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Edinburgh Test Soil

Achieving Test Consistency Across NHS Trusts

Human blood substitutes used for washer/disinfector process cleaning challenges have always been a matter for debate, with even the official recipe for Edinburgh test soil from the HTM itself recently coming into question.

There are several alternatives to the Edinburgh soil that have been available for many years and are established consumable product accepted by most Authorized Engineers. By far the most popular is the dehydrated Browne test soil with the ‘just add water’ simplicity, lack of blood content and long shelf life.

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Pre-applied test soils such as the Pereg TOSI test strip and Browne STF Load check are also long standing consumables, the obvious benefit of these being the lack of any mixing and the specific surrogate devices available to increase the challenge.

The HTM Edinburgh test soil however, is the definitive human blood substitute for validation of washer/disinfectors, with the recipe laid out in the legislation.

The ingredients required however are not exactly ‘off the shelf’ products with supply obtained only from recognized Laboratory stockists which then need to be safely contained upon reaching site, mixed accordingly and the short seven day shelf life monitored.

Several companies, including Isopharm, have recognized the problems involved in adhering to this particular aspect of the HTM and offer supply of ready-mixed Edinburgh soil delivered refrigerated on the day of use.

The ease of purchasing such a unique product has proved extremely popular with Isopharm clients who are not able to use the alternative products in their validation procedure.

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There is however, an issue with regards to the HTM recipe and consistency. Clients who have compared Edinburgh soil from various sources have commented on consistency of colour and of viscosity. The HTM Edinburgh soil consists of an ingredient ratio of 10ml defibrinated horse blood, 2g Hog Mucin and 100ml egg yolk and herein lies the consistency problem: these are natural products.

  • Colour consistency can be affected by both the horse blood oxygenation at time of extraction, and the egg yolk which everyone knows varies through many shades of yellow!
  • As for thickness, it is the Hog mucin that predominantly dictates the viscosity of the soil. The HTM recipe simply refers to ‘hog mucin’, however this product is available in different refinements, Type II and Type III.

It is worth noting that there was an amendment in HTM2030 with regards to blood quantity within the Edinburgh soil that also affected the soil viscosity. The original recipe called for 100ml of horse blood, whereas the latest edition states only 10ml should be used.

The fact of the matter is that the HTM Edinburgh soil does not yield an entirely consistent soil by the very nature of the ingredients used, but at the same time provides an excellent efficacy challenge regardless of these slight variations. It is impossible to visually compare batches, the only comparison can be on the efficacy of the test.

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Due diligence dictates that your supplier should provide you with a certificate of conformance to the HTM with every bottle of Edinburgh test soil provided, showing ingredients, batch numbers and expiry dates.

Isopharm manufactures Edinburgh test soil in accordance with the HTM requirement and can provide a certificate of legislative conformance with every shipment.