This article presents results and conclusions obtained from the interlaboratory comparison studies of the luminescent bacteria toxicity assay performed during three consecutive years. This assay is one of the recommended bioassays used in the current Spanish and Catalonian regulations for toxicity assessment of industrial discharges. It is used routinely to assess the level of contamination of industrial wastewaters, and to calculate a levy related to volume and contaminant load of industrial wastewater discharges.
The objective of these exercises was to evaluate the repeatability of results obtained using this bioassay when used to assess toxicity of diverse wastewaters. Differences in bacterial sensitivity toward chemical substances, storage conditions, reagents, and sample handling, and minute differences in test procedure, may result in increased deviations when testing identical samples in different laboratories. A discussion on experimental conditions and test variables that may affect variability of results of this bioassay is provided.
Studies were performed yearly from 1993 to 1995, using four series of five samples in each exercise. Samples tested included standard solutions of pure compounds, and effluents collected either from industrial or municipal discharges.
( ) The number of participants increased steadily 11 in 1993, 19 in 1994, 24 in 1995 , which indicates increasing awareness of private and public laboratories in this type of quality assurance exercise. Overall, the relative standard deviations were respectively 48.8, 42.2, and 35.5%, which indicates not only a better knowledge of the nature of the assay, but also a trend of improvement in experimental procedures and quality assurance programs within each laboratory.