Quality control of fibre counting – the work of the Central Reference Laboratory

The standard method used to measure airborne asbestos fibre concentrations involves drawing a known volume of dust-laden air through a membrane filter, clearing and mounting the filter on a glass slide and counting the fibres using a phase contrast optical microscope. However, application of this method can lead to large differences in results being produced by different operators.In 1978, the UK Government’s Advisory Committee on Asbestos recommended that laboratories using this method should accept the need for regular check counting within and between laboratories with the aim of minimising systematic and random errors. It was also recommended that a Central Reference Laboratory (CRL) be established to carry out such inter-comparisons. Consequently, the Health and Safety Executive and the Asbestosis Research Council jointly placed a contract with the Institute of Occupational Medicine to operate a Central Reference Scheme (CRS) and set up a Steering Committee to oversee the work.The CRL’s initial activities were directed towards quantifying the differences in counting levels that existed between five experienced member laboratories and establishing the causes of such differences. A review of methodology used in the five laboratories showed a number of differences in the detailed procedures used to evaluate samples.A slide exchange was conducted to determine the inter-laboratory variation arising from these ‘in-house’ methods.Up to 2.5 fold systematic differences in counts were observed between laboratories, with the ratio of highest to lowest count being up to 12:1 for individual slides.Special exercises were then conducted to identify possible causes of these differences, including methodological effects such as microscope specification, eyepiece graticule, mounting procedures and counting rules. The major contribution to the variation arose, however, from the subjectivity involved in the visual counting procedure. A reference method was formulated and, following a training course attended by all participating microscopists, further slide exchanges were carried out to assess the reproducibility of the CRS reference method. Not surprisingly, inter-laboratory variation was reduced; systematic differences were less than 2-fold, with the ratio of the highest to lowest counts not exceeding 3:1. Thus, the work of the CRL quantified not only the variation being achieved by experienced laboratories but also the achievable variations.Despite the technical superiority of the CRS reference method, as indicated by the results of an international inter-laboratory comparison trial conducted by the Institute on behalf of the Commission of European Communities, the method published by the Asbestos International Association was adopted in 1983 as the European Reference Method. The work of the CRS led to the Regular Inter-laboratory Counting Exchange (RICE) scheme being set up in 1982 to minimise inter- laboratory variations in counting nationally. This scheme makes use of reference counts produced by the Magiscan imatge analyser using software known as the Manchester Asbestos Program (MAP). RICE is a quality control scheme, with satisfactory performance criteria derived initially on the basis of the counting performance of the CRS group of laboratories using their own routine methodologies. More stringent criteria have recently been introduced. Present membership of RICE comprises some 320 laboratories. Participation in RICE and the achievement and maintenance of satisfactory performance are amongst the requirements of laboratory accreditation for asbestos fibre counting set out by the National Measurement Accreditation Service (NAMAS).An international scheme, the Asbestos Fibre Regular Informal Counting Arrangement (AFRICA), similar to RICE was also established in 1984. Membership currently comprises 40 laboratories representing a total of 20 countries. Unlike RICE, AFRICA is simply a comparison scheme, with the RICE satisfactory performance criteria being used as guidelines only.Some ideas on future developments in RICE and AFRICA are also presented in this report.