is a large gap for critical species requiring hospital infection control. Although FI calculation is difficult, antibiograms must be calculated as FI for proper preemptive antimicrobial therapy because FI provides proper antimicrobial susceptibility data.