THE USE OF SPIKED ASBESTOS SAMPLES TO DETERMINE ANALYTICAL PROFICIENCY OF COMMERCIAL LABORATORIES DOING FIBER COUNTING

A series of proficiency testing samples containing one of two levels o f chrysotile fibers was prepared by an outside laboratory for use in a ssessing the competence of commercial laboratories doing asbestos fibe r counting for various corporate locations throughout the world. Sampl es were prepared at levels of 768 and 203 fibers/mm2, with standard de viations of 65 and 18 fibers/mm2, respectively, according to the prepa rer. A third set of samples with only a background material loaded on it was also prepared. The samples, which were submitted blind along wi th unknowns, were analyzed with phase contrast microscopy used by comm ercial laboratories in various parts of the United States. The results reported for the knowns were then analyzed statistically. Very few of the results were identified as outliers at a 98% confidence level, ei ther within a given laboratory using the Dixon test or among laborator ies using the Grubbs' test. The performances of the individual laborat ories were also compared against each other using Tukey Pairwise Compa risons. Among those laboratories that analyzed at least four samples a t a particular level, there was no difference among the laboratories' performances (at a 95% confidence level). Despite the failure of the s tatistical evaluation to distinguish among the laboratories' performan ces, the misidentification of spiked sample results by two laboratorie s proved useful in eliminating laboratories with procedural flaws from future use by the corporation.