The performance of laboratories enrolled in three of the Centers for Disease Control microbiology performance evaluation programs was studied in relation to the number of actual patient specimens tested by the laboratories. Laboratories were grouped according to the number of specimens tested weekly to compare performance among the groups. Results of the study showed that laboratories, as a group, that tested large numbers of specimens performed better than laboratories that tested small numbers. The infrequency of performing certain tests or identifying certain microbial species can be an important factor in laboratory error. One strategy for laboratories that cannot resolve internal problems associated with low testing volumes would be to limit their testing to procedures that are done well.

In 1976, the Centers for Disease Control initiated an external quality control program for the isolation and identification of Neisseria gonorrhoeae. This program required microbial samples of sufficient stability for shipment to laboratories throughout the United States. The Centers for Disease Control undertook studies to determine the most appropriate media for the propagation of strains for freeze-drying, the cell-suspending media that would afford protection during and after freeze-drying, the most favorable growth conditions, the proper times and methods for harvesting cells, the appropriate lyophilization conditions, the critical residual moisture content, and the stability of samples. These studies resulted in the development of methods for preparing and testing freeze-dried samples suitable for shipment.

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a Percentages in boldface denote results with pure-culture type samples; all other results were with mixed-culture type samples. b Percentages reflect correct identification of genus, genus and group, genus and species, or serotype.

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.