LETTER

We read with interest the recently published article from Alonso-Echanove et al. (1) on the proficiency of Spanish clinical laboratories in detecting vancomycin-resistant enterococci (VRE). In their conclusions, they found an overall acceptable proficiency in the detection of such strains by 22 participating laboratories and recommend that studies on validation of clinical laboratories of microbiology be conducted in order to prevent and control the spread of resistance. Dealing with this objective, and since 1989, the Sociedad Española de Enfermedades Infecciosas y Microbiología Clínica (SEIMC) has sponsored a quality control and proficiency program (Programa de Control de Calidad SEIMC). Specifically, in September 1998, a vancomycin-resistant clinical strain of Enterococcus faecalis displaying the VanA phenotype and for which the MIC of vancomycin was >256 µg/ml was sent to 223 laboratories participating in our program. These laboratories covered all regions of Spain as well as the complete spectrum of laboratory levels, from tertiary, university-affiliated hospitals to primary-care and private laboratories (Table 1). Along with the strain, a summary of clinical history clearly explaining that the isolate was hospital acquired was sent to all participants, who were asked to identify the strain and to perform susceptibility studies with those antimicrobials they considered suitable.

A total of 189 responses were returned to the society offices, and, of these, 176 laboratories (93.1%) sent results on vancomycin susceptibility, meaning that most of the clinical laboratories in our country were aware of the need to follow up on vancomycin resistance. Of those laboratories performing susceptibility tests with this glycopeptide, a total of 171 (97.2%) found the strain to be resistant, 1 gave an intermediate susceptibility test result, and 4 considered it to be susceptible. The four very major errors (susceptible instead of resistant) were obtained by the disk-diffusion method in two instances and by the Vitek system (bioMérieux, Madrid, Spain) in the other two. Neither the inhibition zone diameters nor the MICs obtained by these participants were reported to us, although those laboratories performing quantitative techniques were specifically asked to provide MIC results. Therefore, we were unable to discern if errors came from the result itself or from a misinterpretation of the laboratory test results. The minor error (intermediate instead of resistant) was reported by one laboratory using the Sceptor (Becton Dickinson, Madrid, Spain) microdilution system, which obtained a MIC of 16 µg/ml. In this case, the interpretation of the laboratory result was in agreement with the criteria of the National Committee for Clinical Laboratory Standards (2).

In conclusion, our survey shows that a vast proportion of the Spanish clinical laboratories can detect high-level vancomycin resistance in strains in an appropriate clinical context. In fact, this is the highest rate of concordance with the reference laboratory recorded in our quality control program when susceptibility to a particular antimicrobial was the primary goal. However, the results from our colleagues for strains with intermediate-level resistance (only 11 out of 22 participants in their study gave concordant results) are a matter of concern, although technical variability in MIC determinations (±1 twofold dilution) and the low number of participants might account for some of this discordance. Besides these limitations, the results of the participating laboratories clearly point out the need to perform proficiency testing with intermediate and low-level-resistant strains in the future. More information on the quality control program and on this survey in particular is available at the SEIMC Web site (3).