Within-laboratory quality control of asbestos counting

Our plain-English summary of Within-laboratory quality control of asbestos counting, by T.L. Ogden, T. Shenton-Taylor, J.W. Cherrie, et al (The Annals of Occupational Hygiene, 1986).

The study

• Authors: T.L. Ogden, T. Shenton-Taylor, J.W. Cherrie, N.P. Crawford, S. Moorcroft, M.J. Duggan, P.A. Jackson, R.D. Treble
• Journal: The Annals of Occupational Hygiene
• Published: 1986;30(4):411-25
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Internal quality control schemes for asbestos counting are presented for four laboratories handling different sample types and with different organizations and facilities. The purpose is to present these not as perfect models, but as sources of ideas and experience which other laboratories can use in order to plan schemes best suited to their own needs. Common problems which emerge are the determination of reference counts for low-density slides and the handling of the density dependence of counter performance on such slides. One solution is a stock of permanently mounted reference slides, slowly replaced by random selection from routine slides. The reference result can be determined by multiple counting over a period, and the density dependence can be allowed for by using OGDEN'S method [OGDEN (1982) Health and Safety Executive Research Paper, No. 18. ISBN 0 7176 0101 3] of expressing deviations as multiples of a calculated 'expected standard deviation'. A link to the national mean can be provided through the Regular Inter-laboratory Counting Exchange (RICE) inter-laboratory quality control scheme.

PubMed has no abstract for this study. This is the authors' abstract, copied from the article.

In plain English

This was not a health study. It described the internal quality control schemes four UK laboratories used in early 1985 to check staff who count asbestos fibres on membrane filters under a microscope. The four were the Health and Safety Executive, the Institute of Occupational Medicine, the Greater London Council and the Laboratory of the Government Chemist. At the Institute of Occupational Medicine, the coefficient of variation (a measure of spread) of counters' results on the same slides at first ranged up to about 100% for chrysotile and 50% for amosite. After retraining in 1979 these ranged up to about 50% and 20%, and the average count level rose. Below about 50 fibres counted, precision depended on how many were counted. At the Greater London Council laboratory, three counters averaged between 20% below and 20% above the group mean over six months. The authors concluded that none of the schemes was perfect. They suggested a stock of permanently mounted slides, slowly replaced from routine slides, with counters' deviations scaled to allow for how variability changes with the number of fibres counted.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Descriptive report (progress report) of the internal quality control schemes for asbestos fibre counting in four laboratories
• Who was studied: Four UK laboratories (Health and Safety Executive, Institute of Occupational Medicine, Greater London Council, Laboratory of the Government Chemist) and their fibre counters, as of early 1985
• Exposure: No health exposure was studied. The subject was the counting of asbestos fibres by the membrane filter method, using samples from asbestos manufacturing (textiles, friction material, cement), asbestos removal and clearance testing, and pure asbestos samples including chrysotile and amosite
• Main results:
  • Institute of Occupational Medicine: before retraining, sample coefficients of variation ranged up to about 100% for chrysotile and 50% for amosite
  • Institute of Occupational Medicine: after the 1979 training course, sample coefficients of variation ranged up to about 50% for chrysotile and 20% for amosite
  • Institute of Occupational Medicine: the laboratory's mean count level approximately doubled within 6 months of the training course, then settled at about 50% above the pre-training level
  • Once fewer than about 50 fibres were counted, the achievable coefficient of variation depended on the number of fibres counted
  • Institute of Occupational Medicine: one counter (A) counted about twice as high as another (B) on denser slides but about half as high on low-density slides
  • Greater London Council laboratory, three counters over the first 6 months: averages ranged from 20% below to 20% above the group mean, individual coefficients of variation were 15-20%, and the total coefficient of variation was 25%
  • Greater London Council laboratory: the Magiscan image analyser may have overestimated by up to a factor of 5 relative to visual counts at lower densities
  • Laboratory of the Government Chemist: expressing deviations in 'expected standard deviation' units worked for low-density slides, and a mean deviation of less than one expected standard deviation was achievable
• The authors' conclusion: The authors concluded that none of the four schemes fully met the criteria. They suggested that the best scheme might use a stock of permanently mounted slides, slowly replaced by random selection from routine slides, with the Ogden method or similar used to allow for variation of the coefficient of variation with count. They added that each laboratory should decide what suits its own needs
• Limitations the authors state:
  • The authors described the schemes as imperfect and offered them as a progress report
  • The HSE scheme gave no direct control of performance on low-concentration samples
  • Six months of Greater London Council data may have been too short to detect changes over time
  • The Greater London Council scheme initially used too few slides, so counters might come to recognize them
  • Internal schemes compare counters only with each other within one laboratory

Related studies

• A PC-based quality control (QC) program for an industrial hygiene laboratory — D.K. Verma, K. Ferguson, et al. (2006)
• A proposal for harmonising laboratory performance assessment criteria in national asbestos fibre counting schemes — M.C. Arroyo, J.M. Rojo (2001)
• Statistical uncertainties in asbestos-laboratory classification through the RICE scheme — T.L. Ogden (1984)
• Quality control of airborne asbestos fibre counts in the United Kingdom--the present position — N.P. Crawford, A.J. Cowie (1984)
• A comparison of fibre counting across three European national proficiency testing schemes — A.D. Jones, M.C. Arroyo, et al. (2005)
• An inter-laboratory study to determine the effectiveness of procedures for discriminating amphibole asbestos fibers from amphibole cleavage fragments in fiber counting by phase-contrast microscopy — M. Harper, E.G. Lee, et al. (2012)

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