Precision and accuracy of asbestos fiber counting by phase contrast microscopy.

A new method was developed to prepare permanent asbestos slides with relocatable fields of view by imprinting these fields directly on the cleared wedges of filters containing asbestos fibers. The image quality of the fibers is comparable with that of the fibers prepared by the acetone/triacetin and the dimethyl formamide/Euparal method. The slides are suitable for evaluating the intercounter precision and accuracy of fiber counts by phase contrast microscopy. Seventeen chrysotile and 16 amosite slides, prepared from American Industrial Hygiene Association/National Institute for Occupational Safety and Health Proficiency Analytical Testing program samples, were evaluated by 58 analysts of 38 government and private laboratories in Canada. By asking the analysts to examine the same fields of view of the slides, the present study found that when examined at 400x, an average of 59.2 fiber counting errors were made for every 100 chrysotile fibers reported and 24.4 errors for 100 amosite fibers reported. The chrysotile fibers were underestimated by 25.0%, but there was no bias in counting amosite fibers. Remedial steps have been proposed to control the major source of errors, which is the subjective ability of the analyst to observe and size fibers. The slides may also be used to harmonize various phase contrast optical microscopy methods and proficiency testing programs.