General
An accredited laboratory should expect and require that any calibration tools they use (such as a Test Slide) are fit for purpose, are verifiable, and appropriately certified by a certifier with proven performance.
Primarily, Pickford Phase Contrast Test Slides are fit for purpose for airborne asbestos analysis by the phase contrast microscope method in any country because similar certification protocols are in place to those used for certifying the UK HSE/NPL Mark II Test Slides. In practice, this is achieved by the application of rigorous testing protocols by the Environmental Analysis Laboratory, Southern Cross University (SCU), Australia which means that the Pickford test slide is fit for purpose because it is compared to, and shown to be equivalent in performance to that of the HSE/NPL Mark II Slide. This is in accordance with United Kingdom Health and Safety (UK HSE) HSG248, 2021 requirements.
Metrological traceability is one possible path to establish certification verifiability, but discussions with the Australian Government National Measurement Institute (NMI), have confirmed that this cannot be used to assess human visual performance operating at its lowest limit of visibility. Instead, traceability is assured because the Australian certification uses one of several Pickford Test Slides that have been tested and passed as satisfactory by the UK HSE, as well as using a certified HSE/NPL Mark II Test Slide.
The SCU certifiers work in a University laboratory accredited by NATA for a large number of environmental tests, and subject to ISO/IEC 17025 requirements. In addition, the Certifier’s optical performance is routinely tested for visual acuity and contrast sensitivity.
United Kingdom
The HSE/NPL Mark II (and Mark III (Band 5)) Phase Contrast Test Slide is specified in the UK HSE ‘Asbestos: the Analysts’ Guide’, HSG248 (https://www.hse.gov.uk/pubns/books/hsg248.htm) and mandated in UK airborne asbestos laboratories.
All HSE/NPL Mark II and Mark III (Band 5) Test Slides are tested by the UK HSE Science and Research Centre, and if satisfactory issued with a Certificate. The 2021 revision of HSG248 has now enabled certification outside of the UK provided that any new Test Slide is proven to be equivalent in performance to that of blocks 5, 6 and 7 of the HSE/NPL Mark II Test Slide – exactly the same as being equivalent in performance to Sets B, C and D of the Pickford Test slide
Australia and New Zealand
Currently, the WHS regulations in most Australian States and New Zealand require the use of the NOHSC Membrane Filter Method [NOHSC:3003 (2005)], which states ‘…it is necessary that the performance of proposed and existing microscopes be assessed by a Detection Limit Test Slide.’ Appendix F then explains that ‘The standard test slide consists of epoxy replicas of a Master Slide produced and certified by the National Physical Laboratory (UK)’ and that ‘…a satisfactory system will detect Block 5’. There is no explicit requirement that the HSE/NPL Test Slide be used, nor does it specify the version of the Slide – for example only the Mark II and Mark III (Band 5) versions of the HSE/NPL Slides are in fact acceptable whereas the Mark I and Mark III (Band 4) Slides are not acceptable. In May 2024, PhaseSlides and Proficiency Testing Australia (PTA) applied to Safe Work Australia (SWA) to allow the use of any alternative test slide provided it is certified as being equivalent in performance to that of the HSE/NPL Mark II Slide. This is a lengthy process and likely to take some time.
The Australian Institute of Occupational Hygienists (AIOH) 2024 Technical Note ‘Asbestos Analytical Equipment: Calibration Requirements’, available from https://www.aioh.org.au/product/asbestos-analytical-equipment-calibration-requirements-2024/, permits an alternative Test Slide providing it is certified as being equivalent in performance to that of the HSE/NPL Mark II Slide, which applies to the Pickford Slide because it is so certified by the Environmental Analysis Laboratory of the Southern Cross University. The Faculty of Asbestos Management of Australia and New Zealand (FAMANZ) also supports the use of the Pickford Test Slide.
United States of America
In USA, the Code of Federal Regulations (CFR) generally requires that a phase contrast test slide should ensure that the microscope is able to detect approximately 3 degrees of phase shift. This applies to the HSE/NPL Mark II Slide, and equally to the Pickford Test Slide. The CFR’s on Asbestos (1910.1001), mandate the use of the most current version of OSHA method ID-160, or the most current version of NIOSH Method 7400. Appendix A, Section 10 requires that microscopes satisfy the requirements of an ‘HSE phase shift test slide’ when conducting airborne asbestos analysis by the membrane filter method, as follows:
· OSHA Method ID-160, July 1997 requires the use of a ‘Phase shift test slide, mark II’.
· NIOSH Method 7400 (Issue 2, 15 Aug 1994) states ‘Check the phase-shift detection limit of the microscope periodically for each analyst/microscope combination’, and specifies the HSE/NPL Mark II phase-contrast test slide. Issue 3 (14 June 2019) only refers to a ‘phase-shift test slide’, plus guidance provided in Reference 19, OSHA Fact Sheet, by Crane and Harper, ‘The Phase Shift Test Slide Interpretation’, 2011. (https://aiha-assets.sfo2.digitaloceanspaces.com/AIHA/registry/aar/Phase-Shift-Interpretation-OSHA-NIOSH_2011_04_11.pdf)
· Crane and Harper (2011) state ‘Should a future technology or product become available for the purpose of assessing phase-shift, it should be acceptable to the extent that it could bracket or measure the phase shift of a microscope to assure that it is operating at about 3 degrees of phase shift.’ The HSE/NPL Mark II, Mark III (green) and Mark III (red) Slide are referred to as being acceptable. Note that the ‘green’ and ‘red’ Slides are now known as Mark III (Band 5) and Mark III (Band 4) Slides respectively. This equally applies to the Pickford Test Slide because its performance is equal to that of the HSE/NPL Mark II Slide.
For further technical information, please refer to the following peer reviewed paper, published in the Journal of Occupational and Environmental Hygiene (JOEH) https://www.tandfonline.com/doi/full/10.1080/15459624.2025.2464587
