Assessment of Testing for and Completeness of Reporting of Vancomycin-Resistant Enterococci -- Connecticut, 1994

From 1989 through 1993, the proportion of enterococcal isolates resistant to vancomycin (VRE) reported to CDC's National Nosocomial Infections Surveillance (NNIS) system increased from 0.3% to 7.9% (1). Since January 1994, clinical laboratories in Connecticut have been required to report all sterile-site VRE isolates to the Connecticut Department of Public Health (CDPH) to determine the epidemiology of VRE infection in the state. In 1995, CDPH surveyed all clinical laboratories in the state to identify microbiologic methods used to determine antimicrobial susceptibility of enterococcal isolates and to assess the completeness of reporting in 1994. This report summarizes the survey findings and the assessment of reporting for VRE, which confirmed for the first time that VRE infections were occurring statewide in Connecticut.

During April 1995, CDPH mailed questionnaires to the laboratory directors at the 125 clinical laboratories in Connecticut and received completed questionnaires from the 46 (37%) laboratories with the capacity to identify enterococcal isolates and perform vancomycin-susceptibility testing of enterococci. A total of 37 laboratories were hospital- affiliated; nine were commercial. Of the 46 laboratories, 33 (72%) tested all enterococcal isolates for vancomycin resistance, and 13 (28%) tested isolates from sterile sites only.

In 1994, these 46 laboratories processed 11,290 enterococcal isolates from both sterile (e.g., blood) and nonsterile (e.g., stool) sites (median: 286 isolates, range: two-1109 isolates per laboratory); of these, 517 (5%) were reported to be vancomycin resistant. A total of 24 (52%) laboratories also performed speciation of enterococci. Of the 3202 isolates identified to species, 2556 (80%) were Enterococcus faecalis, and 646 (20%) were E. faecium; of these, 12 (less than 0.1%) and 120 (19%), respectively, were reported to be vancomycin resistant.

Methods of vancomycin-susceptibility testing varied among laboratories: 25 (54%) used the Kirby-Bauer method; 15 (33%), the automated Microscan (Dade International, West Sacramento, California) * system; nine (20%), the automated Vitek (bioMerieux, Hazlewood, Missouri) system; six (13%), vancomycin screen agar; four (9%), minimum inhibitory concentration panels; two (4%), the automated Sensititre (Accumed International, West Lake, Ohio) system; and two (4%), the automated Uniscept (bioMerieux, Hazlewood, Missouri) system. Nineteen (41%) laboratories used at least one duplicate test. Six laboratories using the Microscan and seven using the Vitek used a second method because of reports of failure to accurately detect antimicrobial resistance in enterococci with these systems (2,3).

To assess completeness of VRE reporting to the state health department, during May-July 1995, CDPH contacted laboratory and infection-control personnel from the 37 hospital-affiliated laboratories to identify sterile-site VRE isolates not previously reported in 1994. Passive reporting identified 34 sterile-site VRE isolates in 1994; the CDPH survey identified an additional 27 isolates, indicating that passive laboratory reporting identified 34 (56%) of 61 sterile-site VRE isolates. Of the 61 sterile-site VRE isolates identified through passive surveillance and the CDPH survey, 47 (77%) were from blood, representing 0.02% of the 238,937 bloodstream pathogens isolated by these laboratories in 1994.

Reported by: ZF Dembek, PhD, ML Cartter, MD, JL Hadler, MD, State Epidemiologist, Connecticut Dept of Public Health. Hospital Infections Program, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Because enterococci commonly are resistant to vancomycin and other widely used antimicrobials, infections with these organisms are virtually untreatable (4). Laboratory-based surveillance is critical in programs to detect, control, and prevent antimicrobial resistance in enterococci and other organisms (5). Connecticut is the first state to require statewide laboratory-based reporting of VRE isolates obtained from sterile sites.

During 1994, only 56% of all sterile-site VRE isolates initially were reported to CDPH. Efforts to increase laboratory reporting in Connecticut have included dissemination to all laboratory directors of CDPH publications that emphasize the importance of reporting and regular communication between CDPH and laboratory directors. These findings also underscore the importance of periodic validation of completeness of reporting of laboratory-based surveillance.

Since the first isolation of VRE in 1988, prevalence of infection has increased in both hospitalized patients and residents of long-term-care facilities (LTCFs), resulting in management and treatment problems (6). Although nosocomial transmission of VRE has been well documented, it is unclear whether the increase in the number of VRE isolates from patients of LTCFs (7) reflects changes in the epidemiology of VRE or increases in admission to LTCFs of patients who have been hospitalized in acute-care hospitals in which VRE is endemic. In response to concerns about admission of VRE-positive patients to LTCFs, CDPH has collaborated with infection- control personnel to develop guidelines for prevention of VRE infection and management of persons who are infected or colonized with VRE.

This report also highlights two issues for laboratories. First, because methods used to test vancomycin susceptibility in enterococci vary widely, as in Connecticut, and some methods fail to detect antimicrobial resistance (2,3), proficiency testing and standardization of acceptable methods may be appropriate for laboratories performing vancomycin- susceptibility testing of enterococci. Second, laboratories that test for vancomycin susceptibility should consider testing isolates to the species level. In Connecticut, only 52% of the laboratories surveyed performed species identification. Species identification is important in assessing the accuracy of susceptibility determinations, understanding the epidemiology of different enterococci strains, and measuring the prevalence of previously unknown clinical pathogens (e.g., E. galinerum, which is known to intrinsically have at least intermediate resistance to vancomycin {2}).

References

1. CDC. Nosocomial enterococci resistant to vancomycin -- United States, 1989-1993. MMWR 1993;42:597-9.

2. Tenover FC, Tokars J, Swenson J, Paul S, Spitalny K, Jarvis B. Ability of clinical laboratories to detect antimicrobial agent-resistant enterococci. J Clin Microbiol 1993;31:1695-9.

3. Zabransky RJ, DiNuzzo AR, Huber MB, Woods GL. Detection of vancomycin resistance in enterococci by the Vitek AMS system. Diagn Microbiol Infect Dis 1994;20:113-6.

4. Spera RV, Farber BF. Multiply resistant Enterococcus faecium: the nosocomial pathogen of the 1990's. JAMA 1992;268:2563-4.

5. CDC. Statewide surveillance for antibiotic-resistant bacteria -- New Jersey, 1992-1994. MMWR 1995;44:504-6.

6. Kaplan AH, Gilligan PH, Facklarn RR. Recovery of resistant enterococci during vancomycin prophylaxis. J Clin Microbiol 1988;26:1216-8.

7. Korten V, Murray BE. The nosocomial transmission of enterococci. Current Opinion in Infectious Diseases 1993;6:498-505.

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