An easy-to construct, easy-to-operate standardized system was developed for determining the residual biological contamination of surgical instruments, endoscopes and other medical appliances subjected to hospital cleansing and/or disinfection. It consists of standard-sized pieces of glass, metal or endoscope plastic--dirt carriers--either bare or enclosed in truncated Eppendorf caps to simulate hard-to-access conditions. The surface of the carriers is covered with model dirt simulating biological contamination and the carriers are then affixed to sturdy metal holders. Conventional model dirt were found to peel or flake off the carrier surface, lowering the precision of residual soil determination. A newly developed model dirt consisting of liver mash, lactose and sunflower oil and exhibiting low tendency to peel off surfaces was therefore used. The whole setup was subjected to chemical or enzymic cleansing programs at elevated temperature in hospital washer-disinfectors of two types, and the residual dirt after cleansing was determined by three methods. The method using toxicant-doped dirt that quenches the luminescence of an indicator bacterium Photobacterium phosphoreum gave satisfactory data under laboratory conditions but with hospital-washed samples it exhibited excessive fluctuations caused by bacterium--dirt interactions and by physical influences. Both other methods gave better results but displayed some process sensitivity. The luciferin-luciferase-based ATP bioluminescence assay sometimes gave low or even negative dirt level values and showed a low effect of reduced dirt accessibility on cleansing of metal carriers. The Bradford protein assay showed about equal cleansing efficiency for both easily and poorly accessible carriers after enzymic cleansing. Our system can be used for determining low levels of residual contamination of medical appliances after cleansing/disinfection and assessing the efficiency of commercial washer-disinfectors; its efficiency can be further increased by using a cleansing process-insensitive method for soil detection and quantification.
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