Abstract

Reference materials containing Listeria monocytogenes were tested in a collaborative study held in june 1989. Fourty laboratories participated. Each laboratory examinated 25 samples by 2 methods. For each method 5 of the 25 test samples were negative control samples (negative for Listeria). One method was standardized and used by all laboratories (Standard Method), the other was a method chosen by participating laboratory (OWN method). The median contamination level of the capsules was 9.27. Based on a lognormal distribution at least 19 out of 20 isolations should be made. With the Standard Method 97% of the capsules were found positive and 85% of the laboratories made at least 19 isolations. The OWN method gave significantly less isolations ; 85% of the capsules were found positive and 55% of the laboratories made at least 19 isolations. Four laboratories (10%) reported false positive isolations with the negative control samples. Concluded was that the Listeria monocytogenes reference materials are suitable for testing laboratory performance.