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Good day V Mallessan,
I am always thrilled when people question if basic assumptions have a basis in science.
J. H. Bowie and J. Dick from the Department of Microbiology, Royal Infirmary, Edinburgh, Scotland, developed this test to challenge the air removal portion of the sterilizer cycle. The technical definition of the Bowie-Dick test pack per AAMI: "[C]onsists of folded 100% cotton surgical towels. They should be freshly laundered, but not ironed. The towels must be folded to a size not smaller than 9 inches (23cm) in one direction and 12 inches (30 cm) in the other direction, and placed one above the other. The height of the test pack must be between 10 to 11 inches (25 to 28 cm). The total number of towels may vary from test to test, depending on towel thickness and wear." The test sheet is placed in the center of the pack before loosely wrapping with a single wrapper. An even color change on the test sheet with a 3 minutes, 30 second exposure time shows the sterilizer can remove air from the test pack.
These standard challenge test pack size and times were developed using 100% cotton surgical towels. This is convenient because almost all hospital sterilizing units have access to surgical towels. 100% cotton surgical towels have the density of material needed to challenge the sterilizer in the size outlined above (23cm x 30cm x 25 cm minimum). If you use material that is less dense than surgical towels, your test pack must be larger. If you use material denser than surgical towels, your pack must be smaller to avoid false positives. Most hospitals do not have the resources to invest in testing other materials to determine the proper density/size to match the 'freshly laundered surgical towel' standard challenge test size. Several commercial concerns have done the necessary research and sell pre-made, disposable Bowie-Dick style tests that use material other than 100% cotton surgical towels. (I particularly like the pre-made, disposable Bowie-Dick style tests using test sheets with Integrator ink that changes from yellow to blue in color. These ended all my arguments with maintenance personnel about how different shades of gray could be and remain acceptable. We all agree on the difference between yellow and blue!)
Regards, Pete Bobb
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