Effect of duplicate isolates of methicillin-susceptible and methicillin-resistant Staphylococcus aureus on antibiogram data

Duplicate Staphylococcus aureus isolates were analyzed to determine the impact of multiple isolates from the same patient on annual antibiogram data. During a 6-year period (1996 to 2001), 3,227 patients with 4,844 S. aureus isolates were evaluated. A total of 39% of patients with methicillin-resistant S. aureus (MRSA) (n = 860) and 23% of patients with methicillin-susceptible S. aureus (MSSA) (n = 2,367) infections had duplicate isolates. Cumulative data show that 91% of the patients during this 6-year period with duplicate isolates (2 to 13 duplicates/year) did not switch between MSSA and MRSA but retained the original S. aureus strain whether it was MSSA or MRSA. Rates of MRSA were calculated for each year by using all isolates and then eliminating duplicates. The impact of duplicate MRSA and MSSA isolates was evaluated by using the ratio of isolates per patient such that ratios of >1.0 indicate >1 isolate per patient. The 6-year ratio for MRSA was 1.90 isolates/patient, and the ratio for MSSA was 1.35. A significant difference (P < 0.05) was noted in the MRSA rates in 4 of 6 years when duplicate isolates were removed. Common phenotypic antibiogram patterns were compared for all MRSA isolates during the 6-year period, and 64% were of a single antibiogram phenotype. Eighty-eight percent of patients with duplicate MRSA isolates had phenotypically identical multiple isolates. The rate of MRSA differs when duplicate isolates are removed from the antibiogram data.