Five years of experience with a national external quality control program for the culture and identification of Neisseria gonorrhoeae

Abstract

In response to a need for monitoring the proficiency of public health laboratories in isolating and identifying Neisseria gonorrhoeae, a national external quality control program was developed. Essentially, three types of freeze-dried samples, representing different levels of challenge for identification, were sent to laboratories for testing. The quality of the samples was confirmed by external reference laboratories, and stability of the samples was confirmed by thermal degradation tests before the samples were sent to laboratories enrolled in the program. By analyzing laboratory results, we identified common errors and chronic problems in testing samples. As a group, laboratories testing small numbers of actual patient specimens did not perform as well in the program as did laboratories testing large numbers of specimens; however, the performance of laboratories testing small numbers of specimens improved over time. Overall, laboratories experienced the most difficulty with samples containing N. gonorrhoeae mixed with other microbial species. Laboratories that performed confirmatory tests committed fewer errors than did laboratories that performed presumptive tests only, but the failure to use pure cultures of gonococci for inoculation of cystine tryptic digest agar appeared to be a chronic problem in confirmatory carbohydrate testing. A review of the use of different plating media and confirmatory tests showed that the use of certain media and tests changed over time.