Helix vs Bowie-Dick test?
Complete air removal is particularly difficult with porous loads such as linen and with hollow items such as cannulated instruments, dental handpieces or re-usable aspirator tips. However, without complete air removal, the item will not be sterile. Since the killing effectiveness of steam sterilisation results from the transfer of the latent heat of condensation that occurs when the steam comes into direct contact with a micro-organism, it is imperative the steam directly contacts all surfaces of the item being sterilised. Physical debris such as blood, body fluids or body tissues will act to insulate the surface of the item from direct contact with the steam as will any air trapped within the steriliser chamber or instrument.
The purpose of air removal and steam penetration tests (so named in AS/NZS 4815:2006) is to prove that your steriliser continues to be capable of removing all of the air from both the chamber and all parts of the load including from within porous or hollow items so that steam can effectively penetrate all parts of the load. The tests are designed to simulate air removal from a full load and must therefore be processed in an empty chamber.
Air removal and steam penetration tests are available in two types based on the type of load to be sterilised. The manufacturer of the test will indicate which type of test it is either on the packaging or on the accompanying documentation.
The first type of tests are commonly known as Bowie-Dick tests, named after the doctors who originally developed the ‘chemical indicator wrapped in towels’ version of this test. These tests are specific for porous loads such as fabric. You would use this type of test to confirm air removal and steam penetration if your loads include medical drapes or other linen. Various formats of this test are available from different manufacturers, most based on the principle of removing air through the equivalent of thick fabric layers, allowing steam to penetrate to the centre of the load. Most of these tests are single use.
The second type of tests are Helix PCD tests. These tests are more rigorous than the porous test as they are designed to demonstrate that the steriliser can effectively remove the air from a Hollow A device as defined in AS/NZS 4815:2006. These tests normally incorporate a long, fine calibrated tube of specific internal diameter and specific length to provide the equivalent of a Hollow A device. Most of these tests are a re-usable device with single use indicator that is fitted into one end of the device.
Note: Two other types of PCD similar in appearance to the Hollow A tests are available and care should be taken to ensure they are not mistakenly used for this testing. One type, usually matched to particular brands of sterilisers, are calibrated to show that the steriliser continues to meet the performance specifications set down by the steriliser manufacturer. These may not necessarily be designed to show air removal and steam penetration equivalent to Hollow A. The other PCD’s are batch control devices designed to be processed in each cycle with a normal load. These are for load monitoring purposes only and are not designed for the testing described here.
Both the porous and Hollow A air removal and steam penetration tests include a Class 2 indicator strip within the test that requires exposure to steam to achieve a colour change. These tests are to demonstrate effective air removal and steam penetration only, not to prove that the strip has been subjected to an adequate sterilising time to ensure sterilisation.
It is important to note that while the Australian Standard has adopted the cycle type definitions and load type definitions of the European Standard, it does not provide the clarification seen in the European Standard regarding which type of air removal and steam penetration tests should be used, only stating that this test should be carried out daily on all B-class sterilisers. It would seem logical to follow the European guidelines that if your load types include porous items such as linen drapes, then you should be testing your steriliser with a test that confirms the air removal and steam penetration of porous loads. Similarly, if your load types include Hollow A items such as cannulated medical devices, dental handpieces or re-usable aspirator tips, then you should be testing your steriliser with a test that confirms the air removal and steam penetration of Hollow A loads.
Daily testing of your steriliser using the appropriate test will provide you with the assurance that the vacuum system on your steriliser is functioning correctly to allow proper sterilisation of your loads throughout the day.
Please note that this information is general in nature and is based on the Australian Standards listed above and should only be taken as a guide and not advice. Please refer to product manufacturers instructions for detailed use.